Hint of crack in standard model vanishes in LHC data

A once-promising hint of new physics from the Large Hadron Collider (LHC), the world’s largest particle accelerator, has melted away, quashing one of physicists’ best hopes for a major discovery.

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

But the latest results from the LHCb experiment at CERN, Europe’s particle-physics laboratory 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 robust than before,” says Florencia Canelli, an experimental particle physicist at the University of Zurich in Switzerland who is a senior member of a separate LHC experiment. It has revealed how a number of surprising subtleties had conspired to produce an apparent anomaly, she says.

Renato Quagliani, an LHCb physicist at the Swiss Federal Polytechnic Institute (EPFL) in Lausanne, reported the results at CERN on 20 December, in a seminar that also attracted more than 700 viewers online. The LHCb collaboration also posted two preprints on the arXiv repository^1,2.

LHCb first reported a tenuous discrepancy in the production of muons and electrons in 2014. When collisions of protons produced massive particles called B mesons, these quickly decayed. The most frequent decay pattern produced another type of meson, called a kaon, plus pairs of particles and their antiparticles — either an electron and a positron or a muon and an antimuon. The standard model predicted that the two types of pairs should occur with roughly the same frequency, but LHCb data suggested that the electron-positron pairs occurred more often.

Particle-physics experiments frequently produce results that slightly deviate from the standard model, but which turn out to be statistical flukes as the experiments collect more data. Instead, in subsequent years, the B-meson anomaly had seemed to become more conspicuous, reaching a confidence level known as 3 sigma — although it still did not reach the level of significance required to claim a discovery, which is 5 sigma. A number of related measurements on B mesons also revealed deviations from the theoretical predictions based on the standard model of particle physics.

The latest results included more data than the previous LHCb measurements of the B meson decays, and also a more thorough study of possible confounding factors. The apparent discrepancies in the earlier measurements involving kaons were in part the result of misidentifying some other particles as electrons, says LHCb spokesperson Chris Parkes, a physicist at the University of Manchester, UK. While LHC experiments are good at catching muons, electrons are trickier for them to detect.

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

Although it had been rumoured for months, the latest result comes as a surprise, says Gino Isidori, a theoretical physicist at the University of Zurich who was at the CERN talk, because a coherent picture seemed to be emerging from related anomalies. This could have pointed to the existence of previously unseen elementary particles that could be affecting the decays of B mesons. Isidori credits the LHCb collaboration for being “honest” in admitting that their previous analyses had problems, but he says regrets the fact that it took so long for the collaboration to find them.

On the other hand, some of the other anomalies, including in B-meson decays that do not involve kaons, could still turn out to be real, Isidori adds. “Not all is lost.” Marcella Bona, an experimental physicist at Queen Mary University of London who is part of yet another LHC experiment, agrees. “It looks like theorists are already thinking about how to console themselves and refocus.”

The remaining hopeful hints of new physics include a measurement that found the mass of a particle called the W boson to be greater than expected, announced in April. But a separate anomaly, also involving muons, could also be going away. The muon’s magnetic moment had appeared to be stronger than predicted by the standard model, but the latest theoretical calculations suggest that it is not, after all. Instead, the discrepancy could have originated from miscalculations of the standard model’s predictions.