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’.
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.
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.
A table showing searches for particles associated with different post-standard-model theories (orange labels on the left). The bars show the energy levels up to which the ATLAS detector at the Large Hadron Collider has not found the particles. Table: ATLAS Collaboration/CERN
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.
It finally happened! The particle-smasher known as the Large Hadron Collider is back online after more than two years, during which its various components were upgraded to make it even meaner. A team of scientists and engineers gathered at the collider’s control room at CERN over the weekend – giving up Easter celebrations at home – to revive the giant machine so it could resume feeding its four detectors with high-energy collisions of protons.
Before the particles enter the LHC itself, they are pre-accelerated to 450 GeV by the Super Proton Synchrotron. At 11.53 am (CET), the first beam of pre-accelerated protons was injected into the LHC at Point 2 (see image), starting a clockwise journey. By 11.59 am, it’d been reported crossing Point 3, and at 12.01 pm, it was past Point 5. The anxiety in the control room was palpable when an update was posted in the live-blog: “The LHC operators watching the screen now in anticipation for Beam 1 through sector 5-6”.
Beam 1 going from Point 2 to Point 3 during the second run of the Large Hadron Collider’s first day in action. Credit: CERN
Finally, at 12.12 pm, the beam had crossed Point 6. By 12.27, it had gone a full-circle around the LHC’s particles pipeline, signalling that the pathways were defect-free and ready for use. Already, as and when the beam snaked through a detector without glitches, some protons were smashed into static targets producing a so-called splash of particles like sparks, and groups of scientists erupted in cheers.
Both Rolf-Dieter Heuer, the CERN Director-General, and Frederick Bordry, Director for Accelerators and Technology, were present in the control room. Earlier in the day, Heuer had announced that another beam of protons – going anti-clockwise – had passed through the LHC pipe without any problems, providing the preliminary announcement that all was well with the experiment. In fact, CERN’s scientists were originally supposed to have run these beam-checks a week ago, when an electrical glitch spotted at the last minute thwarted them.
In its new avatar, the LHC sports almost double the energy it ran at, before it shut down for upgrades in early-2013, as well as more sensitive collision detectors and fresh safety systems. For the details of the upgrades, read this. For an ‘abridged’ version of the upgrades together with what new physics experiments the new LHC will focus on, read this. Finally, here’s to another great year for high-energy physics!
CERN has announced the restart schedule of its flagship science “project”, the Large Hadron Collider, that will see the giant machine return online in early 2015. I’d written about the upgrades that could be expected shortly before it shut down in 2012. They range from new pixel sensors and safety systems to facilities that will double the collider’s energy and the detectors’ eyes for tracking collisions. Here’s a little timeline I made with Timeline.js, check it out.
(It’s at times like this that I really wish WP.com would let bloggers embed iframes in posts.)
This reconstructed image of two high-energy protons colliding at the LHC shows a B_s meson (blue) produced that then decays into two muons (pink), about 50 mm from the collision point. Image: LHCb/CERN
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.)
On December 10, 2012, I spoke to a bunch of physicists attending the Frontiers of High-energy Physics symposium at the Institute of Mathematical Sciences, Chennai. They included Rahul Sinha, G. Rajasekaran, Tom Kibble, Sheldon Stone, Marina Artuso, M.V.N. Murthy, Kajari Mazumdar, and Hai-Yang Cheng, amongst others.
All their talks, obviously, focused on either the search for the Higgs boson or the search for dark matter, with the former being assured and celebratory and the latter, contemplative and cautious. There was nothing new left to be said – as a peg for a news story – given that what of 2012 had gone before that day had already read hundreds of stories on the two searches.
Most of the memorable statements by physicists I remember from that day came from Dr. Sheldon Stone, Syracuse University, and member, LHCb collaboration.
A word on the LHCb before I go any further: It’s one of the seven detector-experiments situated on the Large Hadron Collider’s (LHC’s) ring. Unlike the ATLAS and CMS, whose focus is on the Higgs boson, the LHCb collaboration is studying the decay of B-mesons and signs of CP-symmetry violations at high energies.
While he had a lot to say, he also best summed up what physicists worldwide might’ve felt when the theorised set of particles’ rules called the Standard Model (SM) was having its predictions validated one after the other, leaving no room for a new theory to edge its way in. While very elegant by itself, the SM has no answers to some of the more puzzling questions, such as that of dark matter or of mass-hierarchy problem.
In other words, the more it stands validated, the fewer cracks there are for a new and better theory, like Supersymmetry, to show itself.
In Dr. Stone’s words, “It’s very depressing. The Standard Model has been right on target, and so far, nothing outside the model has been observed. It’s very surprising that everything works, but at the same time, we don’t know why it works! Everywhere, physicists are depressed and clueless, intent on digging deeper, or both. I’m depressed, too, but I also want to dig deeper.”
In answer to some of my questions on what the future held, Dr. Stone said, “Now that we know how things actually work, we’re starting to play some tricks. But beyond that, moving ahead, with new equipment, etc., is going to cost a lot of money. We’ve to invest in the collider, in a lot of detector infrastructure, and computing accessories. In 2012, we had a tough time keeping with the results the LHC was producing. For the future, we’re counting on advancements in computer science and the LHC Grid.”
One interesting thing that he mentioned in one of his answers was that the LHC costs less than one aircraft-carrier. I thought that’d put things in perspective – how much some amount of investment in science could achieve when compared to what the same amount could achieve in other areas. This is not to discourage the construction of aircraft carriers, but to rethink the opportunities science research has the potential to unravel.
(This blog post first appeared at The Copernican on December 22, 2012.)
A version of this article, as written by me, appeared in The Hindu on November 22, 2012.
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On November 12, at the first day of the Hadron Collider Physics Symposium at Kyoto, Japan, researchers presented a handful of results that constrained the number of hiding places for a new theory of physics long believed to be promising.
Members of the team from the LHCb detector on the Large Hadron Collider (LHC) experiment located on the border of France and Switzerland provided evidence of a very rare particle-decay. The rate of the decay process was in fair agreement with an older theory of particles’ properties, called the Standard Model (SM), and deviated from the new theory, called Supersymmetry.
“Theorists have calculated that, in the Standard Model, this decay should occur about 3 times in every billion total decays of the particle,” announced Pierluigi Campana, LHCb spokesperson. “This first measurement gives a value of around 3.2 per billion, which is in very good agreement with the prediction.”
The result was presented at the 3.5-sigma confidence level, which corresponds to an error rate of 1-in-2,000. While not strong enough to claim discovery, it is valid as evidence.
The particle, called a Bs0 meson, decayed from a bottom antiquark and strange quark pair into two muons. According to the SM, this is a complex and indirect decay process: the quarks exchange a W boson particle, turn into a top-antitop quark pair, which then decays into a Z boson or a Higgs boson. The boson then decays to two muons.
This indirect decay is called a quantum loop, and advanced theories like Supersymmetry predict new, short-lived particles to appear in such loops. The LHCb, which detected the decays, reported no such new particles.
The solid blue line shows post-decay muons from all events, and the red dotted line shows the muon-decay event from the B(s)0 meson. Because of a strong agreement with the SM, SUSY may as well abandon this bastion.
At the same time, in June 2011, the LHCb had announced that it had spotted hints of supersymmetric particles at 3.9-sigma. Thus, scientists will continue to conduct tests until they can stack 3.5 million-to-1 odds for or against Supersymmetry to close the case.
As Prof. Chris Parkes, spokesperson for the UK participation in the LHCb experiment, told BBC News: “Supersymmetry may not be dead but these latest results have certainly put it into hospital.”
The symposium, which concluded on November 16, also saw the release of the first batch of data generated in search of the Higgs boson since the initial announcement on July 4 this year.
The LHC can’t observe the Higgs boson directly because it quickly decays into lighter particles. So, physicists count up the lighter particles and try to see if some of those could have come from a momentarily existent Higgs.
These are still early days, but the data seems consistent with the predicted properties of the elusive particle, giving further strength to the validity of the SM.
Dr. Rahul Sinha, a physicist at the Institute of Mathematical Sciences, Chennai, said, “So far there is nothing in the Higgs data that indicates that it is not the Higgs of Standard Model, but a conclusive statement cannot be made as yet.”
The scientific community, however, is disappointed as there are fewer channels for new physics to occur. While the SM is fairly consistent with experimental findings, it is still unable to explain some fundamental problems.
One, called the hierarchy problem, asks why some particles are much heavier than others. Supersymmetry is theoretically equipped to provide the answer, but experimental findings are only thinning down its chances.
Commenting on the results, Dr. G. Rajasekaran, scientific adviser to the India-based Neutrino Observatory being built at Theni, asked for patience. “Supersymmetry implies the existence of a whole new world of particles equaling our known world. Remember, we took a hundred years to discover the known particles starting with the electron.”
With each such tightening of the leash, physicists return to the drawing board and consider new possibilities from scratch. At the same time, they also hope that the initial results are wrong. “We now plan to continue analysing data to improve the accuracy of this measurement and others which could show effects of new physics,” said Campana.
So, while the area where a chink might be found in the SM armour is getting smaller, there is hope that there is a chink somewhere nonetheless.
