At the smallest scales, everything in the universe can be broken down into fundamental morsels called particles. The Standard Model of particle physics—the reigning theory of these morsels—describes a small collection of known species that combine in myriad ways to build the matter around us and carry the forces of nature. Yet physicists know that these particles cannot be all there is—they do not account for the dark matter or dark energy that seem to contribute much of the universe’s mass, for example. Now two experiments have observed particles misbehaving in ways not predicted by any known laws of physics, potentially suggesting the existence of some new type of particle beyond the standard zoo. The results are not fully confirmed yet, but the fact that two experiments colliding different types of particles have seen a similar effect, and that hints of this behavior also showed up in 2012 at a third particle collider, has many physicists animated. “It’s really bizarre,” says Mark Wise, a theorist at the California Institute of Technology who was not involved in the experiments. “The discrepancy is large and it seems like it’s on very sound footing. It’s probably the strongest, most enduring deviation we’ve seen from the Standard Model.” Finding such a crack in the Standard Model is exciting because it suggests a potential path toward expanding the model beyond those particles currently known.
The eyebrow-raising results come from the LHCb experiment at the Large Hadron Collider (LHC) in Switzerland and the Belle experiment at the High Energy Accelerator Research Organization (KEK) in Japan. Both observed an excess of certain types of leptons compared to others produced when particles called B mesons (made of a bottom quark and an antiquark) decay. Leptons are a category of particles that includes electrons, as well as their heavier cousins muons and taus. A Standard Model principle known as lepton universality says that all leptons should be treated equally by the weak interaction, the fundamental force responsible for radioactive decay. But when the experiments observed a large number of B meson decays, which should have produced equal numbers of electrons, muons and taus among their final products (after the different masses of the particles are taken into account), the decays actually made more taus.
The LHC collides protons with protons, whereas the Belle accelerator smashes electrons into their antimatter counterpart, positrons. Both types of collisions sometimes result in B mesons, however, allowing each to measure the end products when the unstable mesons decay. In a paper published in the September 11 issue of Physical Review Letters, the LHCb team announced that they had observed a potential excess of taus about 25 to 30 percent greater than the frequency predicted by the Standard Model. Belle saw a similar, but less pronounced, effect, in data reported in a paper under review at Physical Review D. Both teams shared their findings in May at the Flavor Physics & CP Violation 2015 conference in Nagoya, Japan.
Intriguingly, both results also agree with earlier findings from 2012 (and expanded on in 2013) made by the BaBar experiment at the SLAC National Accelerator Laboratory in Menlo Park, Calif. “By itself neither the Belle result nor the LHCb result is significantly off from the Standard Model,” says Belle team member Tom Browder of the University of Hawaii, who is also spokesperson of its successor project, Belle II. “Together with BaBar we can make a ‘world average’ (combining all results), which is 3.9 sigma off from the Standard Model.” Sigma refers to standard deviations—a statistical measurement of a divergence—and the usual threshold among physicists for declaring a discovery is five sigma. Although a 3.9 sigma difference does not quite hit the mark, it indicates that the chance of this effect occurring randomly is just 0.011 percent. “Right now we have three suggestive but not yet conclusive hints of an extremely interesting effect,” says theorist Zoltan Ligeti of Lawrence Berkeley National Laboratory, who was not involved in the experiments. “We should know the answer definitively in a few years” as the experiments collect more data.
If the discrepancy is real, rather than a statistical fluke, researchers will then face the tough challenge of figuring out what it means. “This effect is really not the kind that most physicists would have expected,” Ligeti says. “It is not easy to accommodate in the most popular models. In that sense it is quite surprising.”
For instance, the darling of so-called “new physics,” or beyond-the-Standard-Model, ideas—supersymmetry—does not usually predict an effect quite like this. Supersymmetry posits a host of undiscovered particles to mirror the ones already known. Yet none of its predicted particles easily produce this kind of violation of lepton universality. “I don’t think at this point we can say that this points to supersymmetry,” says Hassan Jawahery, a physicist at the University of Maryland and a member of the LHCb collaboration, “but it doesn’t necessarily violate supersymmetry.”
Yet if the signal is real, then some kind of new particle is probably implicated. In all B meson decays, at one point a heavier “virtual” particle is created and then quickly disappears—a strange phenomenon allowed by quantum mechanics. In the Standard Model this virtual particle is always a W boson (a particle that carries the weak force), which interacts equally with all leptons. But if the virtual particle were something more exotic that interacts with each lepton differently, depending on its mass, then more taus could be created at the end because taus are the heaviest leptons (and thus might interact more strongly with the virtual particle).
New Higgs or leptoquark?
One potentially appealing candidate for the virtual particle is a new type of Higgs boson that would be heavier than the particle discovered to much fanfare in 2012 at the LHC. The known Higgs boson is thought to give all other particles their mass. The new Higgs, in addition to being heavier than this known particle, would have other differing qualities—for example, to affect the B meson decays, it would have to have electromagnetic charge, where the known Higgs has none. “It would mean that the one Higgs we found so far is not the only one that is responsible for generating the mass for all the particles,” Jawahery says. Supersymmetry, in fact, predicts additional Higgs bosons beyond the one we know. Yet in most formulations of the model, these predicted Higgs particles would not create a discrepancy as large as the one showing up in the experiments.
Another option is an even more exotic hypothetical particle called a leptoquark—a composite of a quark and a lepton, which has never been seen in nature. This particle, too, would interact more strongly with the tau than the muon and the electron. “Leptoquarks can occur very naturally in certain types of models,” Ligeti says. “But there is no reason to expect them to be as low-mass as what would be needed to explain these data. I think most theorists would not consider these models particularly compelling right now.”
In fact, all of the explanations theorists can think of so far for the observations leave something to be desired—and do not do much to solve any of the larger outstanding problems of physics, such as the question of what makes dark matter or dark energy. “There’s nothing nice about these models—they’re just sort of cooked up to explain this fact, not to get at the trouble with other facts,” Wise says. “But just because the theorists are not comfortable with it, nature will do what nature does.”
There is also a chance, albeit slim, that physicists have incorrectly calculated the Standard Model’s predictions, and that the reigning rules still apply. “It’s possible the Standard Model calculation is not correct, but recent calculations have not revealed any serious problem there,” says Michael Roney of the University of Victoria in Canada, spokesperson for the BaBar Experiment. “It is also conceivable that the experiments have missed some more conventional explanation, but the experimental conditions at LHCb and BaBar are very different. In BaBar we have been continuing to mine our data in different ways but the effect persists.”
Physicists are optimistic the mystery will be sorted out soon with more data. In April the LHC started running collisions at higher energy, which for LHCb translates to more B mesons produced, and more chances to look for the discrepancy. Belle, meanwhile, is planning an upgraded experiment with an improved detector called Belle II scheduled to start collecting data in 2018. Both experiments should eventually find more data to confirm the effect, or see it fizzle if it was a statistical fluke. “If it is there then we have a huge program ahead of us for the next decade to study it in even more detail,” Jawahery says. “By then we would hopefully know what it also means, not just that it is there.”