The decay of neutral B-meson deviated from the predictions of the Standard model

Physicists have conducted the most accurate to date analysis of measurements of the decay of B mesons at the LHCb detector. The experimental data are not consistent with the theoretical predictions of the Standard model, but the magnitude of this discrepancy depends on the choice of the unknown parameters. Preprint published on the document server, CERN.

One of the main objectives of the LHCb detector (Large Hadron Collider beauty experiment) — search for observed deviations from the Standard model in decays of beautiful hadrons — that is, of elementary particles, which include the lovely (beauty, b-) quark or its antiquark. To this class of particles applies to B0-meson is electrically neutral unstable system of b-antiquark and a d quark. In experiments LHCb B0-mesons are produced in proton-proton collisions and shortly after that break up in various modes (i.e., different means). In one of the scenarios of collapse are born neutral kaon in the excited state (K*0), and a pair of muon and anti (μ– and μ+). Registering the decay products on the detector, it is possible to compare their distribution in the direction of the forecast, which yields the Standard model. The main problem lies in the statistical processing of data: to compare experimental and theoretical data, it is necessary to correctly determine the errors of determination of those or other quantities.

Scientists from the LHCb collaboration since the direction of steering One (Roel Aaij) from the National Institute for subatomic physics Nikhef in Amsterdam has conducted the most accurate to date analysis of measurements of decays of B0-meson with the formation of K*0 and a pair of μ+μ–. Corresponding experiments on the accelerator was carried out in 2011, 2012 and 2016.

First of all, the scientists got rid of the distortion data, which lead to background events, i.e., reactions to form other particles. The authors exclude from consideration the energy ranges found in other decay channels, and for the rest of the background — used a computer algorithm, which allowed to suppress 97% of the noise while preserving 85% of the signal. To estimate the systematic errors responsible inaccuracies in a particular setting, the researchers did a series of pseudoexperiments — that is, simulations based on real experience, which measures the difference of the total data variation of the input variables. Theoretical calculations based on the Standard model was taken into account corrections to the form factors of the particles and the corresponding effects of quantum chromodynamics for the characteristic distances.

In the result, the physicists found that the deviation of the experimental from the theoretical values is about 3.3 σ from a statistical point of view this puts the predictions of the Standard model in serious question. The authors note that the magnitude of the deviation depends on the choice of energy ranges. So, if you do not take into account data from the region with the largest shift with respect to the forecasts, the difference is reduced to 2.7 σ. In addition, the result can affect handling “nuisance” parameters of the model — an auxiliary unknown variables that are not the desired values.

Recent LHCb data also helped to achieve other important scientific results. So, last year, physicists announced the opening of a new pentaquark and for the first time noticed the asymmetry between matter and antimatter in decays of D-meson.

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