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At half past six on the evening of 20 January 2021, amid the gloom of a long winter lockdown, a small team met on Zoom to share a moment they knew might change physics forever. 鈥淚 was literally shaking,鈥� says . He and his team were about to 鈥渦nblind鈥� a long-awaited measurement from the LHCb experiment at the CERN particle physics laboratory near Geneva, Switzerland 鈥� one that might, at long last, break the standard model, our current best picture of nature鈥檚 fundamental workings.

The measurement concerns subatomic particles known as 鈥渂eauty鈥� or 鈥渂ottom鈥� quarks. Over the past few years, their behaviour has hinted at forces beyond our established understanding. Now, with the hints continuing to firm up, and more results imminent, it鈥檚 crunch time. If these quarks are acting as they appear to be, then we are not only seeing the influence of an unknown force of nature, but perhaps also the outline of a new, unified theory of particles and forces.

That is a big if 鈥� but many particle physicists are on tenterhooks, myself included.鈥滻鈥檝e never seen something like this,鈥� says , a theorist at the University of Zurich, Switzerland. 鈥淚鈥檝e never been so excited in my life.鈥�

For all its dazzling success in describing the basic ingredients of our universe, the standard model of particle physics has many shortcomings. It can鈥檛 explain dark matter, the invisible stuff that keeps galaxies from flying apart, or dark energy, which seems to be driving the accelerating expansion of the universe. Nor can it tell us how matter survived the big bang, rather than being annihilated by an equal amount of antimatter. What鈥檚 more, it has several apparently arbitrary features that beg deeper explanations. Clearly, the standard model isn鈥檛 the whole picture. To complete it, we need to break it.

The saga of the beauty quarks began in the mid-2000s when Gudrun Hiller, a theoretical physicist then at the University of Munich, Germany, was panning for insights in a flood of data from the Belle experiment in Japan and the BaBar experiment in California. These 鈥淏-factories鈥� produced beauty quarks by colliding electrons with their antiparticles, positrons. The beauty quarks would live for an instant 鈥� around 1.5 trillionths of a second, on average 鈥� before decaying into other particles.

A strange beauty

Hiller was particularly interested in an extremely rare decay where a beauty quark transforms into a strange quark, the third heaviest of six types of quark. In doing so, it emits two oppositely charged muons, heavier versions of electrons. Rare decays such as these are very valuable, as they could be strongly influenced by unknown forces of nature, should they exist. The idea is to make the most precise measurement possible of such decays and compare them with the most precise predictions theorists can muster using the standard model. If the two disagree, you have evidence for a new force.

The trouble was, theoretical predictions of how often a beauty quark should transform into a strange quark and two muons were plagued by uncertainties from quantum chromodynamics (QCD), the theory of the strong force that governs how quarks interact with one another within the standard model. This made it very hard to make any meaningful comparison with experimental measurements 鈥� any discrepancy could be down to the imprecision of the predictions. 鈥淲e realised that we hit a wall,鈥� says Hiller.

Undeterred, she and her collaborator Frank Kr眉ger realised that if you look at how often this decay occurred compared with a similar decay that spits out electrons instead, the nasty uncertainties from QCD cancelled out. The ratio of the two decays could be predicted very precisely 鈥� but would be sensitive only to forces pulling on the electrons and muons with differing strength. That was a long shot. All known forces pull on the two particles equally, and the assumption was that any undiscovered forces would do so too, meaning Hiller and Kr眉ger鈥檚 ratio wouldn鈥檛 reveal anything new.

A decade later, collisions at CERN鈥檚 Large Hadron Collider (LHC) began producing a torrent of beauty quarks, which were recorded and analysed by the LHCb experiment, one of four large particle detectors on the 27-kilometre accelerator ring beneath the French-Swiss border. Now, physicists could really start to put these rarest decays under the microscope. As they did so, intriguing anomalies began to emerge.

The first came when early measurements suggested that decays producing a strange quark and two muons happened less often than the standard model predicted. Then, in 2013, the LHCb experiment released a new measurement that analysed the angles that the particles produced in these decays went flying out at. This time, there were even stronger hints of deviations from the standard model. And yet there were still sufficient theoretical uncertainties to leave room to quibble.

Could Hiller and Kr眉ger鈥檚 ratio help? In 2014, LHCb released the first measurement comparing how often beauty quarks decayed into muons and electrons. To almost everyone鈥檚 surprise, the data once more disagreed with the standard model. Beauty quarks appeared to be decaying to muons less often than to electrons. Analysis concluded there was less than a 1 per cent chance the deviation was purely down to some random statistical wobble in the data. This was still a long way short of the gold-standard statistical significance required to declare a discovery in particle physics, which corresponds to a 1 in 3.5 million chance of the result being a fluke.

Strong deviations

Still, when you combined the measurements of the muon-to-electron ratio, the angles and how often the decays happened, a coherent picture did seem to be emerging. Since then, almost every time a measurement has been updated with yet more beauty quark data, the deviations from theory have become stronger.

Almost, because there was one notable exception. When the Hiller-Kr眉ger ratio was updated with more data in 2019, the measured value moved towards the standard model value. 鈥淲e really thought we had it,鈥� says Patel, who led the work. 鈥淲e ended up feeling gutted.鈥� So, when Patel and his colleagues met on Zoom in January 2021 to unveil a new measurement, emotions were running high.

鈥淭hese anomalies could be the real deal鈥�

University of Cambridge experimental physicist . The measured value of the ratio had stayed almost exactly the same, but the error on it had shrunk, creating an unmistakable tension with the standard model prediction. There was now less than a 1 in 1000 chance the discrepancy was a statistical fluke. Everyone on the call erupted. 鈥淭here was an awful lot of swearing,鈥� says Patel. However, the team also felt the weight of responsibility; they knew the result would create huge excitement. As Alvarez Cartelle puts it: 鈥淵ou don鈥檛 want to think, 鈥業 just broke the standard model鈥�, but at the same time you鈥檙e a bit, 鈥極h shit!鈥�.鈥�

Anomalies come and go in particle physics, and no measurement of the muon-electron ratio on its own has yet crossed the threshold of statistical certainty for it to be regarded as a definitive discovery. But there is a coherency to what have become known as the 鈥淏 anomalies鈥� that has led a growing number of physicists to regard this as the real deal. 鈥淚鈥檝e turned into a believer,鈥� says , a theorist at the University of Cambridge. 鈥淭here鈥檚 always healthy scepticism, but the fact that it鈥檚 coming from lots of different angles and saying the same thing is pretty convincing.鈥�

In which case, what could be causing these anomalies? Allanach has spent the past few years trying to figure that out. For him, the most promising candidate is a force carried by a hypothetical particle known as a Z prime. This would be very heavy, electrically neutral and, crucially, would interact with electrons and muons with different strengths. This could explain why beauty quarks decay into muons less often than to electrons 鈥� the Z prime is stopping them.

This could also explain one of the most mysterious, seemingly arbitrary features of the standard model: the fact that matter particles come in three 鈥済enerations鈥�. The first comprises the familiar particles that make up most ordinary matter: the electron, the electron neutrino and the up and down quarks. The second contains heavier copies of these particles: the muon, muon neutrino, charm and strange quarks. And the third generation is heavier still: the tau, tau neutrino, top (or 鈥渢ruth鈥�) and beauty quarks. The existence of these generations has long been a puzzle, as has the peculiar fact that the masses of the matter particles vary so wildly, with the top quark being around 350,000 times heavier than the electron.

The different generations could be explained if the beauty quark anomalies are revealing the presence of a new force that acts almost exclusively on the third generation of particles. 鈥淭he model I鈥檓 working on contains a symmetry which means that if you squint a bit, only the third generation is allowed to have a mass,鈥� says Allanach 鈥� which would explain why these particles are so heavy.

The implications of this new force wouldn鈥檛 end there. In the second half of the 20th century, physicists discovered that the three forces of nature described by the standard model 鈥� the strong and weak forces and electromagnetism 鈥� could each be described using a mathematical symmetry. In the 1970s, there was a big push to bring all three forces together under a single bigger symmetry, to create a so-called grand unified theory, which promised to unify these forces and the matter particles into one elegant structure.

The problem was that the various grand unified theories predicted that protons should decay, while every experiment performed failed to see any sign of that. What鈥檚 more, the energies required to probe these theories are over a trillion times higher than even the LHC can achieve, meaning that the new particles they predict are well out of experimental reach. As a result, the quest to unify the forces and the matter particles has been stalled for decades.

The B anomalies appear to be resurrecting aspects of the old grand unified theories, but at far lower energies than anyone had expected. 鈥淲hat we鈥檙e doing is putting in a tiny bit of symmetry 鈥� it鈥檚 an element of a grand unified theory, but it鈥檚 only a little one,鈥� says Allanach. He believes that the hints of a new force we are seeing at the moment could be a low-energy remnant of a much grander symmetry that only becomes apparent at very high energies. In other words, we might be catching a glimpse of the edge of a grand unified theory.

Hiller pioneered an alternative explanation for the B anomalies that goes further still 鈥� a particle known as a leptoquark. Again, a leptoquark would be the carrier of a new force. This force would transform quarks directly into electrons, muons and taus, collectively known as leptons 鈥� hence the particle鈥檚 name. Unlike Z prime models, leptoquark models also aim to explain a second set of anomalies that have appeared in another type of beauty quark decay, this time to charm quarks, while pointing to a unified theory that鈥檚 much closer at hand in terms of energy scales.

The colour violet

Isidori is a proponent of leptoquarks. He says the models represent a 鈥渃hange of paradigm鈥� compared with the old grand unified theories. While the old ones looked for symmetries that unified all three forces, the modern leptoquark models instead unify leptons with quarks.

They do this by differing from the standard model in a crucial way. In the standard model, the equivalent of electric charge for the strong force, which acts on quarks, is known as 鈥渃olour鈥�. It comes in three varieties, red, green and blue. Leptons don鈥檛 carry colour, so they don鈥檛 feel the strong force. In leptoquark models, however, there is a fourth colour, sometimes labelled violet, which arises from an enlarged version of the symmetry that describes the strong force. This larger symmetry then breaks down into the usual three-colour strong force with red, green and blue quarks, while the leftover fourth colour is carried by the leptons. Leptons are really just differently coloured quarks.

This is heady stuff 鈥� but the challenge now is to prove that these anomalies are the real deal. Isidori, for one, is convinced. 鈥淔or me, the evidence is already very solid,鈥� he says. But not everyone agrees. Although a series of unfortunate statistical flukes now seems like a very unlikely explanation given the range of different anomalies, the looming spectre is the chance of a conspiracy of missed biases, either in the theoretical predictions or the experimental measurements, or perhaps both.

New measurements are already under way at LHCb to confirm the picture and test for hidden experimental effects. In October 2021, my University of Cambridge colleague John Smeaton and I performed . It revealed very similar effects to those seen in March, strengthening the case for a new force.

Meanwhile, the growing excitement around the anomalies has awoken the two big beasts of the LHC, the ATLAS and CMS experiments. In 2012, they discovered the Higgs boson, the long-predicted standard-model particle that gives all other fundamental particles their mass, and are now beginning to think about ways they might spy the predicted Z primes or leptoquarks. In Japan, the Belle II experiment is gradually accumulating data that will allow it to independently check several of LHCb鈥檚 results. Later this year, an upgraded LHCb will begin collecting data at a far higher rate than before, allowing us to seek out even rarer decays where the anomalies could be even stronger.

If the emerging picture is confirmed, we are in for a revolution in our understanding of the constituents of nature that could reveal a deeper structure beneath the standard model, while perhaps even giving us a handle on the nature of dark matter or the strange properties of the Higgs boson. If that happens, it will be the greatest discovery in fundamental physics since the standard model was put together. The stakes are high and the game is on.

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