Lepton universality restored

The LHCb collaboration © 2022 CERN

The LHCb collaboration © 2022 CERN

A study led by EPFL scientists within the LHCb collaboration solves a long-debated issue in particle physics.

Renato Quagliani, a scientist at LPHE-OS, has presented a long-awaited measurement of lepton universality from the LHCb collaboration [1,2] at a dedicated LHC seminar at CERN [3] on Tuesday December 20, bringing a bright conclusion to the long-standing puzzle which was generating a lot of debates and excitement in particle physics [4].

The measurement is set up to provide a firm answer with the current available data at LHCb to a set of deviations from the theory predictions which were reported in the measurements of the rare decays of a B meson (containing a beauty quark, b) to a K+(0*) meson and a pair of electrons or muons. In the Standard Model of particle physics, these decay rates should be equal due to the same interaction strength of all leptons with force carriers, a property called lepton universality. At the same time, several previous measurements indicated that the decays to electrons might be more frequent than those to muons. Lepton universality violation would imply physics beyond the Standard Model such as a new fundamental force in addition to the known gravitational, electromagnetic, weak and strong interactions. That’s why getting new measurements of such rare decays is of utmost importance.

Invariant mass distribution of the signal B meson decays with two electrons in the final state [1]. Peaking structures in the background components show the contribution of the hadrons misidentified as electrons. © 2022 LHCb collaboration

The new analysis is led by Renato Quagliani and is carried out in collaboration between scientists from many LHCb institutes, namely LPNHE in Paris, IJClab in Orsay, RWTH Aachen, Dortmund University, Birmingham University, Tsinghua University, and EPFL. It is based on the full data set recorded by the LHCb detector, and it represents the first simultaneous measurement of two different B meson decays in two kinematic regimes, providing in total four RX measurements, where RX is the ratio between the B meson decay probabilities to muons and to electrons. Such a comprehensive approach allows a better understanding of the LHCb detector performance and the identification of a new source of background in the electron channel mimicking the signal of the rare process. Very rarely, it can happen that a charged hadron, such as a pion or a kaon, is misidentified as an electron in the detector. In case of two such misidentifications happening simultaneously, very abundant fully-hadronic B meson decays can look like a rare process with electrons, as shown in the above figure. EPFL scientists Renato Quagliani and Lesya Shchutska (LPHE-LS) developed a dedicated method to precisely estimate the contribution of such processes to the explored dataset. After inclusion of the new background component to the overall model, the RX ratios are found to be consistent with the predictions of the Standard Model within the precision attainable with the current LHCb dataset.

Measured values of lepton universality observables in the considered decays and their overall compatibility with the standard model [2].© 2022 LHCb collaboration

The new result provides an answer to one set of the so-called flavour anomalies of particle physics, observed in comparing the B meson decay rates to muons and electrons, however other observables, namely the angular distributions of muons in such decays and their differential branching fractions, still exhibit tensions with the theory predictions at the level of about 3 standard deviations. Therefore, further, more detailed, investigations of these processes are planned with the larger data set to be collected by the LHCb detector after its major upgrade which has been finalized this year. These efforts should be accompanied with developments on the theory side in order to find answers to the remaining puzzles of the Standard Model.