Crack index in the Standard Model disappears in LHC data

Workers complete the assembly of the LHCb, Upstream Tracker, C-side detector with its silicon bars, electronics and infrastructure.

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

A once-promising clue to new physics from the Large Hadron Collider (LHC), the world’s largest particle accelerator, has melted, 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 result from the decay of certain particles.

But the latest results from the LHCb experiment at CERN, the European particle physics laboratory that hosts the LHC near Geneva, Switzerland, suggest that electrons and muons are being 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, a senior member of a separate LHC experiment. It revealed how a number of startling subtleties had conspired to produce an apparent anomaly, she said.

Renato Quagliani, LHCb physicist at the Swiss Federal Institute of Technology (EPFL) in Lausanne, presented the results at CERN on 20 December, during a seminar which also attracted more than 700 viewers online. The LHCb collaboration has also published two preprints on the arXiv repository1,2.

LHCb first reported a small gap in muon and electron production in 2014. When proton collisions produced massive particles called B mesons, these rapidly decayed. 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 antimuon. The Standard Model predicted that both types of pairs should occur at about the same frequency, but the LHCb data suggested that electron-positron pairs occurred more often.

Particle physics experiments frequently produce results that deviate slightly from the Standard Model, but turn out to be statistical flukes as the experiments collect more data. Instead, over the next few years, the B-meson anomaly seemed to become more visible, reaching a level of confidence known as 3 sigma – although it still hadn’t reached the level of significance. required to claim a discovery, which is 5 sigma. A number of related B meson measurements have also revealed deviations from theoretical predictions based on the Standard Model of particle physics.

The latest results included more data than previous LHCb measurements on B meson decays, as well as further investigation of possible confounders. According to LHCb spokesman Chris Parkes, a physicist at the University of Manchester, UK, the apparent discrepancies in earlier measurements involving kaons were partly the result of misidentifying other particles as than electrons. 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 come up with models that could explain the anomalies. “I’m sure people would have liked us to find a flaw in the Standard Model,” Parkes says, but in the end, “you do the best analysis with the data you have, and you see what nature gives you.” , he said. . “That’s really how science works.”

Although it’s been rumored 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 conference, as a consistent picture seemed to emerge from the associated abnormalities. This could have indicated the existence of novel elementary particles that could affect B meson decays. Isidori credits the LHCb collaboration for being “honest” in admitting that their previous analyzes had problems, but regrets that ‘t took the collaboration so long to find them.

On the other hand, some of the other anomalies, including in B meson decays that don’t involve kaons, could still turn out to be real, adds Isidori. “All is not lost.” Marcella Bona, an experimental physicist at Queen Mary University of London who is involved in another LHC experiment, agrees. “It seems that theorists are already thinking about how to console and refocus.”

The remaining hints of hope from the new physics include a measurement that found the mass of a particle called the W boson to be larger than previously announced in April. But a separate anomaly, also involving muons, could also disappear. The muon’s magnetic moment seemed 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 stem from miscalculations of the Standard Model predictions.

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