The hint of a crack in the Standard Model disappears in the LHC data

The LHCb experiment detector is under construction.Credit: Brice, Maximilien; CERN

A promising hint of new physics has faded from the Large Hadron Collider (LHC), the world’s largest particle accelerator, dashing one of physicists’ best hopes for a big discovery.

The apparent anomaly was the unexpected difference in the behavior of electrons and their more massive cousins, muons, when they arise from the decay of certain particles.

But recent results from the LHCb experiment at CERN, the particle-physics laboratory in Europe that hosts the LHC near Geneva, Switzerland, suggest that electrons and muons are produced at the same rate after all.

“My first impression is that the analysis is much more powerful than before,” says Florencia Canelli, an experimental particle physicist at the University of Zurich in Switzerland and a lead member of a separate Large Hadron Collider experiment. She revealed how a number of surprisingly subtle details conspired to produce an apparent anomaly, she says.

Renato Quagliani, an LHCb physicist at the Swiss Federal Polytechnic Institute (EPFL) in Lausanne, announced the results at CERN on December 20, in a symposium that also drew more than 700 viewers online. The LHCb collaboration has also published two first editions on the arXiv repository1And the2.

LHCb first reported a weak discrepancy in the production of muons and electrons in 2014. When colliding protons create massive particles called B mesons, these particles quickly decay. The most common decay pattern produced another type of meson, called a kaon, as well as pairs of particles and their antiparticles—either an electron and a positron or a muon and an antidote. The Standard Model predicted that both types of pairs should occur at about the same frequency, but the LHCb data indicated that electron-positron pairs occur more often.

Particle physics experiments often yield results that deviate slightly from the Standard Model, but turn out to be statistical luck because the experiments collect more data. Instead, in later years, the B-meson anomaly seemed to become more pronounced, reaching a confidence level known as 3 sigma—although it still did not reach the significance level required to claim discovery, which is 5 sigma. A number of related measurements on B mesons have also revealed deviations from theoretical predictions based on the Standard Model of particle physics.

The latest results included more data from previous LHCb measurements of meson B decay, as well as a more comprehensive study of potential confounding factors. The apparent discrepancies in previous measurements involving kaons were partly caused by some other particles being misidentified as electrons, says LHCb spokesman Chris Parkes, a physicist at the University of Manchester, UK. While the LHC experiments are good at catching muons, detecting electrons is even harder for them.

The result is likely to disappoint many theorists who have spent time trying to come up with models that can explain anomalies. “I’m sure people would have liked us to find a crack in the Standard Model,” Parks says, but in the end, “you do the best analysis with the data you have, and see what nature gives you,” he says. “It’s really how science works.”

Although it had been rumored for months, the latest finding was surprising, says Gino Isidori, a theoretical physicist at the University of Zurich who was present at the CERN lecture. This could indicate the existence of previously unseen elementary particles that could influence the decay of B mesons. Isidori credits the LHCb collaboration for being “honest” in acknowledging that their previous analyzes had problems, but says he regrets the fact that it took the collaboration so long to find on her.

On the other hand, Isidori adds, some other anomalies, including in B-meson decays that do not involve kaons, could still be real. “All is not lost.” Marcella Bona, an experimental physicist at Queen Mary University of London who is part of another Large Hadron Collider experiment, agrees. “Theorists seem to be already thinking about how to console themselves and refocus.”

The remaining hopeful hints of the new physics include a measurement that found the mass of a particle called the W boson larger than expected, announced in April. But a separate anomaly, which also includes muons, can also disappear. The muon’s magnetic moment appeared to be stronger than predicted by the Standard Model, but recent theoretical calculations indicate that it is not, after all. Alternatively, the discrepancy could have arisen from a miscalculation of the Standard Model’s predictions.

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