The Large Hadron Collider (LHC) International Collaboration today provided new measurements of rare particle transitions, or decays, that provide one of the highest resolution tests yet of a key property of the Standard Model of particle physics, known as lepton flavor. International.
Previous studies of these anomalies have indicated interesting tensions with theoretical predictions, possibly due to the effects of novel particles or forces. The results of an improved, large-scale analysis based on the full LHC data set collected by the Run 1 and Run 2 experiment, presented at a symposium at CERN this morning, are in line with Standard Model predictions.
A central puzzle of particle physics is why the twelve quarks and leptons are arranged in pairs across three generations that are identical in all but mass, with ordinary matter made up of particles of the first and lightest generation. The universality of the Lipton flavor states that the essential powers are blind to which generation Lipton belongs to.
However, in recent years, accumulating results from LHCb and experiments in Japan and the United States have suggested that this may not be the case, prompting a stir of caution among physicists about a more fundamental theory—perhaps shedding light on the enigmatic Standard Model structure of flavor—that might reveal about the same at the LHC.
Interest in “flavor anomalies” peaked in March 2021, when LHCb presented new results comparing the rates of decay of some B mesons, complex particles containing beauty quarks, into muons and electrons. According to the theory, deflections involving muons and electrons should occur at the same rate, once differences in the masses of leptons are accounted for. But the LHCb results hinted that B mesons decay into muons at a lower rate than expected, as indicated by the statistical significance of the results of 3.1 standard deviations from the Standard Model prediction.
The new LHCb analysis, which has been ongoing for the past five years, is even more comprehensive. It takes into account two different B-meson decay modes simultaneously for the first time and provides better control over background processes that can simulate the decay of B mesons of electrons. In addition, the two decay modes are measured in two different regions of the cluster, resulting in four independent comparisons of decay. The results, which supersede previous comparisons, are in excellent agreement with Lipton’s universal flavor principle.
“Measurements of the ratios of rare B-meson decays to electrons and muons have generated great interest in recent years because they are theoretically ‘clean’ and show consistency with the anomaly pattern seen in other flavoring processes,” explains LHCb spokesman Chris Parkes. The University of Manchester and CERN. “The results presented today are the product of a comprehensive study of the two main situations using our full data sample and the application of new, more robust techniques. These results are consistent with the expectations of our theory.”
The new datasets will allow LHCb — one of the LHC’s four large experiments at CERN — to further investigate the universality of lepton flavour, as well as to conduct a broader research program involving studies of new hadrons, including searches for tetraquarks and exotic pentavalents and investigations of differences between matter and antimatter. . An upgraded version of the experiment now running for LHC Run 3 will collect larger data sets that will allow for higher-resolution tests of the decay of rare particles.
“Previous LHCb indications of anomalies regarding the universality of lepton flavor have generated excitement,” says theoretical physicist Michelangelo Mangano of CERN.
“Such potentially real anomalies show just how much remains unknown, as theoretical explanations have revealed countless potentially unexpected phenomena. LHCb’s latest discoveries take nothing away from our mission to push the boundaries of our knowledge even further, And the search for anomalies continues, guided by empirical hints.”
Related research is also available at arXiv Prepress server.
The LHCb collaboration, measuring global lepton parameters in b+→ k+ℓ+ℓ– and web0→ k∗ 0ℓ+ℓ– decay arXiv (2022). doi: 10.48550/arxiv.2212.09153
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