Tag: muons

• 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

• Challenging the neutrino signal anomaly

  A gentle reminder before we begin: you’re allowed to be interested in particle physics. 😉

  Neutrinos are among the most mysterious particles in physics. They are extremely light, electrically neutral, and interact so weakly with matter that trillions of them pass through your body each second without leaving a trace. They are produced in the Sun, nuclear reactors, the atmosphere, and by cosmic explosions. In fact neutrinos are everywhere — yet they’re almost invisible.

  Despite their elusiveness, they have already upended physics. In the late 20th century, scientists discovered that neutrinos can oscillate, changing from one type to another as they travel, which is something that the simplest version of the Standard Model of particle physics — the prevailing theory of elementary particles — doesn’t predict. Because oscillations require neutrinos to have mass, this discovery revealed new physics. Today, scientists study neutrinos for what they might tell us about the universe’s structure and for possible hints of particles or forces yet unknown.

  

  However, detecting neutrinos is very hard. Because they rarely interact with matter, experiments must build massive detectors filled with dense material in the hopes that a small fraction of neutrinos will collide inside with atoms. One way to detect such collisions uses Cherenkov radiation, a bluish glow emitted when a charged particle moves through a medium like water or mineral oil faster than light does in that medium.

  (This is allowed. The only speed limit is that of light in vacuum: 299,792,458 m/s.)

  The MiniBooNE experiment at Fermilab used a large mineral-oil Cherenkov detector. When neutrinos from the Booster Neutrino Beamline struck the atomic nuclei in the mineral oil, the interaction released charged particles, which sometimes produced rings of Cherenkov radiation (like ripples) that the detector recorded. In MiniBooNE’s data, the detection events were classified by the type of light ring produced. An “electron-like” event was one that looked like it had been caused by an electron. But because photons can also produce nearly identical rings when they strike the nuclei, the detector couldn’t always tell the difference. A “muon-like” event, on the other hand, had the distinctive ring pattern of a muon, which is a subatomic particle like the electron but 200-times heavier, and which travels in a straighter, longer track. To be clear, these labels described the detector’s view; they didn’t  guarantee which particle was actually present.

  MiniBooNE began operating in 2002 to test an anomaly that had been reported at the LSND experiment at Los Alamos. LSND had recorded more electron-like” events than predicted, especially at low energies below about 600 MeV. This came to be called the “low-energy excess” and has become one of the most puzzling results in particle physics. It raised the possibility that neutrinos might be oscillating into a hitherto unknown neutrino type, sometimes called the sterile neutrino — or it might have been a hint of unexpected processes that produced extra photons. Since MiniBooNE couldn’t reliably distinguish electrons from photons, the mystery remained unresolved.

  To address this, scientists built the MicroBooNE experiment at Fermilab. It uses a very different technology: the liquid argon time-projection chamber (LArTPC). In a LArTPC, charged particles streak through an ultra-pure mass of liquid argon, leaving a trail of ionised atoms in their wake. An applied electric field causes these trails to drift towards fine wires, where they are recorded. At the same time, the argon emits light that provides the timing of the interaction. This allows the detector to reconstruct interactions in three dimensions with millimetre precision. Crucially, it lets physicists see where the particle shower begins, so they can tell whether it started at the interaction point or some distance away. This capability prepared MicroBooNE to revisit the “low-energy excess” anomaly.

  MicroBooNE also had broader motivations. With an active mass of about 90 tonnes of liquid argon inside a 170-tonne cryostat, and 8,256 wires in its readout planes, it was the largest LArTPC in the US when it began operating. It served as a testbed for the much larger detectors that scientists are developing for the Deep Underground Neutrino Experiment (DUNE). And it was also designed to measure the rate at which neutrinos interacted with argon atoms, to study nuclear effects in neutrino scattering, and to contribute to searches for rare processes such as proton decay and supernova neutrino bursts.

  (When a star goes supernova, it releases waves upon waves of neutrinos before it releases photons. Scientists were able to confirm this when the star Sanduleak -69 202 exploded in 1987.)

  

  Initial MicroBooNE analyses using partial data already challenged the idea that MiniBooNE’s excess was due to the anomaly. However, the collaboration didn’t cover the full range of parameters until recently. On August 21, MicroBooNE published results from five years of operations, corresponding to 1.11 x 10²¹ protons on target, which was about a 70% increase over previous analyses. This complete dataset together with higher sensitivity and better modelling has provided the most decisive test so far of the anomaly.

  The MicroBooNE detector recorded neutrino interactions from the Booster Neutrino Beamline, a setup that produces neutrinos, using its LArTPC detector, which operated at about 87 K inside a cryostat. Charged particles from neutrino interactions produced ionisation electrons that drifted across the detector and were recorded by the wire. Simultaneous flashes of argon scintillation light, seen by photomultiplier tubes, gave the precise time of each interaction.

  In neutrino physics, a category of events grouped by what the detector sees in the final state is called a channel. Researchers call it a signal channel when it matches the kind of event they are specifically looking for, as opposed to background signals from other processes. With MicroBooNE, the team stayed on the lookout for two signal channels: (i) one electron and no visible protons or pions (abbreviated as 1e0p0π) and (ii) one electron and at least one proton above 40 MeV (1eNp0π). These categories reflect what MiniBooNE would’ve seen as electron-like events while exploiting MicroBooNE’s ability to identify protons.

  One important source of background noise the team had to cut from the data was cosmic rays — high-energy particles from outer space that strike Earth’s atmosphere, creating particle showers that can mimic neutrino signals. In 2017, MicroBooNE added a suite of panels around the detector. For the full dataset, the panels cut an additional 25.4% of background noise in the 1e0p0π channel while preserving 98.9% of signal events.

  

  In the final analysis, the MicroBooNE data showed no evidence of an anomalous excess of electron-like events. When both channels were combined, the observed events matched the expectations of the Standard Model of particle physics well. The agreement was especially strong in the 1e0p0π channel.

  In the 1eNp0π channel, MicroBooNE actually detected slightly fewer events than the Model predicted: 102 events v. 134. This shortfall, of about 24%, is however not enough to claim a new effect but enough to draw attention. But rather than confirming MiniBooNE’s excess, this result suggests there’s some tension in the models the scientists use to simulate how the neutrinos and argon atoms will interact. Argon has a large and complex nucleus, which makes accurate predictions challenging. The scientists have in fact stated in their paper that the deficit may reflect these uncertainties rather than new physics.

  The new MicroBooNE results have far-reaching consequences. Foremost, the results reshape the sterile-neutrino debate. For two decades, the LSND and MiniBooNE anomalies had been cited together as signs that the neutrino was oscillating into a previously undetected state. By showing that MiniBooNE’s excess was not due to extra electron-like interactions, MicroBooNE shows that the ‘extra’ events were not caused by excess electron neutrinos. This in turn casts doubt on the simplest explanation, of sterile neutrinos.

  As a result, theoretical models that once seemed straightforward now face strong tension. While more complex scenarios remain possible, the easy explanation is no longer viable.

  

  Second, they demonstrate the maturity of the LArTPC technology. The MicroBooNE team successfully operated a large detector for years, maintaining the argon’s purity and low-noise electronics required for high-resolution imaging. Its performance validates the design choices for larger detectors like DUNE, which use similar technology but at kilotonne scales. The experiment also showcases innovations such as cryogenic electronics, sophisticated purification systems, protection against cosmic rays, and calibration with ultraviolet lasers, proving that such systems can deliver reliable data over long periods of operation.

  Third, the modest deficit in the 1eNp0π channel points to the importance of better understanding neutrino-argon interactions. Argon’s heavy nucleus produces complicated final states where protons and neutrons may scatter or be absorbed, altering the visible event. These nuclear effects can lead to mismatches between simulation and data (possibly including the 24% deficit in the 1eNp0π signal channel). For DUNE, which will also use argon as its target, improving these models is critical. MicroBooNE’s detailed datasets and sideband constraints will continue to inform these refinements.

  Fourth, the story highlights the value of complementary detector technologies. MiniBooNE’s Cherenkov detector recorded more events but couldn’t tell electrons from photons; MicroBooNE’s LArTPC recorded fewer events but with much greater clarity. Together, they show how one experiment can identify a puzzle and another can test it with a different method. This multi-technology approach is likely to continue as experiments worldwide cross-check anomalies and precision measurements.

  Finally, the MicroBooNE results show how science advances. A puzzling anomaly inspired new theories, new technology, and a new experiment. After five years of data-taking and with the most complete analysis yet, MicroBooNE has said that the MiniBooNE anomaly was not due to electron-neutrino interactions. The anomaly itself remains unexplained, but the field now has a sharper focus. Whether the cause lies in photon production, detector effects or actually new physics, the next generation of experiments can start on firmer footing.

  2025.09.06

• Technical foundation for a muon collider laid at J-PARC

  A particle collider is a machine that energises two beams of subatomic particles and smashes them head on.

  The Large Hadron Collider (LHC) in Europe is the world’s largest and most famous particle collider. It accelerates (with the effect of energising) two beams of protons to nearly the speed of light and has them collide. When they do, energy is released in the same way the collision of two cars releases sound, heat, and kinetic energy. The existing kinetic energy of the beams is redistributed into the mass and kinetic energy of new particles. By studying this process, physicists can learn a lot about their properties.

  For example, this is how they made one of the headline discoveries of the 21st century: the Higgs boson particle in 2012. Proving the particle exists allowed physicists to confirm that their theory about how subatomic particles get mass is right. That theory is in turn related to many properties of our universe, including its size, the formation of galaxies, and the inner lives of all the universe’s stars, including our sun. For first proposing that theory in 1964 (together with four others), Peter Higgs and François Englert were awarded the physics Nobel Prize in 2013.

  This said, the properties of the Higgs boson, which physicists have since examined in more detail, have raised more questions about the universe. Two examples include the mysterious nature of dark matter and why neutrinos have mass even though the theory that explains all subatomic particles says they shouldn’t.

  While scientists have built and are operating clever experiments to test different explanations for these anomalous entities, they are also discussing the possibility of building more powerful colliders. The LHC has been able to access a collision energy of up to 13.6 TeV, or about 14,000-times the energy of a proton at rest. Scientists are currently deliberating proposals for colliders that can do better.

  The machines in these proposals have taken three forms: a linear electron-positron collider, a circular electron-positron collider, and a circular proton-proton collider. Each of these machines will cost several billion dollars to build and will require many countries to fund and manage them.  So scientists have to be able to justify which collider they’d like to build and then convince governments to pay.

  The most common argument has been that participating in such sophisticated experiments will also lead to spin-off benefits that will give countries the edge in other spheres, including in medical diagnostics and materials of the future. Increasingly, the question of scientific leadership has also become relevant: India is looking for some of it en route to its goal to become an economically developed country by mid-century; the US is trying to not lose it to China; China is working to take more of it from the US; and so on.

  The point is that there is more at stake here than ‘simple’ problems in physics, although these questions are weighty in their own right.

  The problem currently is that all three types of machines — a linear/circular electron-positron collider or a circular proton-proton collider — are beset by important disadvantages of their own, and different scientists have focused on them (in addition to their price tags) as they try to decide the way forward.

  A circular proton-proton collider like the LHC but bigger can scale a collision energy of 100 TeV. However, it will need to deal with the fact that protons are composite particles, i.e. they’re made up of smaller particles. When they collide head on, only a small fraction of energy is used to ‘make’ new particles; the rest is exchanged between the constituent particles.

  Both electrons and positrons are elementary particles on the other hand and generate ‘clean’ collision data. But when an electron (or a positron) is circulated in a magnetic field through the collider while it’s being accelerated, its small mass means it releases much of the energy it acquires as light. Thus circular electron accelerators consume a lot of energy to achieve their results.

  

  A linear collider doesn’t have this problem since the particles are accelerated in a straight line, but because they can’t go round and round to accelerate more and more, the machine needs to be really long. In some designs they are a few tens of kilometres long: finding a suitably large patch of land is difficult, and maintaining the integrity of the beam across that distance more so. And because each group of particles collides only once and is dumped, the collider must produce, accelerate, and dispose of fresh ultra-intense particle beams at a high frequency, increasing its wall-plug power demand.

  In this scenario, some scientists are also mulling a new type of collider that hasn’t been built before — one for muons. Unlike protons and like electrons, muons are elementary particles and thus lead to clean collisions. A muon is also about 200-times heavier than an electron, so it loses more than a billion-times less energy as light when it’s being accelerated in a circle.

  Thus, as scientist Diktys Stratakis of Fermilab in the US wrote, “A muon collider ring with a circumference of 10 km could have the same potential as a 100 km proton collider ring, if proven to be feasible.”

  But of course it’s not a silver bullet. Perhaps the single biggest issue is that muons are much less stable than protons or electrons. Each muon has a lifetime of about 2.2 microseconds at rest. So producing a sufficiently dense bunch of muons is difficult. The collider must also be able to create large, powerful magnetic fields fast enough before the muons decay. And when muons do decay, they emit electrons or positrons that the machine’s various components must be shielded from. So building a muon collider entails a lot of innovation first.

  A team of scientists in Japan recently reported in Physical Review Letters that they had taken a crucial step forward: they were able to cool (de-energise) a bunch of muons, then accelerate them for the first time using a device called a radiofrequency cavity. This is significant because this end-to-end feat has never been demonstrated before and as such represents the first major problem to solve when building a muon collider.

  The scientists — from Canada, China, and Japan — performed their feat at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai. They achieved it in six steps.

  1. A beam of 0.003 TeV protons strikes a graphite target and produces ‘hot’ muons.

  2. A slender aluminium foil in front of the target slows them down a little.

  3. The muons are further slowed by an 8-mm thick silicon dioxide aerogel disc. As a muon slows nearly to a halt, it bonds with an electron in the aerogel to create a muonium atom: a positively charged muon plus a negatively charged electron.

  4. An ultraviolet laser knocks off the electrons to free very low energy muons — about as much energy they’d have at room temperature.

  5. Electrostatic lenses and steering plates impart a small amount of energy to the muons and focus them, like getting people at a venue to gather in a single room.

  6. The muons are subjected to electric fields alternating at 324 MHz inside a 3-m-long tube, accelerating them. (This is the radiofrequency stage.)

  The feat is the first ever demonstration that started with muons jiggling around at a room-temperature level of energy (around 25 meV) and ended with muons moving in a common direction with about 100 keV of energy — an energy boost by a factor of 4 million.

  

  While 100 keV is still seven orders of magnitude away from 2 TeV, the Japan team’s feat is remarkable because it ‘solves’ the very first and possibly  hardest challenge presented by a muon collider: catching ‘live’ muons before they ‘die’. The team’s setup stopped fast-moving muons, cooled them to 25 meV, stripped the electrons, and injected them into a radiofrequency cavity in 2.28 microseconds, i.e. within the muons’ lifetime. In a manner of speaking, if a 2-TeV muon collider is a skyscraper, the study lays the foundation.

  The J-PARC team was also able to cut the transverse emittance — a measure of how the beam spreads — by 200-times horizontally and 400-times vertically relative to the raw muon beam at the beginning. This two-order-of-magnitude reduction is paramount for the lightly energised muon beam to enter the next, more powerful accelerators.

  “Although the beam produced by the J-PARC team is of good quality (in terms of having low emittance), its energy and intensity are not yet high enough for the experiments that researchers eventually hope to make. Nevertheless, the demonstration of the potential to re-accelerate cold muons is an exciting step forward,” Chris Rogers, a scientist with the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in the UK, wrote in Physics.

  Featured image: All matter around us is made of elementary particles, the building blocks of matter. These particles occur in two basic types called quarks and leptons. Each group consists of six particles, which are related in pairs, or ‘generations’. The muon is a type of lepton, denoted by the letter µ. Credit: CERN.

  2025.06.24

• 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