Hint of Crack in Standard Model Vanishes in LHC Data

Hint of Crack in Standard Model Vanishes in LHC Data

: Hint of Crack in Standard Model Disappears


Hopes for new physics were raised by the discrepancy in measuring a particular type of particle decay.

Construction of a detector for the LHCb experiment. Credit: Maximilien Brice/CERN

One of the most promising hints of new physics, once from the Large Hadron Collider, the world’s largest particle accelerator has melted away, ending one of physicists best hopes for a major breakthrough.

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

The latest results of the LHCb experiment at CERN, Europe’s particle-physics lab near Geneva, Switzerland, suggest that electrons as well as muons are being produced at the same time.

Florencia Canelli is an experimental particle physicist from the University of Zurich, Switzerland. She is also a senior member of an independent LHC experiment. She says it has shown how many surprising subtleties conspired together to produce an anomaly.

Renato Quagliani (a Swiss Federal Institute of Technology in Lausanne) reported the results at CERN on December 20th in a seminar that attracted over 700 viewers online. Two preprints were also published by the LHCb collaboration on arXiv.

Unbalanced decay

LHCb reported a slight discrepancy between the production of electrons and muons for the first time in 2014. Massive particles were created when protons collide with each other.BThese mesons 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. LHCb data suggested that both types of pair occur at roughly the same frequency. However, the standard model predicted that they would occur with similar frequency.

Particle-physics experiments often produce early results that slightly differ from the standard model. However, these are statistical flukes because the experiments collect more data. This time, however, that didn’t happen. Instead, as the years passed, theBThe -meson anomaly became more prominent, reaching a confidence level of 3 sigma, although it did not reach the level that is used to claim a discovery, 5 sigma. There are many related measurements hereBMesons also showed deviations from theoretical predictions based upon the standard model for particle physics.

This week’s results included more data than previous LHCb measurements.B-meson decays and a more detailed study of possible confounding variables. According to Chris Parkes (LHCb spokesperson), a physicist from the University of Manchester, UK, the apparent discrepancies in earlier measurements involving kaons were caused by misidentifying other particles as electrons. While LHC experiments are effective at catching muons and electrons, they are more difficult to detect.

Redirecting the search

Many theorists who spent hours trying to find models that could explain these anomalies will be disappointed by the result. Parkes says that although “I’m sure people would like us to find a crack” in the standard model, in the end “you do the best analysis with what data you have, you see what nature has for you,” he said. It’s how science works.

The latest result was rumoured for many months. However, it is a surprise. Gino Isidori, an academic theoretical physicist at University of Zurich, said that the confirmation came as a surprise because a coherent picture appeared to be emerging from related anomalies. This could have indicated the existence of previously unknown elementary particles that influence the decay of.Bmesons. Isidori credits the LHCb collaboration for being “honest” in admitting its previous analyses had issues, but he regrets that it took so much time for the collaboration to find them.

There are however other anomalies, including some that were recorded inBIsidori says that even if a -meson is not involved in kaons, it could still be true. “Not all is lost.”

Marcella Bona is an experimental physicist at Queen Mary University of London, who is also part of another LHC experiment. “It seems like theorists are already thinking of how to console themselves, and refocus.”

One of the remaining promising hints of new Physics is a measurement that foundThe mass of a particle is called theWBoson to be higher than expectedAnnounced in April. A separate anomaly, also involving muons could be disappearing. Although the muon’s magnetic moment seemed stronger than expected by the standard model, it was still a strong one.According to the latest theoretical calculations, it is not, after all. The discrepancy could be due to miscalculations of standard model predictions.

This article was reproduced with permission.First publishedon December 20 2022.


    Davide CastelvecchiIs a staff reporter atNatureHe has been obsessed with quantum spin his entire life. Follow him on Twitter@dcastelvecchi

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