Tag: Standard Model of particle physics

• Worlds between theory and experiment

  Once Isaac Newton showed that a single gravitational law plus his rules of dynamics could reproduce the orbits of planets that Johannes Kepler had predicted, explain tides on Earth, and predict that a comet that had passed by once would return again, physicists considered Newtonian mechanics and gravitation to have been completely validated. After these successful tests, they didn’t wait to test every other possible prediction of Newton’s ideas before they considered them to be legitimate.

  When Jean Perrin and others carefully measured Brownian motion and extracted Avogadro’s number in the early 20th century, they helped cement the science of the kinetic theory of gases and statistical mechanics that Ludwig Boltzmann and Josiah Willard Gibbs had developed. As with Newtonian mechanics, physicists didn’t also require every single consequence of kinetic theory to be rechecked from scratch. They considered them all to be fully and equally legitimate then on.

  Similarly, in 1886-1889, Heinrich Hertz produced and detected electromagnetic waves in the laboratory, measured their speed and other physical properties, and showed that they behaved exactly as James Clerk Maxwell had predicted based on his (famous) equation. Hertz’s experiments didn’t test every possible configuration of charges and fields that Maxwell’s equations allowed, yet what they did test and confirm sufficed to convince all physicists that Maxwell’s theory could be treated as the correct classical theory of electromagnetism.

  In all these cases, a theory won broad acceptance after scientists validated only a small (yet robust) subset of its predictions. They didn’t have to validate every single prediction in distinct experiments.

  However, there are many ideas in high-energy particle physics that, even as they are derived from other theoretical constructs that have been tested to extreme precision, physicists insist on testing them anew as well. Why are they going to this trouble now?

  “High-energy particle physics” is a four-word label for something you’ve likely already heard of: the physics of the search for the subatomic particles like the Higgs boson and the efforts to identify their properties.

  In this enterprise, many scientific ideas follow from theories that have been validated by very large amounts of experimental data. Yet physicists want to test them at every single step because of the way such theories are built and the way unknown effects can hide inside their structures.

  The overarching theory that governs particle physicists is called, simply, the Standard Model. It’s a quantum field theory, i.e. a theory that combines the precepts of quantum mechanics and special relativity*. Because the Standard Model is set up in this way, it makes predictions about the relations between different observable quantities, e.g. the mass of a subatomic particle called the W boson with a parameter that’s related to the decay of other particles called muons. Some of these relations connect measured quantities with others that have not yet been probed, e.g. the mass of the muon with the rate at which Higgs bosons decay to pairs of muons. (Yes, it’s all convoluted.) These ‘extra’ relations often depend on assumptions that go beyond the domains that experiments have already explored. New particles and new interactions between them can change particular parts of the structure while leaving other parts nearly unchanged.

  (* Quantum field theory gives physicists a single, internally consistent framework in which they can impose both the rules of quantum theory and the requirements of special relativity, such as that information or matter can’t travel faster than light and that our spacetime conserves energy and momentum together, for example. However, quantum field theory does not unify quantum theory with general relativity; that’s the monumental and still unfinished purpose of the quantum gravity problem.)

  For a more intricate example, consider the gauge sector of the Standard Model, i.e. the parts of the Model involving the gluons, W and Z bosons, and photons, their properties, and their interactions with other particles. The gauge sector has been thoroughly tested in experiments and is well-understood. Now, the gauge sector also interacts with the Higgs sector, and the Higgs sector interacts with other sectors. The result is new possibilities involving the properties of the Higgs boson, their implications for the gauge sector, and so on that — even if physicists have tested the gauge sector — need to be tested separately. The reason is that none of these possibilities follow directly from the basic principles of the gauge sector.

  The search for ‘new physics’ also drives this attitude. ‘New physics’ refers to measurable entities and physical phenomena that lie beyond what the Standard Model can currently describe. For instance, most physicists believe a substance called dark matter exists (in order to explain some anomalous observations about the universe), but they haven’t been able to confirm what kind of particles dark matter is made of. One popular proposal is that dark matter is made of hitherto unknown entities called weakly interacting massive particles (WIMPs). The Standard Model in its contemporary form doesn’t have room for WIMPs, so the search for WIMPs is a search for new physics.

  Physicists have also proposed many ways to ‘extend’ the Standard Model to accommodate new kinds of particles that ‘repair’ the cracks in reality left by the existing crop of particles. Some of these extensions predict changes to the Model that are most pronounced in sectors that are currently poorly pinned down by existing data. This means even a sizeable deviation from the Model’s structure in this sector would still be compatible with all current measurements. This is another important reason physicists want to collect more data and with ever-greater precision.

  Earlier experience also plays an important role. Physicists may make some assumptions because they seem safe in some year but new data collected in the next two decades might reveal that they were mistaken. For instance, physicists believed neutrinos didn’t have mass, like photons, because that idea was consistent with many existing datasets. Yet dedicated experiments contradicted their belief (and won their performers the 2015 physics Nobel Prize).

  (Aside: High-energy particle physics uses large machines called particle colliders to coerce subatomic particles into configurations where they interact with each other, then collect data of those interactions. Operating these instruments demands hundreds of people working together, using sophisticated technologies and substantial computing resources. Because the instruments are so expensive, these collaborations aim to collect as much data as possible, then maximise the amount of information they extract from each dataset.)

  Thus, when a theory like the Standard Model predicts a specific process, that process becomes a thing to test. But even if the prediction seems simple or obvious, actually measuring it can still rule out whole families of rival theories offering to explain the same process. It also sharpens physicists’ estimates of the theory’s basic parameters, which then makes other predictions more precise and helps plan the next round of experiments. This is why, in high-energy physics, even predictions that follow from other, well-tested parts of a theory are expected to face experimental tests of their own. Each successful test can reduce the space for new physics to hide in — or in fact could reveal it.

  A study published in Physical Review Letters on December 3 showcases a new and apt example of testing predictions made by a theory some of whose other parts have already survived testing. Tests at the Large Hadron Collider (LHC) — the world’s largest, most powerful particle collider — had until recently only weakly constrained the Higgs boson’s interaction with second-generation leptons (a particle type that includes muons). The new study provides strong, direct evidence for this coupling and significantly narrows that gap.

  The LHC operates by accelerating two beams of protons in opposite directions to nearly the speed of light and smashing them head on. Its operation is divided into segments called ‘runs’. Between runs, the collaboration that manages the machine conducts maintenance and repair work and, sometimes, upgrades its detectors.

  One of the LHC’s most prominent detectors is named ATLAS. To probe the interactions between Higgs bosons and leptons, the ATLAS collaboration collected and analysed data from the LHC’s run 2 and run 3. The motivation was to obtain direct evidence for Higgs bosons’ coupling to muons and to measure its strength. And in the December 3 paper, the collaboration reported that the coupling parameters were consistent with the Standard Model’s predictions.

  So that’s one more patch of the Standard Model that has passed a test, and one more door to ‘new physics’ that has closed a little more.

  ---

  Featured image: A view of the Large Hadron Collider inside its tunnel. Credit: CERN.

  2025.12.06

• Is the Higgs boson doing its job?

  At the heart of particle physics lies the Standard Model, a theory that has stood for nearly half a century as the best description of the subatomic realm. It tells us what particles exist, how they interact, and why the universe is stable at the smallest scales. The Standard Model has correctly predicted the outcomes of several experiments testing the limits of particle physics. Even then, however, physicists know that it’s incomplete: it can’t explain dark matter, why matter dominates over antimatter, and why the force of gravity is so weak compared to the other forces. To settle these mysteries, physicists have been conducting very detailed tests of the Model, each of which has either tightened their confidence in a hypothetical explanation or has revealed a new piece of the puzzle.

  A central character in this story is a subatomic particle called the W boson — the carrier of the weak nuclear force. Without it, the Sun wouldn’t shine because particle interactions involving the weak force are necessary for nuclear fusion to proceed. W bosons are also unusual among force carriers: unlike photons (the particles of light), they’re massive, about 80-times heavier than a proton. This mass difference — of a massless photon and a massive W boson — arises due to a process called the Higgs mechanism. Physicists first proposed this mechanism in 1964 and confirmed it was real when they found the Higgs boson particle at the Large Hadron Collider (LHC) in 2012.

  

  But finding the Higgs particle was only the beginning. To prove that the Higgs mechanism really works the way the theory says, physicists need to check its predictions in detail. One of the sharpest tests involves how W bosons scatter off each other at high energies. The key to achieving this is the W boson’s polarisation states. Both photons and W bosons have a property called quantum spin, but whereas for photons its value is zero, for W bosons its non-zero. The spin also has a direction. If it points sideways, the W boson is said to be transverse polarised; if it’s pointing along the particle’s direction of travel, the W boson is said to be longitudinally polarised. The longitudinal ones are special because their behaviour is directly tied to the Higgs mechanism.

  Specifically, if the Higgs mechanism and the Higgs boson don’t exist, calculations involving the longitudinal W bosons scattering off of each other quickly give rise to nonsensical mathematical results in the theory. The Higgs boson acts like a regulator in this engine, preventing the mathematics from ‘blowing up’. In fact, in the 1970s, the theoretical physicists Benjamin Lee, Chris Quigg, and Hugh Thacker showed that without the Higgs boson, the weak force would become uncontrollably powerful at high energies, leading to the breakdown of the theory. Their work was an important theoretical pillar that justified building the colossal LHC machine to search for the Higgs boson particle.

  https://withspeak.wpcomstaging.com/2025/06/24/technical-foundation-for-a-muon-collider-laid-at-j-parc/

  The terms Higgs boson, Higgs field, and Higgs mechanism describe related but distinct ideas. The Higgs field is a kind of invisible medium thought to fill all of space. Particles like W bosons and Z bosons interact with this field as they move and through that interaction they acquire mass. This is the Higgs mechanism: the process by which particles that would otherwise be massless become heavy.

  The Higgs boson is different: it’s a particle that represents a vibration or a ripple in the Higgs field, just as a photon is a ripple in the electromagnetic field. Its discovery in 2012 confirmed that the field is real and not just something that appears in the mathematics of the theory. But discovery alone doesn’t prove the mechanism is doing everything the theory demands. To test that, physicists need to look at situations where the Higgs boson’s balancing role is crucial.

  The scattering of longitudinally polarised W bosons is a good example. Without the Higgs boson, the probabilities of the scatterings occurring uncontrollably at higher energy, but with the Higgs boson in the picture, they stay within sensible bounds. Observing longitudinally polarised W bosons behaving as predicted is thus evidence for the particle as well as a check on the field and the mechanism behind it.

  Imagine a roller-coaster without brakes. As it goes faster and faster, there’s nothing to stop it from flying off the tracks. The Higgs mechanism is like the braking system that keeps the ride safe. Observing longitudinally polarised W bosons in the right proportions is equivalent to checking that the brakes actually work when the roller-coaster speeds up.

  

  Another path that physicists once considered and that didn’t involve a Higgs boson at all was called technicolor theory. Instead of a single kind of Higgs boson giving the W bosons their mass, technicolor proposed a brand-new force. Just as the strong nuclear force binds quarks into protons and neutrons, the hypothetical technicolor force would bind new “technifermion” particles into composite states. These bound states would mimic the Higgs boson’s job of giving particles mass, while producing their own new signals in high-energy collisions.

  The crucial test to check whether some given signals are due to the Higgs boson or due to technicolor lies in the behaviour of longitudinally polarised W bosons. In the Standard Model, their scattering is kept under control by the Higgs boson’s balancing act. In technicolor, by contrast, there is no Higgs boson to cancel the runaway growth. The probability of the scattering of longitudinally polarised W bosons would therefore rise sharply with more energy, often leaving clearly excessive signals in the data.

  Thus, observing longitudinally polarised W bosons at consistent with the predictions of the Standard Model, and not finding any additional signals, would also strengthen the case for the Higgs mechanism and weaken that for technicolor and other “Higgs-less” theories.

  At the Large Hadron Collider, the cleanest way to study look for such W bosons is in a phenomenon called vector boson scattering (VBS). In VBS, two protons collide and the quarks inside them emit W bosons. These W bosons then scatter off each other before decaying into lighter particles. The leftover quarks form narrow sprays of particles, or ‘jets’, that fly far forward.

  If the two W bosons happen to have the same electric charge — i.e. both positive or both negative — the process is even more distinctive. This same-sign WW scattering is quite rare and that’s an advantage because then it’s easy to spot in the debris of particle collisions.

  Both ATLAS and CMS, the two giant detectors at the LHC, had previously observed same-sign WW scattering without breaking down the polarisation. In 2021, the CMS detector reported the first hint of longitudinal polarisation but at a statistical significance only of 2.3 sigma, which isn’t good enough (particle physicists prefer at least 3 sigma). So after the LHC completed its second run in 2018, collecting data from around 10 quadrillion collisions between protons, the ATLAS collaboration set out to analyse it and deliver the evidence. This group’s study was published in Physical Review Letters on September 10.

  

  The challenge of finding longitudinally polarised W bosons is like finding a particular needle in a very large haystack where most of the needles look nearly identical. So ATLAS designed a special strategy.

  When one W boson decays, the result is one electron or muon and one neutrino. If the W boson is positively charged, for example, the decay could be to one anti-electron and one electron-neutrino or to one anti-muon and a muon-neutrino. Anti-electrons and anti-muons are positively charged. If the W boson is negatively charged, the products could one electron and one electron-antineutrino or one muon and one muon-antineutrino. So first, ATLAS zeroed in on the fact that it was looking for two electrons, two muons, or one of each, both carrying the same electric charge. Neutrinos however are really hard to catch and study, so the ATLAS group look for their absence rather than their presence. In all these particle interactions, the law of conservation of momentum holds — which means in a given interaction, a neutrino’s presence can be elucidated when the momenta of the electrons or muons add up to be slightly lower than that of the W boson; the missing amount would have been carried away by the neutrino, like money unaccounted for in a ledger.

  This analysis also required an event of interest to have at least two jets (reconstructed from streams of particles) with a combined energy above 500 GeV and separated widely in rapidity (which is a measure of their angle relative to the beam). This particular VBS pattern — two electrons/muons, two jets, and missing momentum — is the hallmark of same-sign WW scattering.

  Second, even with these strict requirements, impostors creep in. The biggest source of confusion is WZ production, a process in which another subatomic particle called the Z boson decays invisibly or one of its decay products goes unnoticed, making the event resemble WW scattering. Other sources include electrons having their charges mismeasured, jets can masquerading as electrons/muons, and some quarks producing electrons/muons that slip into the sample. To control for all this noise, the ATLAS group focused on control regions: subsets of events that produced a distinct kind of noise that the group could cleanly ‘subtract’ from the data to reveal same-sign WW scattering, thus also reducing uncertainty in the final results.

  Third, and this is where things get nuanced: the differences between transverse and longitudinally polarised W bosons show up in distributions — i.e. how far apart the electrons/muons are in angle, how the jets are oriented, and the energy of the system. But since no single variable could tell the whole story, the ATLAS group combined them using deep neural networks. These machine-learning models were fed up to 20 kinematic variables — including jet separations, particle angles, and missing momentum patterns — and trained to distinguish between three groups:

  (i) Two transverse polarised W bosons;

  (ii) One transverse polarised W boson and one longitudinally polarised W boson; and

  (iii) Both longitudinally polarised W bosons

  Fourth, the group combined the outputs of these neural networks and fit with a maximum likelihood method. When physicists make measurements, they often don’t directly see what they’re measuring. Instead, they see data points that could have come from different possible scenarios. A likelihood is a number that tells them how probable the data is in a given scenario. If a model says “events should look like this,” they can ask: “Given my actual data, how likely is that?” And the maximum likelihood method will help them decide the parameters that make the given data most likely to occur.

  For example, say you toss a coin 100 times and get 62 heads. You wonder: is the coin fair or biased? If it’s fair, the chance of exactly 62 heads is small. If the coin is slightly biased (heads with probability 0.62), the chance of 62 heads is higher. The maximum likelihood estimate is to pick the bias, or probability of heads, that makes your actual result most probable. So here the method would say, “The coin’s bias is 0.62” — because this choice maximises the likelihood of seeing 62 heads out of 100.

  In their analysis, the ATLAS group used the maximum likelihood method to check with the LHC data ‘preferred’ a contribution from longitudinal scattering, after subtracting what background noise and transverse-only scattering could explain.

  The results are a milestone in experimental particle physics. In the September 10 paper, ATLAS reported evidence for longitudinally polarised W bosons in same-sign WW scattering with a significance of 3.3 sigma — sufficiently close to 4, which is the calculated significance based on the predictions of the Standard Model. This means the data behaved as theory predicted, with no unexpected excess or deficit.

  It’s also bad news for technicolor theory. By observing longitudinal W bosons at exactly the rates predicted by the Standard Model, and not finding any additional signals, the ATLAS data strengthens the case for the Higgs mechanism providing the check on the W bosons’ scattering probability, rather than the technicolor force.

  The measured cross-section for events with at least one longitudinally polarised W boson was 0.88femtobarns, with an uncertainty of 0.3 femtobarns. These figures essentially mean that there were only a few hundred same-sign WW scattering events in the full dataset of around 10 quadrillion proton-proton collisions. The fact that ATLAS could pull this signal out of such a background-heavy environment is a testament to the power of modern machine learning working with advanced statistical methods.

  The group was also able to quantify the composition of signals. Among others:

  1. About 58% of events were genuine WW scattering
  2. Roughly 16% were from WZ production
  3. Around 18% arose from irrelevant electrons/muons, charge misidentification or the decay of energetic photons

  One way to appreciate the importance of these findings is by analogy: imagine trying to hear a faint melody being played by a single violin in the middle of a roaring orchestra. The violin is the longitudinal signal; the orchestra is the flood of background noise. The neural networks are like sophisticated microphones and filters, tuned to pick out the violin’s specific tone. The fact that ATLAS couldn’t only hear it but also measured its volume to match the score written by the Standard Model is remarkable.

  Perhaps in the same vein, these results are more than just another tick mark for the Standard Model. It’s a direct test of the Higgs mechanism in action. The discovery of the Higgs boson particle in 2012 was groundbreaking but proving that the Higgs mechanism performs its theoretical role requires demonstrating that it regulates the scattering of W bosons. By finding evidence for longitudinally polarised W bosons at the expected rate, ATLAS has done just that.

  The results also set the stage for the future. The LHC is currently being upgraded to a form called the High-Luminosity LHC and it will begin operating later this decade, collect datasets about 10x larger than what the LHC did in its second run. With that much more data, physicists will be able to study differential distributions, i.e. how the rate of longitudinal scattering varies with energy, angle or jet separation. These patterns are sensitive to hitherto unknown particles and forces, such as additional Higgs-like particles or modifications to the Higgs mechanism itself. That is, even small deviations from the Standard Model’s predictions could hint at new frontiers in particle physics.

  Indeed, history has often reminded physicists that such precision studies often uncover surprises. Physicists didn’t discover neutrino oscillations by finding a new particle but by noticing that the number of neutrinos arriving from the Sun at detectors on Earth didn’t match expectations. Similarly, minuscule mismatches between theory and observations in the scattering of W bosons could someday reveal new physics — and if they do, the seeds will have been planted by studies like that of the ATLAS group.

  https://withspeak.wpcomstaging.com/2025/09/06/challenging-the-neutrino-signal-anomaly/

  On the methodological front, the analysis also showcases how particle physics is evolving. ‘Classical’ analyses once banked on tracking single variables; now, deep learning has played a starring role by combining many variables into a single discriminant, allowing ATLAS to pull the faint signal of longitudinally polarised W bosons from the noise. This approach could only become more important as both datasets and physicists’ ambitions expand.

  Perhaps the broadest lesson in all this is that science often advances by the unglamorous task of verifying the details. The discovery of the Higgs boson answered one question but opened many others; among them, measuring how it affects the scattering of W bosons is one of the ore direct ways to probe whether the Standard Model is complete or just the first chapter of a longer story. Either way, the pursuit exemplifies the spirit of checking, rechecking, testing, and probing until scientists truly understand how nature works at extreme precision.

  Featured image: The massive mural of the ATLAS detector at CERN painted by artist Josef Kristofoletti. The mural is located at the ATLAS Experiment site and shows on two perpendicular walls the detector with a collision event superimposed. The event on the large wall shows a simulation of an event that would be recorded in ATLAS if a Higgs boson was produced. The cavern of the ATLAS Experiment with the detector is 100 m directly below the mural. The height of the mural is about 12 m. The actual ATLAS detector is more than twice as big. Credit: Claudia Marcelloni, Michael Barnett/CERN.

  2025.09.12

• Found: clue to crack the antimatter mystery

  Imagine you’ve put together a torchlight. You know exactly how each part of the device works. You know exactly how they’re all connected togetger. Yet when you put in fresh batteries and turn it on, the light flickers. You take the torchlight apart, check each component piece by piece. It’s all good. The batteries are fully charged as well. Then you put it back together and turn it — and the light flickers still.

  This torchlight is the Standard Model of particle physics. It’s the main theory of its field: it ties together the various properties of all the subatomic particles scientists have found thus far. It organises them into groups, describes how the groups interact with each other, and makes predictions about particles that have been tested to extraordinary precision. And yet, the Standard Model can’t explain what dark matter is, why the Higgs boson is so light or how neutrinos have mass.

  Physicists are thus looking for ‘new physics’: a hitherto unseen part of the torchlight’s internal apparatus that causes its light to flicker, i.e. some new particle or force that completes the Standard Model, closing the gaps that the current crop of particles and forces haven’t been able to.

  This search for new physics received a boost yesterday when the physicists working with one of the detectors of the Large Hadron Collider reported that they had observed CP violations in baryons. This phenomenon is required to explain why the universe has more matter than antimatter today even though it was assumed to have been born with equal quantities of both. Baryons are particles made up of three quarks, like protons and neutrons.

  CP symmetry is the idea that the laws of physics should be the same if you swap all particles with their antiparticles and flip left and right, like looking in a mirror. Thus CP violation in baryons means if swapped a baryon with the corresponding anti-baryon and swapped left and right, the laws of physics won’t be the same, i.e. the laws treat matter and antimatter differently.

  I wrote about this finding and its implications — including its place in the Sakharov conditions and what the results mean for the Standard Model — for The Hindu. Do read it.

  I’ve found it’s one of those things you don’t read because it has anything to say about saving money or living longer. By reminding you that there’s a natural universe out there worth exploring and discovering and that it contains no sign or imprint of the false justifications humans have advanced for their crimes, perhaps it can help you live better. As I’ve said before, if you’re not interested in particle physics, that’s fine. But remember that you can be.

  Featured image: A view of the LHCb detector at the LHC as seen through a fisheye lens. Credit: CERN.

  2025.07.17

• New LHC data puts ‘new physics’ lead to bed

  One particle in the big zoo of subatomic particles is the B meson. It has a very short lifetime once it’s created. In rare instances it decays to three lighter particles: a kaon, a lepton and an anti-lepton. There are many types of leptons and anti-leptons. Two are electrons/anti-electrons and muons/anti-muons. According to the existing theory of particle physics, they should be the decay products with equal probability: a B meson should decay to a kaon, electron and anti-electron as often as it decays to a kaon, muon and anti-muon (after adjusting for mass, since the muon is heavier).

  In the last 13 years, physicists studying B meson decays had found on four occasions that it decayed to a kaon, electron and anti-electron more often. They were glad for it, in a way. They had worked out the existing theory, called the Standard Model of particle physics, from the mid-20th century in a series of Nobel Prize-winning papers and experiments. Today, it stands complete, explaining the properties of a variety of subatomic particles. But it still can’t explain what dark matter is, why the Higgs boson is so heavy or why there are three ‘generations’ of quarks, not more or less. If the Standard Model is old physics, particle physicists believe there could be a ‘new physics’ out there – some particle or force they haven’t discovered yet – which could really complete the Standard Model and settle the unresolved mysteries.

  Over the years, they have explored various leads for ‘new physics’ in different experiments, but eventually, with more data, the findings have all been found to be in line with the predictions of the Standard Model. Until 2022, the anomalous B meson decays were thought to be a potential source of ‘new physics’ as well. A 2009 study in Japan found that some B meson decays created electron/anti-electrons pairs more often than muons/anti-muon pairs – as did a 2012 study in the US and a 2014 study in Europe. The last one involved the Large Hadron Collider (LHC), operated by the European Organisation for Nuclear Research (CERN) in France, and a detector on it called LHCb. Among other things, the LHCb tracks B mesons. In March 2021, the LHCb collaboration released data qualitatively significant enough to claim ‘evidence’ that some B mesons were decaying to electron/anti-electron pairs more often than to muon/anti-muon pairs.

  But the latest data from the LHC, released on December 20, appears to settle the question: it’s still old physics. The formation of different types of lepton/anti-lepton particle pairs with equal probability is called lepton-flavour universality. Since 2009, physicists had been recording data that suggested that one type of some B meson decays were violating lepton-flavour university, in the form of a previously unknown particle or force acting on the decay process. In the new data, physicists analysed B meson decays in the current as well as one of two other pathways, and at two different energy levels – thus, as the official press release put it, “yielding four independent comparisons of the decays”. The more data there is to compare, the more robust the findings will be.

  This data was collected over the last five years. Every time the LHC operates, it’s called a ‘run’. Each run generates several terabytes of data that physicists, with the help of computers, comb through in search of evidence for different hypotheses. The data for the new analysis was collected over two runs. And it led physicists to conclude that B mesons’ decay does not violate lepton-flavour universality. The Standard Model still stands and, perhaps equally importantly, a 13-year-old ‘new physics’ lead has been returned to dormancy.

  The LHC is currently in its third run; scientists and engineers working with the machine perform maintenance and install upgrades between runs, so each new cycle of operations is expected to produce more as well as more precise data, leading to more high-precision analyses that could, physicists hope, one day reveal ‘new physics’.

  2022.12.20

• US experiments find hint of a break in the laws of physics

  

  At 9 pm India time on April 7, physicists at an American research facility delivered a shot in the arm to efforts to find flaws in a powerful theory that explains how the building blocks of the universe work.

  Physicists are looking for flaws in it because the theory doesn’t have answers to some questions – like “what is dark matter?”. They hope to find a crack or a hole that might reveal the presence of a deeper, more powerful theory of physics that can lay unsolved problems to rest.

  The story begins in 2001, when physicists performing an experiment in Brookhaven National Lab, New York, found that fundamental particles called muons weren’t behaving the way they were supposed to in the presence of a magnetic field. This was called the g-2 anomaly (after a number called the gyromagnetic factor).

  An incomplete model

  Muons are subatomic and can’t be seen with the naked eye, so it could’ve been that the instruments the physicists were using to study the muons indirectly were glitching. Or it could’ve been that the physicists had made a mistake in their calculations. Or, finally, what the physicists thought they knew about the behaviour of muons in a magnetic field was wrong.

  In most stories we hear about scientists, the first two possibilities are true more often: they didn’t do something right, so the results weren’t what they expected. But in this case, the physicists were hoping they were wrong. This unusual wish was the product of working with the Standard Model of particle physics.

  According to physicist Paul Kyberd, the fundamental particles in the universe “are classified in the Standard Model of particle physics, which theorises how the basic building blocks of matter interact, governed by fundamental forces.” The Standard Model has successfully predicted the numerous properties and behaviours of these particles. However, it’s also been clearly wrong about some things. For example, Kyberd has written:

  When we collide two fundamental particles together, a number of outcomes are possible. Our theory allows us to calculate the probability that any particular outcome can occur, but at energies beyond which we have so far achieved, it predicts that some of these outcomes occur with a probability of greater than 100% – clearly nonsense.

  The Standard Model also can’t explain what dark matter is, what dark energy could be or if gravity has a corresponding fundamental particle. It predicted the existence of the Higgs boson but was off about the particle’s mass by a factor of 100 quadrillion.

  All these issues together imply that the Standard Model is incomplete, that it could be just one piece of a much larger ‘super-theory’ that works with more particles and forces than we currently know. To look for these theories, physicists have taken two broad approaches: to look for something new, and to find a mistake with something old.

  For the former, physicists use particle accelerators, colliders and sophisticated detectors to look for heavier particles thought to exist at higher energies, and whose discovery would prove the existence of a physics beyond the Standard Model. For the latter, physicists take some prediction the Standard Model has made with a great degree of accuracy and test it rigorously to see if it holds up. Studies of muons in a magnetic field are examples of this.

  According to the Standard Model, a number associated with the way a muon swivels in a magnetic field is equal to 2 plus 0.00116591804 (with some give or take). This minuscule addition is the handiwork of fleeting quantum effects in the muon’s immediate neighbourhood, and which make it wobble. (For a glimpse of how hard these calculations can be, see this description.)

  Fermilab result

  In the early 2000s, the Brookhaven experiment measured the deviation to be slightly higher than the model’s prediction. Though it was small – off by about 0.00000000346 – the context made it a big deal. Scientists know that the Standard Model has a habit of being really right, so when it’s wrong, the wrongness becomes very important. And because we already know the model is wrong about other things, there’s a possibility that the two things could be linked. It’s a potential portal into ‘new physics’.

  “It’s a very high-precision measurement – the value is unequivocal. But the Standard Model itself is unequivocal,” Thomas Kirk, an associate lab director at Brookhaven, had told Science in 2001. The disagreement between the values implied “that there must be physics beyond the Standard Model.”

  This is why the results physicists announced today are important.

  The Brookhaven experiment that ascertained the g-2 anomaly wasn’t sensitive enough to say with a meaningful amount of confidence that its measurement was really different from the Standard Model prediction, or if there could be a small overlap.

  Science writer Brianna Barbu has likened the mystery to “a single hair found at a crime scene with DNA that didn’t seem to match anyone connected to the case. The question was – and still is – whether the presence of the hair is just a coincidence, or whether it is actually an important clue.”

  So to go from ‘maybe’ to ‘definitely’, physicists shipped the 50-foot-wide, 15-tonne magnet that the Brookhaven facility used in its Muon g-2 experiment to Fermilab, the US’s premier high-energy physics research facility in Illinois, and built a more sensitive experiment there.

  The new result is from tests at this facility: that the observation differs from the Standard Model’s predicted value by 0.00000000251 (give or take a bit).

  The Fermilab results are expected to become a lot better in the coming years, but even now they represent an important contribution. The statistical significance of the Brookhaven result was just below the threshold at which scientists could claim evidence but the combined significance of the two results is well above.

  The Muon g-2 experiment at Fermilab sees fundamental particles called muons behaving in a way not predicted by the Standard Model of particle physics. These results confirm an earlier experiment performed at @BrookhavenLab. #gminus2https://t.co/92KZ5nWzCT pic.twitter.com/eX0ifQcR03

  — Fermilab (@Fermilab) April 7, 2021

  Potential dampener

  So for now, the g-2 anomaly seems to be real. It’s not easy to say if it will continue to be real as physicists further upgrade the Fermilab g-2’s performance.

  In fact there appears to be another potential dampener on the horizon. An independent group of physicists has had a paper published today saying that the Fermilab g-2 result is actually in line with the Standard Model’s prediction and that there’s no deviation at all.

  This group, called BMW, used a different way to calculate the Standard Model’s value of the number in question than the Fermilab folks did. Aida El-Khadra, a theoretical physicist at the University of Illinois, told Quanta that the Fermilab team had yet to check BMW’s approach, but if it was found to be valid, the team would “integrate it into its next assessment”.

  The ‘Fermilab approach’ itself is something physicists have worked with for many decades, so it’s unlikely to be wrong. If the BMW approach checks out, it’s possible according to Quanta that just the fact that two approaches lead to different predictions of the number’s value is likely to be a new mystery.

  But physicists are excited for now. “It’s almost the best possible case scenario for speculators like us,” Gordan Krnjaic, a theoretical physicist at Fermilab who wasn’t involved in the research, told Scientific American. “I’m thinking much more that it’s possibly new physics, and it has implications for future experiments and for possible connections to dark matter.”

  The current result is also important because the other way to look for physics beyond the Standard Model – by looking for heavier or rarer particles – can be harder.

  This isn’t simply a matter of building a larger particle collider, powering it up, smashing particles and looking for other particles in the debris. For one, there is a very large number of energy levels at which a particle might form. For another, there are thousands of other particle interactions happening at the same time, generating a tremendous amount of noise. So without knowing what to look for and where, a particle hunt can be like looking for a very small needle in a very large haystack.

  The ‘what’ and ‘where’ instead come from different theories that physicists have worked out based on what we know already, and design experiments depending on which one they need to test.

  Into the hospital

  One popular theory is called supersymmetry: it predicts that every elementary particle in the Standard Model framework has a heavier partner particle, called a supersymmetric partner. It also predicts the energy ranges in which these particles might be found. The Large Hadron Collider (LHC) in CERN, near Geneva, was powerful enough to access some of these energies, so physicists used it and went looking last decade. They didn’t find anything.

  

  Other groups of physicists have also tried to look for rarer particles: ones that occur at an accessible energy but only once in a very large number of collisions. The LHC is a machine at the energy frontier: it probes higher and higher energies. To look for extremely rare particles, physicists explore the intensity frontier – using machines specialised in generating collisions.

  The third and last is the cosmic frontier, in which scientists look for unusual particles coming from outer space. For example, early last month, researchers reported that they had detected an energetic anti-neutrino (a kind of fundamental particle) coming from outside the Milky Way participating in a rare event that scientists predicted in 1959 would occur if the Standard Model is right. The discovery, in effect, further cemented the validity of the Standard Model and ruled out one potential avenue to find ‘new physics’.

  This event also recalls an interesting difference between the 2001 and 2021 announcements. The late British scientist Francis J.M. Farley wrote in 2001, after the Brookhaven result:

  … the new muon (g-2) result from Brookhaven cannot at present be explained by the established theory. A more accurate measurement … should be available by the end of the year. Meanwhile theorists are looking for flaws in the argument and more measurements … are underway. If all this fails, supersymmetry can explain the data, but we would need other experiments to show that the postulated particles can exist in the real world, as well as in the evanescent quantum soup around the muon.

  Since then, the LHC and other physics experiments have sent supersymmetry ‘to the hospital’ on more than one occasion. If the anomaly continues to hold up, scientists will have to find other explanations. Or, if the anomaly whimpers out, like so many others of our time, we’ll just have to put up with the Standard Model.

  Featured image: A storage-ring magnet at Fermilab whose geometry allows for a very uniform magnetic field to be established in the ring. Credit: Glukicov/Wikimedia Commons, CC BY-SA 4.0.

  The Wire Science
  April 8, 2021

  2021.04.09

• The weekly linklist – July 25, 2020

  I’ve decided to publish this linklist via Substack. Next weekend onwards, it will only be available on https://linklist.substack.com. And this is why the list exists and what kind of articles you can find in it.

  • Want to buy a parrot? Please login via Facebook. – “F-commerce emerged in Bangladesh largely because there was no major e-commerce platform to absorb all the business. But although it’s biggest there, this form of selling isn’t exclusive to the country, or even the region: globally, 160 million small stores operate on Facebook, and in countries like Thailand, almost half of all online sales happen through social media.”
  • The history of climate science – “The fact that carbon dioxide is a ‘greenhouse gas’ – a gas that prevents a certain amount of heat radiation escaping back to space and thus maintains a generally warm climate on Earth, goes back to an idea that was first conceived, though not specifically with respect to CO2, nearly 200 years ago. The story of how this important physical property was discovered, how its role in the geological past was evaluated and how we came to understand that its increased concentration, via fossil fuel burning, would adversely affect our future, covers about two centuries of enquiry, discovery, innovation and problem-solving.”
  • The story of cryptomining in Europe’s most disputed state – “In early 2018, millions of digital clocks across Europe began falling behind time. Few took notice at first as slight disruptions in the power supply caused bedside alarms and oven timers running on the frequency of electric current to begin lagging. … European authorities soon traced the power fluctuations to North Kosovo, a region commonly described as one of Europe’s last ganglands. Since 2015, its major city, Mitrovica, has been under the control of Srpska Lista, a mafia masquerading as a political party. Around the time Srpska came to power, North Kosovo’s electricity consumption surged. Officials at the Kosovo Electricity Supply Company in Prishtina, Kosovo’s capital city, told me that the region now requires 20 percent more power than it did five years ago. Eventually, it became clear why: across the region, from the shabby apartment blocks of Mitrovica to the cellars of mountain villages, Bitcoin and Ethereum rigs were humming away, fueling a shadow economy of cryptocurrency manufacturing.”
  • Electromagnetic pulses are the last thing you need to worry about in a nuclear explosion – “The electromagnetic pulse that comes from the sundering of an atom, potentially destroying electronics within the blast radius with some impact miles away from ground zero, is just one of many effects of every nuclear blast. What is peculiar about these pulses, often referred to as EMPs, is the way the side effect of a nuclear blast is treated as a special threat in its own right by bodies such as the Task Force on National and Homeland Security, which, despite the official-sounding name, is a privately funded group. These groups continue a decadelong tradition of obsession over EMPs, one President Donald Trump and others have picked up on.”
  • India’s daunting challenge: There’s water everywhere, and nowhere – “I am walking across the world. Over the past seven years I have retraced the footsteps of Homo sapiens, who roamed out of Africa in the Stone Age and explored the primordial world. En route, I gather stories. And nowhere on my foot journey—not in any other nation or continent—have I encountered an environmental reckoning on the scale of India’s looming water crisis. It is almost too daunting to contemplate.”
  • Here be black holes – “During the 15th and 16th centuries, when oceans were the spaces between worlds, marine animals, often so prodigious that they were termed sea monsters, were difficult to see and even harder to analyse, their very existence uncertain. Broadly construed, the history of space science is also a story of looking across and into the ocean – that first great expanse of space rendered almost unknowable by an alien environment. Deep space, like the deep sea, is almost inaccessible, with the metaphorical depth of space echoing the literal depth of oceans. These cognitive and psychic parallels also have an analogue in the practicalities of survival, and training for space missions routinely includes stints under water.”
  • Birds bear the warnings but humans are responsible for the global threat – “Bird omens of a sort are the subject of two recent anthropological studies of avian flu preparedness in Asia. Both Natalie Porter, in Viral Economies, and Frédéric Keck, in Avian Reservoirs, convey the ominousness suffusing poultry farming, using birds as predictors. As both demonstrate, studying how birds interact with human agriculture can provide early warnings of a grim future. Indeed, Keck in Avian Reservoirs explicitly compares public-health surveillance (which he studies in the book) to augury, tracing ‘the idea that birds carry signs of the future that humans should learn to read … back to Roman divination.’”
  • Fiction as a window into the ethics of testing the Bomb – “The stuff that surprised me was on the American side. For example, the assessment by Curtis LeMay [the commander who led US air attacks on Japan] where he basically says, “We’ve bombed the shit out of Japan. Hurry up with your atomic bomb, because there’s going to be nothing left if you don’t.” That shocked me, and also that they deliberately left those cities pristine because they wanted to show the devastation. They wanted, I believe, to kill innocent people, because they were already moving on to the Cold War.”
  • The idea of entropy has led us astray – “Perhaps physics, in all its rigors, is deemed less susceptible to social involvement. In truth, though, Darwinian and thermodynamic theories served jointly to furnish a propitious worldview—a suitable ur-myth about the universe—for a society committed to laissez-faire competition, entrepreneurialism, and expanding industry. Essentially, under this view, the world slouches naturally toward a deathly cold state of disorder, but it can be salvaged—illuminated and organized—by the competitive scrabble of creatures fighting to survive and get ahead.”
  • How massive neutrinos broke the Standard Model – “Niels Bohr … had the radical suggestion that maybe energy and momentum weren’t really conserved; maybe they could somehow be lost. But Wolfgang Pauli had a different — arguably, even more radical — thought: that perhaps there was a novel type of particle being emitted in these decays, one that we simply didn’t yet have the capacity to see. He named it “neutrino,” which is Italian for “little neutral one,” and upon hypothesizing it, remarked upon the heresy he had committed: ‘I have done a terrible thing, I have postulated a particle that cannot be detected.’”
  • How a small Arab nation built a Mars mission from scratch in six years – “When the UAE announced in 2014 that it would send a mission to Mars by the country’s 50th birthday in December 2021, it looked like a bet with astronomically tough odds. At the time, the nation had no space agency and no planetary scientists, and had only recently launched its first satellite. The rapidly assembled team of engineers, with an average age of 27, frequently heard the same jibe. ‘You guys are a bunch of kids. How are you going to reach Mars?’ says Sarah Al Amiri, originally a computer engineer and the science lead for the project.”
  • The pandemic has made concentrated reading difficult. How are book reviewers dealing with this? – “To read good and proper, I needed to disconnect from the terrible reality of the present – wishful thinking with the always-on-alert mode that the pandemic thrust upon us. A few pages in, my mind would wander, snapping out of the brief, quiet moment and I’d find myself reaching for my phone. … But as neuroscientists world over have told us, it’s been hard for most people to focus, with our brain in fight-or-flight mode to the threat of the virus. An activity like deep reading is especially difficult because it requires a high level of engagement and quiet. So it wasn’t just me.”
  • Facebook’s employees reckon with the social network they’ve built – “Why was Zuckerberg only talking about whether Trump’s comments fit the company’s rules, and not about fixing policies that allowed for threats that could hurt people in the first place, he asked. ‘Watching this just felt like someone was sort of slowly swapping out the rug from under my feet,’ Wang said. ‘They were swapping concerns about morals or justice or norms with this concern about consistency and logic, as if it were obviously the case that ‘consistency’ is what mattered most.’”

  2020.07.25

• ‘Weak charge’ measurement holds up SM prediction

  Various dark matter detectors around the world, massive particle accelerators and colliders, powerful telescopes on the ground and in space all have their distinct agendas but ultimately what unites them is humankind’s quest to understand what the hell this universe is on about. There are unanswered questions in every branch of scientific endeavour that will keep us busy for millennia to come.

  Among them, physics seems to be sufferingly uniquely, as it stumbles even as we speak through a ‘nightmare scenario’: the most sensitive measurements we have made of the physical reality around us, at the largest and smallest scales, don’t agree with what physicists have been able to work out on paper. Something’s gotta give – but scientists don’t know where or how they will find their answers.

  The Qweak experiment at the Jefferson Lab, Virginia, is one of scores of experiments around the world trying to find a way out of the nightmare scenario. And Qweak is doing that by studying how the rate at which electrons scatter off a proton is affected by the electrons’ polarisation (a.k.a. spin polarisation: whether the spin of each electron is “left” or “right”).

  Unlike instruments like the Large Hadron Collider, which are very big, operate at much higher energies, are expensive and are used to look for new particles hiding in spacetime, Qweak and others like it make ultra-precise measurements of known values, in effect studying the effects of particles both known and unknown on natural phenomena.

  And if these experiments are able to find that these values deviate at some level from that predicted by the theory, physicists will have the break they’re looking for. For example, if Qweak is the one to break new ground, then physicists will have reason to suspect that the two nuclear forces of nature, simply called strong and weak, hold some secrets.

  However, Qweak’s latest – and possibly its last – results don’t break new ground. In fact, they assert that the current theory of particle physics is correct, the same theory that physicists are trying to break free of.

  Most of us are familiar with protons and electrons: they’re subatomic particles, carry positive and negative charges resp., and are the stuff of one chapter of high-school physics. What students of science find out quite later is that electrons are fundamental particles – they’re not made up of smaller particles – but protons are not. Protons are made up of quarks and gluons.

  Interactions between electrons and quarks/gluons is mediated by two fundamental forces: the electromagnetic and the weak nuclear. The electromagnetic force is much stronger than the aptly named weak nuclear force. On the other hand, it is agnostic to the electron’s polarisation while the weak nuclear force is sensitive to it. In fact, the weak nuclear force is known to respond differently to left- and right-handed particles.

  When electrons are bombarded at protons, the electrons are scattered off. Scientists at measure how often this happens and at what angle, together with the electrons’ polarisation – and try to find correlations between the two sets of data.

  

  At Qweak, the electrons were accelerated to 1.16 GeV and bombarded at a tank of liquid hydrogen. A detector positioned near the tank picked up on electrons scattered at angles between 5.8º and 11.6º. By finely tuning different aspects of this setup, the scientists were able to up the measurement precision to 10 parts per billion.

  For example, they were able to achieve a detection rate of 7 billion per second, a target luminosity of 1.7 x 10³⁹ cm^-2 s^-1 and provide a polarised beam of electrons at 180 µA – all considered high for an experiment of this kind.

  The scientists were looking for patterns in the detector data that would tell them something about the proton’s weak charge: the strength with which it interacts with electrons via the weak nuclear force. (Its notation is Q_weak, hence the experiment’s name.)

  At Qweak, they’re doing this by studying how the electrons are scattered versus their polarisation. The Standard Model (SM) of particle physics, the theory that physicists work with to understand the behaviour of elementary particles, predicts that the number of left- and right-handed electrons scattered should differ by one for every 10 million interactions. If this number is found to be bigger or smaller than usual when measured in the wild, then the Standard Model will be in trouble – much to physicists’ delight.

  SM’s corresponding value for the proton’s weak charge is 0.0708. At Qweak, the value was measured to be 0.0719 ± 0.0045, i.e. between 0.0674 and 0.0764, completely agreeing with the SM prediction. Something’s gotta give – but it’s not going to be the proton’s weak charge for now.

  Paper: Precision measurement of the weak charge of the proton

  Featured image credit: Pexels/Unsplash.

  2018.05.17

• Prospects for suspected new fundamental particle improve marginally

  

  On December 15 last year, scientists working with the Large Hadron Collider experiment announced that they had found slight whispers of a possible new fundamental particle, and got the entire particle physics community excited. There was good reason: should such a particle’s existence become verified, it would provide physicists some crucial headway in answering questions about the universe that our current knowledge of physics has been remarkably unable to cope with. And on March 17, members of the teams that made the detection presented more details as well as some preliminary analyses at a conference, held every year, in La Thuile, Italy.

  The verdict: the case for the hypothesised particle’s existence has got a tad bit stronger. Physicists still don’t know what it could be or if it won’t reveal itself to have been a fluke measurement once more data trickles in by summer this year. At the same time, the bump in the data persists in two sets of measurements logged by two detectors and at different times. In December, the ATLAS detector had presented a stronger case – i.e., a more reliable measurement – than the CMS detector; at La Thuile on March 17, the CMS team also came through with promising numbers.

  Because of the stochastic nature of particle physics, the reliability of results is encapsulated by their statistical significance, denoted by σ (sigma). So 3σ would mean the measurements possess a 1-in-350 chance of being a fluke and marks the threshold for considering the readings as evidence. And 5σ would mean the measurements possess a 1-in-3.5 million chance of being a fluke and marks the threshold for claiming a discovery. Additionally, tags called ‘local’ and ‘global’ refer to whether the significance is for a bump exactly at 750 GeV or anywhere in the plot at all.

  And right now, particle physicists have this scoreboard, as compiled by Alessandro Strumia, an associate professor of physics at Pisa University, who presented it at the conference:

  

  Pauline Gagnon, a senior research scientist at CERN, explained on her blog, “Two hypotheses were tested, assuming different characteristics for the hypothetical new particle: the ‘spin 0’ case corresponds to a new type of Higgs boson, while ‘spin 2’ denotes a graviton.” A graviton is a speculative particle carrying the force of gravity. The – rather, a – Higgs boson was discovered at the LHC in July 2012 and verified in January 2013. This was during the collider’s first run, when it accelerated two beams of protons to 4 TeV (1,000 GeV = 1 TeV) each and then smashed them together. The second run kicked off, following upgrades to the collider and detectors during 2014, with a beam energy of 6.5 TeV.

  Although none of the significances are as good as they’d have to be for there to be a new ‘champagne bottle boson’moment (alternatively: another summertime hit), it’s encouraging that the data behind them has shown up over multiple data-taking periods and isn’t failing repeated scrutiny. More presentations by physicists from ATLAS and CMS at the conference, which concludes on March 19, are expected to provide clues about other anomalous bumps in the data that could be related to the one at 750 GeV. If theoretical physicists have such connections to make, their ability to zero in on what could be producing the excess photons becomes much better.

  But even more than new analyses gleaned from old data, physicists will be looking forward to the LHC waking up from its siesta in the first week of May, and producing results that could become available as early as June. Should the data still continue to hold up – and the 5σ local significance barrier be breached – then physicists will have just what they need to start a new chapter in the study of fundamental physics just as the previous one was closed by the Higgs boson’s discovery in 2012.

  For reasons both technical and otherwise, such a chapter has its work already cut out. The Standard Model of particle physics, a theory unifying the behaviours of different species of particles and which requires the Higgs boson’s existence, is flawed despite its many successes. Therefore, physicists have been, and are, looking for ways to ‘break’ the model by finding something it doesn’t have room for. Both the graviton and another Higgs boson are such things although there are other contenders as well.

  The Wire
  March 19, 2016

   

  2016.03.19

• Physicists could have to wait 66,000 yottayears to see an electron decay

  The longest coherently described span of time I’ve encountered is from Hindu cosmology. It concerns the age of Brahma, one of Hinduism’s principal deities, who is described as being 51 years old (with 49 more to go). But these are no simple years. Each day in Brahma’s life lasts for a period called the kalpa: 4.32 billion Earth-years. In 51 years, he will actually have lived for almost 80 trillion Earth-years. In a 100, he will have lived 157 trillion Earth-years.

  157,000,000,000,000. That’s stupidly huge. Forget astronomy – I doubt even economic crises have use for such numbers.

  On December 3, scientists announced that we’ve all known something that will live for even longer: the electron.

  Yup, the same tiny lepton that zips around inside atoms with gay abandon, that’s swimming through the power lines in your home, has been found to be stable for at least 66,000 yottayears – yotta- being the largest available prefix in the decimal system.

  In stupidly huge terms, that’s 66,000,000,000,000,000,000,000,000,000 (66,000 trillion trillion) years. Brahma just slipped to second place among the mortals.

  But why were scientists making this measurement in the first place?

  Because they’re desperately trying to disprove a prevailing theory in physics. Called the Standard Model, it describes how fundamental particles interact with each other. Though it was meticulously studied and built over a period of more than 30 years to explain a variety of phenomena, the Standard Model hasn’t been able to answer few of the more important questions. For example, why is gravity among the four fundamental forces so much weaker than the rest? Or why is there more matter than antimatter in the universe? Or why does the Higgs boson not weigh more than it does? Or what is dark matter?

  Silence.

  The electron belongs to a class of particles called leptons, which in turn is well described by the Standard Model. So if physicists are able to find that an electron is less stable the model predicts, it’d be a breakthrough. But despite multiple attempts to find an equally freak event, physicists haven’t succeeded – not even with the LHC (though hopeful rumours are doing the rounds that that could change soon).

  The measurement of 66,000 yottayears was published in the journal Physical Review Letters on December 3 (a preprint copy is available on the arXiv server dated November 11). It was made at the Borexino neutrino experiment buried under the Gran Sasso mountain in Italy. The value itself is hinged on a simple idea: the conservation of charge.

  If an electron becomes unstable and has to break down, it’ll break down into a photon and a neutrino. There are almost no other options because the electron is the lightest charged particle and whatever it breaks down into has to be even lighter. However, neither the photon nor the neutrino has an electric charge so the breaking-down would violate a fundamental law of nature – and definitely overturn the Standard Model.

  The Borexino experiment is actually a solar neutrino detector, using 300 tonnes of a petroleum-based liquid to detect and study neutrinos streaming in from the Sun. When a neutrino strikes the liquid, it knocks out an electron in a tiny flash of energy. Some 2,210 photomultiplier tubes surrounding the tank amplify this flash for examination. The energy released is about 256 keV (by the mass-energy equivalence, corresponding to about a 4,000th the mass of a proton).

  However, the innards of the mountain where the detector is located also produce photons thanks to the radioactive decay of bismuth and polonium in it. So the team making the measurement used a simulator to calculate how often photons of 256 keV are logged by the detector against the ‘background’ of all the photons striking the detector. Kinda like a filter. They used data logged over 408 days (January 2012 to May 2013).

  The answer: once every 66,000 yotta-years (that’s 420 trillion Brahma-years).

  Physics World reports that if photons from the ‘background’ radiation could be eliminated further, the electron’s lifetime could probably be increased by a thousand times. But there’s historical precedent that to some extent encourages stronger probes of the humble electron’s properties.

  In 2006, another experiment situated under the Gran Sasso mountain tried to measure the rate at which electrons violated a defining rule in particle physics called Pauli’s exclusion principle. All electrons can be described by four distinct attibutes called their quantum numbers, and the principle holds that no two electrons can have the same four numbers at any given time.

  The experiment was called DEAR (DAΦNE Exotic Atom Research). It energised electrons and then measured how much of it was released when the particles returned to a lower-energy state. After three years of data-taking, its team announced in 2009 that the principle was being violated once every 570 trillion trillion measurements (another stupidly large number).

  That’s a violation 0.0000000000000000000000001% of the time – but it’s still something. And it could amount to more when compared to the Borexino measurement of an electron’s stability. In March 2013, the team that worked DEAR submitted a proposal for building an instrument that improve the measurement by a 100-times, and in May 2015, reported that such an instrument was under construction.

  Here’s hoping they don’t find what they were looking for?

  2015.12.15

• New Higgs results show signs of SUSY

  Two years ago, physicists working on the Large Hadron Collider first announced the discovery of a Higgs boson-like particle, setting the high-energy physics community atwitter. And it was only a couple weeks ago that physicists also announced that the particle was definitely the one predicted by the sturdy Standard Model of particle physics, the theory that governs the Higgs boson’s properties and behavior.

  But new results from the ongoing International Conference on High Energy Physics in Valencia, Spain, could add a twist to this plot. Physicists announced that they had evidence – albeit not strong enough – that the Higgs boson was showing signs of disobeying the model.

  Members of the ATLAS and CMS collaborations, who work with the detectors of that name, said they had results showing the Higgs boson was decaying into a pair of particles called W bosons at a rate some 20% higher than predicted by the Standard Model. This non-compliance will be a breath of fresh air for physicists who have been faithful to a potent but as yet unobserved theory of new physics called supersymmetry, in short and fondly SUSY.

  The W boson mediates the decay of radioactive substances in nature. At sufficiently high energies, such as produced inside the Large Hadron Collider (LHC), these bosons are produced by a multitude of particle interactions. Since their discovery in 1983, they have been widely studied. In these circumstances, announcing signs of SUSY through Higgs decays into WW pairs provides little room for uncertainties.

  SUSY predicts that for every fermion, or matter particle, of the Standard Model there is a partner particle that is a boson called a sfermion. Conversely, for every boson, or force particle, of the Standard Model there is a partner particle that is a fermion called a bosino. Physicists who believe SUSY is a plausible theory use these extra particles to solve problems that the Standard Model can’t. One of them is that of dark matter; another is to explain why the Higgs boson weighs much lighter than it should.

  Jong Soo Kim et al have described how the anomalous decay rates could be explained using a simple version of SUSY in a pre-print paper uploaded to arXiv on June 27. The paper is playfully titled ‘Stop that ambulance! New physics at the LHC?‘. The ‘Stop’ is a reference to the name of the suppersymmetric partner of the top quark. The authors describe how a combination of supersymmetric particles including the stop boson could explain the new results with only a 1-in-370 chance of error. Even though this means physicists have a confidence of 99.7% in the results, it’s still not high to claim evidence. When the LHC comes online in 2015, physicists will be eager to put these results to the test.

  The paper’s title might also refer to a comment that physicist Chris Parkes, spokesperson for the UK participation in the LHCB experiment at the LHC, made to the BBC during the Hadron Collider Physics Symposium in Kyoto, Japan, in November 2012. Results had been announced of the B_s meson decaying into lighter particles at a rate predicted exactly by the Standard Model, nudging SUSY further toward impossibility. Parkes had said, “Supersymmetry may not be dead but these latest results have certainly put it into hospital.”

  2014.07.05