Editor’s Note (10/19/21): On October 19, 2021, LHCb physicists unveiled two more small anomalies that continue a curious pattern of “missing” muons, which collectively hint that these exotic subatomic particles are being produced at lower-than-expected rates. With further validation, these results could become the most promising pathway toward new physics beyond the Standard Model.
If beauty is in the eye of the beholder, then consider a tantalizing new result beguiling the world’s particle physicists. Specifically, scientists are interested in fresh data from the LHCb (Large Hadron Collider beauty) detector, an experiment studying the decays of B-mesons—particles that contain beauty quarks. During a virtual session of the annual Rencontres de Moriond conference on Tuesday, nearly 1,000 physicists watched as the LHCb collaboration announced evidence for an unexplained discrepancy in the behavior of electrons and their heavier cousins, muons.
Under the Standard Model—the theory that describes elementary particles and the forces they obey, minus gravity—leptons such as electrons and muons are identical except for their mass. So B mesons should decay to a kaon and two muons at the same rate at which they decay to a kaon and two electrons. Yet LHCb is seeing a difference in this rare beauty decay: B mesons seem to decay to muons 15 percent less often than they do to electrons.
“It’s certainly intriguing, this new measurement,” says Monika Blanke, a theoretical physicist at the Karlsruhe Institute of Technology in Germany, who was not involved with the new research. “If it’s eventually confirmed experimentally, then there actually is something beyond the Standard Model that treats the lepton flavors differently.”
Physicists have long wondered if muons, electrons and other leptons possess differences besides their mass; the latest LHCb result suggests the answer might be yes. The finding has a statistical significance of 3.1 sigma, which meets the standard baseline for evidence in particle physics. Precisely speaking, 3.1 sigma means that in the absence of new physics, statistical fluctuations would still lead the researchers to see a discrepancy between electrons and muons of 15 percent or more once every 740 times they performed the experiment. Although this would seem to suggest the observed muon-electron discrepancy is almost certainly more than a mirage, the three-sigma effect, in fact, falls well short of the gold standard of discovery in particle physics: five sigma, which works out to running the experiment 3.4 million times before seeing a statistical fluke that large. (These figures are subtly but importantly different from a one-in-740 or one-in-3.4-million chance of being wrong.)
Why all the fuss about statistics? At LHCb and other experiments, numerous two- and three-sigma discrepancies between electrons and muons have popped up across the years. But so far, none of these results has held up: once more data were collected, the differences between leptons faded away, leaving the Standard Model triumphant.
“If it was only one, I wouldn’t be super excited. I’ve seen other anomalies go away,” says Gino Isidori, a theoretical physicist at the University of Zurich, who was not involved with the research. But he is encouraged by the latest LHCb result because it follows a pattern of other measurements that also hint at differences between electrons and muons. For Isidori and other particle physicists, that is reason enough for cautious excitement.
A Thing of Beauty
Located right on the border of France and Switzerland, LHCb is one of many detectors along the Large Hadron Collider’s (LHC’s) 17-mile loop. Although LHCb also looks at the results from proton-proton collisions, its focus is on extremely rare decays, such as those of B mesons.
“Rare decays are a different way of trying to find heavy particles,” says Patrick Koppenburg, a particle physicist at LHCb. Instead of just smashing protons together and looking for signs of a new particle in the detritus, as the LHC did in its successful brute-force search for the Higgs boson, LHCb looks at minor variations in the one-in-a-million events. That is, a rare decay of a B meson does not directly yield new particles—muons and kaons are old hat—but the rate at which the decay happens can depend on heavy, as-yet-unseen particles influencing the outcome behind the scenes. In the 1960s, for example, rare decays of kaons hinted at the existence of the charm quark before it was directly discovered. LHCb is designed to tease out these needles from the haystack. But even so, the work is difficult and full of experimental uncertainties.
Then there are also theoretical uncertainties to consider: the Standard Model predictions that researchers compare their results against. Part of the excitement surrounding the latest LHCb result is that the specific B meson decay is “clean”—it has a very small theoretical uncertainty. Eliminating one source of error makes it much easier to see if the difference between electrons and muons is genuine.
Since the Standard Model’s inception in the 1970s, theoretical physicists have proposed models that explain this difference in the form of a new particle. Two of the top candidates are the Z’ (pronounced “zee prime”)—a variation on the existing Z boson —and the leptoquark, a particle that would link leptons and quarks. In the coming days and weeks, theoreticians will use the latest result to update their models—and, in fact, three preprint papers were already released within less than 24 hours of the announcement of the LHCb results.
But the physics of this rare decay is far from settled, and much more data are needed before a new particle can be claimed as the culprit. The best option for corroboration will be Belle II, a Japanese experiment. Mikihiko Nakao, a researcher involved in Belle II, expects it will take about five years to catch up to LHCb’s sensitivity.
Currently, LHCb is shut down for maintenance. But when it reopens with an upgraded detector next year, it could double all of the data taken over the past decade in just a single year, according to Koppenburg. In April upcoming results from Muon g-2, an experiment at the Fermi National Accelerator Laboratory in Batavia, Ill., could also shed light on differences between leptons.
Physicists are aware that this latest result—a bump in the data—is quite possibly just a statistical fluctuation. Having been let down several times before, they are now careful to hedge their bets, trying to avoid conveying certitude or undue hype.
But if it is real—well, that would be beautiful.