Hint of crack in Standard Model fades in LHC data

Workers finish assembling the LHCb, Upstream Tracker, detector C side with its silicon staves, electronics and infrastructure.

A detector from the LHCb experiment under construction.Credit: Brice, Maximilian; CERN

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

The apparent anomaly was an unexpected difference in the behavior 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 houses 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. She has revealed how a series of startling 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 December 20, in a seminar that also drew more than 700 viewers online. The LHCb collaboration also published two preprints in the arXiv repository.1,2.

LHCb first reported a slight discrepancy in the production of muons and electrons in 2014. When proton collisions produced massive particles called B mesons, they rapidly decayed. The most frequent 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 antimuon. The standard model predicted that the two types of pairs should occur with roughly the same frequency, but the LHCb data suggested that electron-positron pairs occurred more frequently.

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

The latest results included more data than the previous LHCb measurements of B-meson decays, and also a more thorough study of potential confounding factors. The apparent discrepancies in previous measurements involving kaons were partly the result of misidentifying some other particles as electrons, says LHCb spokesman Chris Parkes, a physicist at the University of Manchester, UK. While LHC experiments are good at capturing muons, electrons are more difficult to detect.

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

Although it had been rumored for months, the latest result is surprising, says Gino Isidori, a theoretical physicist at the University of Zurich who was at the CERN talk, because a consistent picture of related anomalies seemed to be emerging. This could have pointed to the existence of never-before-seen elementary particles that could be affecting the decays of the B mesons. Isidori credits the LHCb collaboration for being “honest” in admitting that their previous analyzes had problems, but says that laments the fact that it took so long for the collaboration to find them.

On the other hand, some of the other anomalies, including B-meson decays that do not involve kaons, could still turn out to be real, Isidori adds. “Not everything is lost”. Marcella Bona, an experimental physicist at Queen Mary University of London who is part of another LHC experiment, agrees. “It seems that theorists are 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 to be larger than expected, announced in April. But a separate anomaly, also involving muons, could also be disappearing. The muon’s magnetic moment appeared to be stronger than predicted by the Standard Model, but the latest theoretical calculations suggest that it isn’t after all. Instead, the discrepancy could have been caused by miscalculations of the Standard Model predictions.

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