Tag: Standard Model

• 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

• 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

• Physics Nobel rewards neutrino work, but has sting in the tail for India

  As neutrino astronomy comes of age, the Nobel Foundation has decided to award Takaaki Kajita and Arthur B. McDonald with the physics prize for 2015 for their discovery of neutrino oscillations – a property which indicates that the fundamental particle has mass.

  Takaaki Kajita is affiliated with the Super-Kamiokande neutrino detector in Japan. He and Yoji Totsuka used the detector to report in 1998 that neutrinos produced when cosmic rays struck Earth’s atmosphere were ‘disappearing’ as they travelled to the detector. Then, in 2002, McDonald of the Sudbury Neutrino Observatory in Canada reported that incoming electron neutrinos from the Sun were metamorphosing into muon- or tau-neutrinos. Electron-neutrino, muon-neutrino and tau-neutrino are three kinds of neutrinos (named for particles they are associated with: electrons, muons and taus).

  What McDonald, Kajita and Totsuka had together found was that neutrinos were changing from one kind to another as they travelled – a property called neutrino oscillations – which is definite proof that the particles have mass. Sadly, Totsuka died in 2009, and may not have been considered for the Nobel Prize for that reason.

  This was an important discovery for astroparticle physics. For one, the Standard Model group of equations that defines the behaviour of fundamental particles hadn’t anticipated it. For another, the discovery also made neutrinos a viable candidate for dark matter, which we’re yet to discover, and for what their having mass implies about the explosive deaths of stars – a process that spews copious amounts of neutrinos.

  Neutrino oscillations were first predicted by the Italian nuclear physicist Bruno Pontecorvo in 1957. In fact, Pontecorvo has laid the foundation of a lot of concepts in neutrino physics whose development has won other physicists the Nobel Prize (in 1988, 1995 and 2002), though he’s never won the prize himself.

  

  Although it was a tremendous discovery that neutrinos have mass, a discovery that forced an entrenched theory of physics to change itself, the questions that Pontecorvo, Kajita, McDonald and others asked have yet to be fully answered: one of the biggest unsolved problems in physics today is what the neutrino-mass hierarchy is. In other words, physicists haven’t yet been able to find out – via theory or experiment – which of the three kinds neutrinos is the heaviest and which the lightest. The implications of the mass-ordering are important for physicists to understand certain fundamental predictions of the Standard Model. As it turns out, the model has many unanswered questions, and some physicists hope that a part of the answer may lie in the unexpected properties of neutrinos.

  

  Exacerbating the scientific frustration is the fact that neutrinos are notoriously hard to detect because they rarely interact with matter. For example, the IceCUBE neutrino observatory operated by the University of Wisconsin-Madison near the South Pole in Antarctica employs thousands of sensors buried under the ice. When a neutrino strikes a water molecule in the ice, the reaction produces a charged lepton – electron, muon or tau, depending on the neutrino. That lepton moves faster through the surrounding ice than the speed of light in ice, releasing energy called Cherenkov radiation that’s then detected by the sensors. Building on similarly advanced principles of detection, India and China are also constructing neutrino detectors.

  At least, India is supposed to be. China on the other hand has been labouring away for about a year now in building the Jiangmen Underground Neutrino Observatory (JUNO). India’s efforts with the India-based Neutrino Observatory (INO) in Theni, Tamil Nadu have, on the other hand, ground to a halt. The working principles behind both INO and JUNO are targeted at answering the mass-ordering questions. And if answered, it would almost definitely warrant a Nobel Prize in the future.

  INO’s construction has been delayed because of a combination of festering reasons with no end in sight. The observatory’s detector is a 50,000-ton instrument called the iron calorimeter that is to be buried underneath a kilometre of rock so as to filter all particles but neutrinos out. To acquire such a natural shield, the principal institutions involved in its construction – the Department of Atomic Energy (DAE) and the Institute of Mathematical Sciences, Chennai (Matscience) – have planned to hollow out a hill and situate the INO in the resulting ‘cave’. But despite clearances acquired from various pollution control boards as well as from the people living in the area, the collaboration has faced repeated resistance from environmental activists as well as politicians who, members of the collaboration allege, are only involved for securing political mileage.

  

  The DAE, which obtained approval for the project from the Cabinet and the funds to build the observatory, has also been taking a hands-off approach and has until now not participated in resolving the face-off between the scientists and the activists.

  At the moment, the construction has been halted by a stay issued by the Madurai Bench of the Madras High Court following a petition filed with the support of Vaiko, founder of the Marugmalarchi Dravida Munnetra Kazhagam. But irrespective of which way the court’s decision goes, members of the collaboration at Matscience say that arguments with certain activists have degenerated of late, eroding their collective spirit to persevere with the observatory – even as environmentalists continue to remain suspicious of the DAE. This is quite an unfortunate situation for a country whose association with neutrinos dates back to the 1960s.

  At that time, a neutrino observatory located at a mine in the Kolar Gold Fields was among the first in the world to detect muon neutrinos in Earth’s atmosphere – the same particles whose disappearance Takaaki Kajita was able to record to secure his Nobel Prize for. Incidentally, a Japanese physicist named Masatoshi Koshiba was spurred by the KGF discovery to build a larger neutrino detector in his country, called Kamioka-NDE, later colloquialised to Kamiokande (Koshiba won the Nobel Prize in 2002 for discovering the opportunities of neutrino astronomy). Kamiokande was later succeeded by Super-Kamiokande, which in the late-1990s became the site of Kajita’s discovery. The KGF observatory, on the other hand, was shut in the 1992 as the mines were closed.

  For the broader physics community, brakes applied on the INO’s progress count for little because there are other neutrino detectors around the world – like JUNO – as well as research labs that can continue to look for answers to the mass-ordering question. In fact, the Nobel Prize awarded to Kajita and McDonald stands testimony to the growing realisation that, like the particles of light, neutrinos can also be used to reveal the secrets of the cosmos. However, for the Indian community, which has its share of talented theoretical physicists, the slowdown signifies a slipping opportunity to get back in the game.

  The Wire
  October 6, 2015

  2015.10.06

• A new particle to break the Standard Model?

  The Wire
  July 2, 2015

  Scientists at the Large Hadron Collider particle-smasher have unearthed data from an experiment conducted in 2012 that shows signs of a new particle. If confirmed, its discovery could herald a new period of particle physics research.

  On June 2, members of the ATLAS detector collaboration uploaded a paper to the arXiv pre-print server discussing the possible sighting of a new particle, which hasn’t been named yet. If the data is to be believed, it weighs as much as about 2,000 protons, making it 12-times heavier than the heaviest known fundamental particle, the top quark. It was spotted in the first place when scientists found an anomalous number of ‘events’ recorded by ATLAS at a particular energy scale, more than predicted by the Standard Model set of theories.

  Actually, the Standard Model is more like a collection of principles and rules that dictate the behaviour of fundamental particles. Since the 1960s, it has dominated particle physics research but of late has revealed some weaknesses by not being able to explain the causes behind some of its own predictions. For example, two physicists – Peter Higgs and Francois Englert – used the Standard Model to predict the existence of a Higgs boson in 1964. The particle was found at the LHC in 2012. However, the model has no explanation for why the particle is much lighter than it was thought to be.

  If its existence is confirmed, the new probable-particle sighted by ATLAS could force the Standard Model to pave way for a more advanced, and comprehensive, theory of physics and ultimately of nature. However, proving that it exists could take at least a year.

  The scientists found the probable-particle in data that was recorded by a detector trained to look for the decays of W and Z bosons. These are two fundamental particles that mediate the weak nuclear force that’s responsible for radioactivity. A particle’s mass is equivalent to its energy, which every particle wants to lose if it has too much of it. So heavier particle often break down into smaller clumps of energy, which manifest as smaller particles. Similarly, at the 2 TeV energy scale, scientists spotted a more-than-predicted clumping of energy that’s often the sign of a new particle, in the W/Z channel.

  The chance of the telltale spike in the data belonging to a fluke or impostor event, on the other hand, was 0.00135 (with 0 being ‘no chance’ and 1, certainty) – enough to claim evidence but insufficient to claim a discovery. For the latter, the chances will have to be reduced to at least 0.000000287. In the future, this is what scientists intent on zeroing in on the particle will be gunning for.

  The LHC shut in early 2013 for upgrades, waking up in May 2015 to smash protons together at almost twice the energy and detect them with twice the sensitivity as before. The ATLAS data about the new particle was gathered in 2012, when the LHC was still smashing protons at a collision energy of 8 TeV (more than 8,000 proton-masses). In its new avatar, it will be smashing them at 13 TeV and with increased intensity as well. As a result, rarer events like this probable-particle’s formation could happen more often, making it easier for scientists to spot and validate them.

  If unfortunately the probable-particle is found to have been something else, particle physicists will be disappointed. Since the LHC kicked off in 2009, physicists have been eager to find some data that will “break” the Standard Model, expose cracks in its foundations, that could be taken advantage of to build a theory that can explain the Higgs boson’s mass or why gravity among the four fundamental forces is so much more weaker than the other three.

  The ATLAS team acknowledges a paper from members of the CMS collaboration, also at the LHC, from last year that found similar but weaker signs of the same particle.

  2015.07.02

• The hunt for supersymmetry: Reviewing the first run – 2

  I’d linked to a preprint paper [PDF] on arXiv a couple days ago that had summarized the search for Supersymmetry (Susy) from the first run of the Large Hadron Collider (LHC). I’d written to one of the paper’s authors, Pascal Pralavorio at CERN, seeking some insights into his summary, but unfortunately he couldn’t reply by the time I’d published the post. He replied this morning and I’ve summed them up.

  Pascal says physicists trained their detectors for “the simplest extension of the Standard Model” using supersymmetric principles called the Minimal Supersymmetric Standard Model (MSSM), formulated in the early 1980s. This meant they were looking for a total of 35 particles. In the first run, the LHC operated at two different energies: first at 7 TeV (at a luminosity of 5 fb^-1), then at 8 TeV (at 20 fb^-1; explainer here). The data was garnered from both the ATLAS and CMS detectors.

  In all, they found nothing. As a result, as Pascal says, “When you find nothing, you don’t know if you are close or far from it!”

  His paper has an interesting chart that summarized the results for the search for Susy from Run 1. It is actually a superimposition of two charts. One shows the different Standard Model processes (particle productions, particle decays, etc.) at different energies (200-1,600 GeV). The second shows the Susy processes that are thought to occur at these energies.

  

  The cross-section of the chart is the probability of an event-type to appear during a proton-proton collision. What you can see from this plot is the ratio of probabilities. For example, stop-stop* (the top quark’s Susy partner particle and anti-particle, respectively) production with a mass of 400 GeV is 10¹⁰ (10 billion) less probable than inclusive di-jet events (a Standard Model process). “In other words,” Pascal says, it is “very hard to find” a Susy process while Standard Model processes are on, but it is “possible for highly trained particle physics” to get there.

  Of course, none of this means physicists aren’t open to the possibility of there being a theory (and corresponding particles out there) that even Susy mightn’t be able to explain. The most popular among such theories is “the presence of a “possible extra special dimension” on top of the three that we already know. “We will of course continue to look for it and for supersymmetry in the second run.”

  2014.05.03

• The hunt for supersymmetry: Reviewing the first run

  What do dark matter, Higgs bosons, the electron dipole moment, topological superconductors and quantum loops have in common? These are exotic entities that scientists have been using to solve some longstanding problems in fundamental physics. Specifically, by studying these entities, they expect to discover new ingredients of the universe that will help them answer why it is the way it is. These ingredients could in come in a staggering variety, so it is important for scientists to narrow down what they’re looking for – which brings us to the question of why these entities are being studied. A brief summary:

  1. Dark matter is an exotic form of matter that is thought to interact with normal matter only through the gravitational force. Its existence was hypothesized in 1932-1933. Its exact nature is yet to be understood.
  2. Quantum loops refer to an intricate way in which some heavier particles decay into sets of lighter particles, involving the exchange of some other extremely short-lived particles. They have been theoretically known to exist for many decades.
  3. Topological superconductors are exotic materials that, under certain conditions, behave like superconductors on their surface and as insulators underneath. They were discovered fairly recently, around 2007, and how they manage to be this way is not fully understood.
  4. The Higgs boson‘s discovery was announced in July 2012 (and verified by March-June 2013). Math worked out on paper predicts that its mass ought to have been very high – but it was found to be much lower.
  5. The electron dipole moment is a measure of how spherical the electron is. Specifically, the EDM denotes how evenly the electron’s negative charge is distributed around it. While the well-understood laws of nature don’t prevent the charge from being uneven, they restrict the amount of unevenness to a very small value. The most precise measurement of this value to date was published in December 2013.

  Clearly, these are five phenomena whose identities are incomplete. But more specifically, scientists have found a way to use advanced mathematics to complete all these identities with one encompassing theory called Supersymmetry (Susy). Unfortunately for them, the mathematics refuses to become real, i.e. scientists have failed to find evidence of Susy in experiments. Actually, that might be an overstatement: different experiments are at different stages of looking for Susy at work in giving these ‘freaks of nature’ a physical identity. On the other hand, it has been a few years since some of these experiments commenced – some of them are quite powerful indeed – and the only positive results they have had have been to say Susy cannot be found in this or that range.

  But if signs of Susy are found, then the world of physics will be in tumult – in a good way, of course. It will get to replace an old theory called the Standard Model of particle physics. The Standard Model is the set of mathematical tools and techniques used to understand how fundamental particles make up different objects, what particles our universe is made of, how quantum loops work, how the Higgs boson could have formed, etc. But it has no answers for why there is dark matter, why the electron is allowed to have that small dipole moment, why topological superconductors work the way they do, why the Higgs boson’s mass is so low, etc.

  Early next year, physicists will turn to the Large Hadron Collider (LHC) – which helped discover the Higgs boson in 2012 – after it wakes up from its two-year slumber to help find Susy, too. This LHC of 2015 will be way more powerful than the one that went dormant in early 2013 thanks to a slew of upgrades. Hopefully it will not disappoint, building on what it has managed to deliver for Susy until now. In fact, on April 28, 2014, two physicists from CERN submitted a preprint paper to the arXiv server summarizing the lessons for Susy from the LHC after the first run.

  2014.05.01

• The hunt for supersymmetry: Is a choke on the cards?

  The Copernican
  April 28, 2014

  “So irrelevant is the philosophy of quantum mechanics to its use that one begins to suspect that all the deep questions are really empty…”

  — Steven Weinberg, Dreams of a Final Theory: The Search for the Fundamental Laws of Nature (1992)

  On a slightly humid yet clement January evening in 2013, a theoretical physicist named George Sterman was in Chennai to attend a conference at the Institute of Mathematical Sciences. After the last talk of the day, he had strolled out of the auditorium and was mingling with students when I managed to get a few minutes with him. I asked for an interview and he agreed.

  After some coffee, we seated ourselves at a kiosk in the middle of the lawn, the sun was setting, and mosquitoes abounded. Sterman was a particle physicist, so I opened with the customary question about the Higgs boson and expected him to swat it away with snowclones of the time like “fantastic”, “tribute to 50 years of mathematics” and “long-awaited”. He did say those things, but then he also expressed some disappointment.

  George Sterman is distinguished for his work in quantum chromodynamics (QCD), for which he won the prestigious J.J. Sakurai Prize in 2003. QCD is a branch of physics that deals with particles that have a property called colour charge. Quarks and gluons are examples of such particles; these two together with electrons are the proverbial building blocks of matter. Sterman has been a physicist since the 1970s, the early years as far as experimental particle physics research is concerned.

  The Standard Model disappoints

  Over the last four or so decades, remarkable people like him have helped construct a model of laws, principles and theories that the rigours of this field are sustaining on, called the Standard Model of particle physics. And it was the reason Sterman was disappointed.

  According to the Standard Model, Sterman explained, “if we gave our any reasonable estimate of what the mass of the Higgs particle should be, it should by all rights be huge! It should be as heavy as what we call the Planck mass.”

  But it isn’t. The Higgs mass is around 125 GeV (GeV being a unit of energy that corresponds to certain values of a particle’s mass) – compare it with the proton that weighs 0.938 GeV. On the other hand, the Planck mass is 10^19 GeV. Seventeen orders of magnitude lie in between. According to Sterman, this isn’t natural. The question is why does there have to be such a big difference in what we can say the mass could be and what we find it to be.

  Martinus Veltman, a Dutch theoretical physicist who won the Nobel Prize for physics in 2003 for his work in particle physics, painted a starker picture, “Since the energy of the Higgs [field] is distributed all over the universe, it should contribute to the curvature of space; if you do the calculation, the universe would have to curve to the size of a football,” in an interview to Nature in 2013.

  Evidently, the Standard Model has many loose ends, and explaining the mass of the Higgs boson is only one of them. Another example is why it has no answer for what dark matter is and why it behaves the way it does. Yet another example is why the four fundamental forces of nature are not of the same order of magnitude.

  An alternative

  Thanks to the Standard Model, some mysteries have been solved, but other mysteries have come and are coming to light – in much the same way Isaac Newton’s ideas struggled to remain applicable in the troubled world of physics in the early 20th century. It seems history repeats itself through crises.

  Fortunately, physicists in 1971-1972 had begun to piece together an alternative theory called supersymmetry, Susy for short. At the time, it was an alternative way of interpreting how emerging facts could be related to each other. Today, however, Susy is a more encompassing successor to the throne that the Standard Model occupies, a sort of mathematical framework in which the predictions of the Model still hold but no longer have those loose ends. And Susy’s USP is… well, that it doesn’t disappoint Sterman.

  “There’s a reason why so many people felt so confident about supersymmetry,” he said. “It wasn’t just that it’s a beautiful theory – which it is – or that it engages and challenges the most mathematically oriented among physicists, but in another sense in which it appeared to be necessary. There’s this subtle concept that goes by the name of naturalness…”

  And don’t yet look up ‘naturalness’ on Wikipedia because, for once, here is something so simple, so elegant, that it is precisely what its name implies. Naturalness is the idea that, for example, the Higgs boson is so lightweight because something out there is keeping it from being heavy. Naturalness is the idea that, in a given setting, the forces of nature all act in equal measure. Naturalness is the idea that causes seem natural, and logically plausible, without having to be fine-tuned in order to explain their effects. In other words, Susy, through its naturalness, makes possible a domesticated world, one without sudden, unexpected deviations from what common sense (a sophisticated one, anyway) would dictate.

  To understand how it works, let us revisit the basics. Our observable universe plays host to two kinds of fundamental particles, which are packets of some well-defined amount of energy. The fermions, named for Enrico Fermi, are the matter particles. Things are made of them. The bosons, named for Satyendra Bose, are the force particles. Things interact with each other by using them as messengers. The Standard Model tells us how bosons and fermions will behave in a variety of situations.

  However, the Model has no answers for why bosons and fermions weigh as much as they do, or come in as many varieties as they do. These are deeper questions that go beyond simply what we can observe. These are questions whose answers demand that we interpret what we know, that we explore the wisdom of nature that underlies our knowledge of it. To know this why, physicists investigated phenomena that lie beyond the Standard Model’s jurisdiction.

  The search

  One such place is actually nothingness, i.e. the quantum vacuum of deep space, where particles called virtual particles continuously wink in and out of existence. But even with their brief life-spans, they play a significant role in mediating the interactions between different particles. You will remember having studied in class IX that like charges repel each other. What you probably weren’t told is that the repulsive force between them is mediated by the exchange of virtual photons.

  Curiously, these “virtual interactions” don’t proliferate haphazardly. Virtual particles don’t continuously “talk” to the electron or clump around the Higgs boson. If this happened, mass would accrue at a point out of thin air, and black holes would be popping up all around us. Why this doesn’t happen, physicists think, is because of Susy, whose invisible hand could be staying chaos from dominating our universe.

  The way it does this is by invoking quantum mechanics, and conceiving that there is another dimension called superspace. In superspace, the bosons and fermions in the dimensions familiar to us behave differently, the laws conceived such that they restrict the random formation of black holes, for starters. In the May 2014 issue of Scientific American, Joseph Lykken and Maria Spiropulu describe how things work in superspace:

  “If you are a boson, taking one step in [superspace] turns you into a fermion; if you are a fermion, one step in [superspace] turns you into a boson. Furthermore, if you take one step in [superspace] and then step back again, you will find that you have also moved in ordinary space or time by some minimum amount. Thus, motion in [superspace] is tied up, in a complicated way, with ordinary motion.”

  The presence of this dimension implies that all bosons and fermions have a corresponding particle called a superpartner particle. For each boson, there is a superpartner fermion called a bosino; for each fermion, there is a superpartner boson called a sfermion (why the confusing titles, though?).

  Physicists are hoping this supersymmetric world exists. If it does, they will have found tools to explain the Higgs boson’s mass, the difference in strengths of the four fundamental forces, what dark matter could be, and a swarm of other nagging issues the Standard Model fails to resolve. Unfortunately, this is where Susy’s credit-worthiness runs into trouble.

  No signs

  “Experiment will always be the ultimate arbiter, so long as it’s science we’re doing.”

  — Leon Lederman & Christopher Hill, Beyond the Higgs Boson (2013)

  Since the first pieces of the Standard Model were brought together in the 1960s, researchers have run repeated tests to check if what it predicts were true. Each time, the Model has stood up to its promise and yielded accurate results. It withstood the test of time – a criterion it shares with the Nobel Prize for physics, which physicists working with the Model have won at least 15 times since 1957.

  Susy, on the other hand, is still waiting for confirmation. The Large Hadron Collider (LHC), the world’s most powerful particle physics experiment, ran its first round of experiments from 2009 to 2012, and found no signs of sfermions or bosinos. In fact, it has succeeded on the other hand to narrow the gaps in the Standard Model where Susy could be found. While the non-empty emptiness of quantum vacuum opened a small window into the world of Susy, a window through which we could stick a mathematical arm out and say “This is why black holes don’t just pop up”, the Model has persistently puttied every other crack we hound after.

  An interesting quote comes to mind about Susy’s health. In November 2012, at the Hadron Collider Physics Symposium in Kyoto, Japan, physicists presented evidence of a particle decay that happens so rarely that only the LHC could have spotted it. The Standard Model predicts that every time the B_s (pronounced “Bee-sub-ess”) meson decays into a set of lighter particles, there is a small chance that it decays into two muons. The steps in which this happens is intricate, involving a process called a quantum loop.

  What next?

  “SUSY has been expected for a long time, but no trace has been found so far… Like the plot of the excellent movie ‘The Lady Vanishes’ (Alfred Hitchcock, 1938)”

  — Andy Parker, Cambridge University

  Susy predicts that some supersymmetric particles should show themselves during the quantum loop, but no signs of them were found. On the other hand, the rate of B_s decays into two muons was consistent with the Model’s predictions. Prof. Chris Parkes, a British physicist, had then told BBC News: “Supersymmetry may not be dead but these latest results have certainly put it into hospital.” Why not: Our peek of the supersymmetric universe eludes us, and if the LHC can’t find it, what will?

  Then again, it took us many centuries to find the electron, and then many decades to find anti-particles. Why should we hurry now? After all, as Dr. Rahul Sinha from the Institute of Mathematical Sciences told me after the Symposium had concluded, “a conclusive statement cannot be made as yet”. At this stage, even waiting for many years might not be necessary. The LHC is set to reawaken around January 2015 after a series of upgrades that will let the machine deliver 10 times more particle collisions per second per unit area. Mayhap a superpartner particle can be found lurking in this profusion by, say, 2017.

  There are also plans for other more specialised colliders, such as Project X in the USA, which India has expressed interest in formally cooperating with. X, proposed to be built at the Fermilab National Accelerator Laboratory, Illinois, will produce high intensity proton beams to investigate a variety of hitherto unexplored realms. One of them is to produce heavy short-lived isotopes of elements like radium or francium, and use them to study if the electron has a dipole moment, or a pronounced negative charge along one direction, which Susy allows for.

  (Moreover, if Project X is realised it could prove extra-useful for India because it makes possible a new kind of nuclear reactor design, called the accelerator-driven sub-critical reactor, which operates without a core of critical-mass radioactive fuel, rendering impossible accidents like Chernobyl and Fukushima, while also being capable of inducing fission reactions using lighter fuel like thorium.)

  Yet another avenue to explore Susy would be looking for dark matter particles using highly sensitive particle detectors such as LUX, XENON1T and CDMS. According to some supersymmetric models, the lightest Susy particles could actually be dark matter particles, so if a few are spotted and studied, they could buffet this theory’s sagging credence.

  … which serves to remind us that this excitement could cut the other way, too. What if the LHC in its advanced avatar is still unable to find evidence of Susy? In fact, the Advanced Cold Molecule Electron group at Harvard University announced in December 2013 that they were able to experimentally rule out that they electron had a dipole moment with the highest precision attained to date. After such results, physicists will have to try and rework the theory, or perhaps zero in on other aspects of it that can be investigated by the LHC or Project X or other colliders.

  But at the end of the day, there is also the romance of it all. It took George Sterman many years to find a theory as elegant and straightforward as Susy – an island of orderliness in the insane sea of quantum mechanics. How quickly would he give it up?

  O Hunter, snare me his shadow!
  O Nightingale, catch me his strain!
  Else moonstruck with music and madness
  I track him in vain!

  — Oscar Wilde, In The Forest

  2014.04.29

• Another window on ‘new physics’ closes

  

  The Standard Model of particle physics is a theory that has been pieced together over the last 40 years after careful experiments. It accurately predicts the behaviour of various subatomic particles across a range of situations. Even so, it’s not complete: it can explain neither gravity nor anything about the so-called dark universe.

  Physicists searching for a theory that can have to pierce through the Standard Model. This can be finding some inconsistent mathematics or detecting something that can’t be explained by it, like looking for particles ‘breaking down’, i.e. decaying, into smaller ones at a rate greater than allowed by the Model.

  The Large Hadron Collider, CERN, on the France-Switzerland border, produces the particles, and particle detectors straddling the collider are programmed to look for aberrations in their decay, among other things. One detector in particular, called the LHCb, looks for signs of a particle called B_s (read “B sub s”) meson decaying into two smaller particles called muons.

  On July 19, physicists from the LHCb experiment confirmed at an ongoing conference in Stockholm that the B_s meson decays to two muons at a rate consistent with the Model’s predictions (full paperhere). The implication is that one more window through which physicists could have a peek of the physics beyond the Model is now shut.

  The B_s meson

  This meson has been studied for around 25 years, and its decay-rate to two muons has been predicted to be about thrice every billion times, 3.56 ± 0.29 per billion to be exact. The physicists’ measurements from the LHCb showed that it was happening about 2.9 times per billion. A team working with another detector, the CMS, reported it happens thrice every billion decays. These are number pretty consistent with the Model’s. In fact, scientists think the chance of an error in the LHCb readings is 1 in 3.5 million, low enough to claim a discovery.

  However, this doesn’t mean the search for ‘new physics’ is over. There are many other windows, such as the search for dark matter, observations of neutrino oscillations, studies of antimatter and exotic theories like Supersymmetry, to keep scientists going.

  The ultimate goal is to find one theory that can explain all phenomena observed in the universe – from subatomic particles to supermassive black holes to dark matter – because they are all part of one nature.

  In fact, physicists are fond of Sypersymmetry, a theory that posits that there is one as-yet undetected particle for every one that we have detected, because it promises to retain the naturalness. In contrast, the Standard Model has many perplexing, yet accurate, explanations that is keeping physicists from piecing together the known universe in a smooth way. However, in order to find any evidence for Supersymmetry, we’ll have to wait until at least 2015, when the CERN supercollider will reopen upgraded for higher energy experiments.

  And as one window has closed after an arduous 25-year journey, the focus on all the other windows will intensify, too.

  (This blog post first appeared at The Copernican on July 19, 2013.)

  2013.07.19

• Where’s all the antimatter? New CERN results show the way.

  If you look outside your window at the clouds, the stars, the planets, all that you will see is made of matter. However, when the universe was born, there were equal amounts of matter and antimatter. So where has all the antimatter gone?

  The answer, if one is found, will be at the Large Hadron Collider (LHC), the world’s most powerful particle physics experiment, now taking a breather while engineers refit it to make it even more powerful by 2015. Then, it will be able to spot tinier, much more shortlived particles than the Higgs boson, which itself is notoriously shortlived.

  While it ran from 2008 to early 2013, the LHC was incredibly prolific. It smashed together billions of protons in each experiment at speeds close to light’s, breaking them open. Physicists hoped the things that’d tumble out might show why the universe has come to prefer matter over antimatter.

  In fact, from 2013 to 2015, physicists will be occupied gleaning meaningful results from each of these experiments because they simply didn’t have enough time to sift through all of them while the machine was running.

  They will present their results as papers in scientific journals. Each paper will will be the product of analysis conducted on experimental data corresponding to some experiment, each with some energy, some luminosity, and other such experimental parameters central to experimental physics.

  One such paper was submitted to a journal on April 23, 2013, titled ‘First observation of CP violation in the decays of B_s mesons‘. According to this paper, its corresponding experiment was conducted in 2011, when the LHC was smashing away at 7 TeV centre-of-mass (c.o.m) collision energy. This is the energy at the point inside the LHC circuit where two bunches of protons collide.

  The paper also notes that the LHCb detector was used to track the results of the collision. The LHCb is one of seven detectors situated on the LHC’s ring. It has been engineered to study a particle known as the beauty quark, which is more than 4.2 times heavier than a proton, and lasts for about one-hundred-trillionth of a second before breaking down into lighter particles, a process mediated by some of nature’s four fundamental forces.

  The beauty is one of six kinds of quarks, and together with other equally minuscule particles called bosons and leptons, they all make up everything in the universe: from entire galaxies to individual atoms.

  For example, for as long as it lives, the beauty quark can team up with another quark or antiquark, the antimatter counterpart, to form particles called mesons. Generally, mesons are particles composed of one quark and one antiquark.

  Why don’t the quark and antiquark meet and annihilate each other in a flash of energy? Because they’re not of the same type. If a quark of one type and an antiquark of another type meet, they don’t annihilate.

  The B_s meson that the April 23 paper talks about is a meson composed of one beauty antiquark and one strange quark. Thus the notation ‘B_s’: A B-meson with an s component. This meson violates a law of the universe physicists long though unbreakable, called the charge-conjugation/parity (CP) invariance. It states that if you took a particle, inverted its charge (‘+’ to ‘-‘ or ‘-‘ to ‘+’), and then interchanged its left and right, its behaviour shouldn’t change in a universe that conserved charge and parity.

  Physicists, however, found in the 2011 LHCb data that the B_s meson was flouting the CP invariance rule. Because of the beauty antiquark’s and strange quark’s short lifetimes, the B_s meson only lasted for so long before breaking down into lighter particles, in this case called kaons and pions.

  When physicists calculated the kaons‘s and pions‘s charges and compared it to the B_s meson’s, they added up. However, when they calculated the kaons‘s and pions‘s left- and right-handednesses, i.e. parities, in terms of which direction they were spinning in, they found an imbalance.

  A force, called the weak force, was pushing a particle to spin one way instead of the other about 27 per cent of the time. According to the physicists’ paper, this result has been reached with a confidence-level of more than 5-sigma. This means that some reading in the data would disagree with their conclusion not more than 0.00001 per cent of the time, sufficient to claim direct evidence.

  Of course, this wouldn’t be the first time evidence of CP violation in B-mesons had been spotted. On 17 May, 2010, B-mesons composed of a beauty antiquark and a down quark were shown shown to decay at a much slower rate than B-antimesons of the same composition, in the process outlasting them. However, this is the first time evidence of this violation has been found in B_s mesons, a particle that has been called “bizarre”.

  While this flies in the face of a natural, intuitive understanding of our universe, it is a happy conclusion because it could explain the aberration that is antimatter’s absence, one that isn’t explained by a theory in physics called the Standard Model.

  Here was something in the universe that was showing some sort of a preference, ready to break the symmetry and uniformity of laws that pervade the space-time continuum.

  Physicists know that the weak force, one of the fundamental forces of nature like gravity is, is the culprit. It has a preference for acting on left-handed particles and right-handed antiparticles. When such a particle shows itself, the weak force offers to mediate its breakdown into lighter particles, in the process resulting in a preference for one set of products over another.

  But in order to fully establish the link between matter’s domination and the weak force’s role in it, physicists have to first figure out why the weak force has such biased preferences.

  —

  This post originally appeared in The Copernican science blog at The Hindu on April 25, 2013.

  2013.04.27

• Where does the Higgs boson come from?

  When the Chelyabinsk meteor – dubbed Chebarkul – entered Earth’s atmosphere at around 17 km/s, it started to heat up due to friction. After a point, cracks already present on the chunk of rock weighing 9,000-tonnes became licensed to widen and eventually split off Chebarkul into smaller parts.

  While the internal structure of Chebarkul was responsible for where the cracks widened and at what temperature and other conditions, the rock’s heating was the tipping point. Once it got hot enough, its crystalline structure began to disintegrate in some parts.

  Spontaneous symmetry-breaking

  About 13.75 billion years ago, this is what happened to the universe. At first, there was a sea of energy, a symmetrically uniform block. Suddenly, this block was rapidly exposed to extreme heat. Once it hit about 10¹⁵ kelvin – 173 billion times hotter than our Sun’s surface – the block disintegrated into smaller packets called particles. Its symmetry was broken. The Big Bang had happened.

  
  The Big Bang splashed a copious amount of energy across the universe, whose residue is perceivable as the CMBR.

  Quickly, the high temperature fell off, but the particles couldn’t return to their original state of perfect togetherness. The block was broken forever, and the particles now had to fend for themselves. There was a disturbance, or perturbations, in the system, and the forces started to act. Physicists today call this the Nambu-Goldstone (NG) mode, named for Jeffrey Goldstone and Yoichiro Nambu.

  In the tradition of particle physics treating with everything in terms of particles, the forces in the NG mode were characterised in terms of NG bosons. The exchange of these bosons between two particles meant they were exchanging forces. Since each boson is also a particle, a force can be thought of as the exchange of energy between two particles or bodies.

  This is just like the concept of phonons in condensed matter physics: when atoms part of a perfectly arranged array vibrate, physicists know they contain some extra energy that makes them restless. They isolate this surplus in the form of a particle called a phonon, and address the entire array’s surplus in terms of multiple phonons. So, as a series of restlessness moves through the solid, it’ll be like a sound wave moving through it. Simplifies the math.

  Anyway, the symmetry-breaking also gave rise to some fundamental forces. They’re called ‘fundamental’ because of their primacy, and because they’re still around. They were born because the disturbances in the energy block, encapsulated as the NG bosons, were interacting with an all-pervading background field called the Higgs field.

  The Higgs field has four components, two charged and two uncharged. Another, more common, example of a field is the electric field, which has two components: some strength at a point (charged) and the direction of the strength at that point (neutral). Components of the Higgs field perturbed the NG bosons in a particular way to give rise to four fundamental forces, one for each component.

  So, just like in Chebarkul’s case, where its internal structure dictated where the first cracks would appear, in the block’s case, the heating had disturbed the energy block to awaken different “cracks” at different points.

  The Call of Cthulhu

  The first such “crack” to be born was the electroweak force. As the surroundings of these particles continued to cool, the electroweak force split into two: electromagnetic (eM) and weak forces.

  The force-carrier for the eM force is called a photon. Photons can exist at different energies, and at each energy-level, they have a corresponding frequency. If a photon happens to be in the “visible range” of energy-levels, then each frequency shows itself as a colour. And so on…

  The force-carriers of the weak forces are the W+, W-, and Z bosons. At the time the first W/Z bosons came to life, they were massless. We know now because of Einstein’s mass-energy equivalence that this means the bosons had no energy. How were they particulate, then?

  Imagine an auditorium where an important lecture’s about to be given. You get there early, your friend is late, and you decide to reserve a seat for her. Then, your friend finally arrives 10 minutes after the lecture’s started and takes her seat. In this scenario, after your arrival, the seat was there all along as ‘friend’s seat’, even though your friend took her time to get there.

  Similarly, the W/Z bosons, which became quite massive later on, were initially massless. They had to have existed when the weak force came to life, if only to account for a new force that had been born. The debut of massiveness happened when they “ate” the NG bosons – the disturbed block’s surplus energy – and became very heavy.

  Unfortunately for them, their snacking was irreversible. The W/Z bosons couldn’t regurgitate the NG bosons, so they were doomed to be forever heavy and, consequently, short-ranged. That’s why the force that they mediate is called the weak force: because it acts over very small distances.

  You’ll notice that the W+, W-, and Z bosons make up for only three components of the Higgs field. What about the fourth component?

  Enter: Higgs boson

  That’s the Higgs boson. And now, getting closer to pinning down the Higgs boson means we’re also getting closer to pinning down the Higgs mechanism as valid, a quantum mechanical formulation within which we understand the behaviours of these particles and forces. This formulation is called the Standard Model.

  (This blog post first appeared at The Copernican on March 8, 2013.)

  2013.03.08