The Standard Model is questioned: is lepton universality maintained when charming mesons decay?

The international collaboration team LHCb (CERN), that includes scientists from the G.I. Budker Institute of Nuclear Physics SB RAS (Novosibirsk Institute of Nuclear Physics) and Novosibirsk State University (NSU), reported the discovery of a noticeable violation of the leptonic universality in the decay of a neutral charming meson (B meson) into an excited kaon and a lepton pair (electron-positron or muon-antimuon). This experimental result deviates from the predictions of the Standard Model, in which the probability of decay with electron-positron and muon-antimuon pairs should be practically equal. The research team emphasizes that the data requires additional verification.

The Standard Model (SM) describes surprisingly well the fundamental interactions of elementary particles, but some astrophysical observations indicate that it does not fully explain the physical picture of the world. In the laboratory, it is possible to verify if there is new physics outside the Standard Model in precision experiments on the properties of elementary particles. To do this,  scientists are building colliders. The most famous of them is the Large Hadron that is located in CERN (Geneva). Its main task is to search for phenomena that go beyond the Standard Model.

Experiment LHCb. In the course of the experiment at the Large Hadron Collider, the LHCb scientists recorded the decay of neutral B mesons into a neutral K * meson and a pair of lepton-antilepton. Lepton universality means that the probability of decay into final states with different generations of leptons is equal. However, scientists have found that for every three events with the appearance of an electron-positron pair, there are only two events when a muon-antimuon pair are observed and that contradicts the SM.

Deputy Director of the Institute of Nuclear Physics SB RAS, Dean of the Novosibirsk State University Physics Department, participant of the LHCb collaboration, Corresponding Member of the Russian Academy of Sciences Alexander Bondar talked about this work, "In this experiment there is a hint of symmetry breaking between leptons of different types. According to the Standard Model asymmetry should not arise in these processes. Perhaps there is a new physics, new particles that interact with leptons of different generations in different ways, that leads to a violation of symmetry between types of leptons. I want to stress this is just a possibility. We must understand this phenomenon, including theoretically. The SM is a very rigid construction so it is extremely difficult to supplement it without violating consistency. Therefore, if in the course of additional measurements, it is discovered that the asymmetry does exist, it will be necessary to modify the SM so that the observed effect fits into a single picture".

New physics or inaccuracy of the experiment? The statistical significance of the difference between any measurement results and the expected value depends on the measurement error and is characterized by the value expressed in the number of so-called standard deviations. In the physics of elementary particles, a reliable difference between the values ​​is considered to be a difference of more than 5 standard deviations (sigma). So far, the results obtained at LHCb do not allow us to talk about the discovery of a new phenomenon because the deviation of the measured value from the calculated value is only about 2.5 sigma. However, recently the data obtained in the study of decay of B mesons into final states containing lepton pairs shows systematic deviations from SM expectations.

The role of experiments on other colliders. When studying the events under discussion in the LHCb experiment, the result obtained a few years ago at the VNPP-4M collider at the INP SB RAS was used for the normalization. "The error in measuring the probability of decay," explains Alexander Bondar, "can be caused not only by a statistical error, but also by the fact that processes with the creation of muons and electrons are detected ith different efficiencies. In the analysis of data, special measures were taken to compensate for possible systematic errors. For example, events from the decay of a neutral B meson into an excited kaon and a J / psi meson, followed by the decay of the J / psi meson into an electron-positron and a muon-antimuon pair, were used to monitor the efficiency. The probability of the decay of the J / psi-meson into pairs of leptons of different generations was measured several years ago at our institute in an experiment with a CEDR detector. It was reliably established that their probabilities are equal. This result was used in the current study of lepton universality in the decay of B mesons.

Hypotheses to explain the experimental data. A possible cause of symmetry breaking between leptons of different types may be the existence of a new particle, the Z'-boson, a heavy virtual particle that is not described by the Standard Model. It can interact differently with the electron and the muon, thereby increasing or, conversely, decreasing the probability of the decay of the B meson into this or that state. Another possible explanation for the observed pattern may be the existence of a hypothetical scalar leptoquark Δ, a particle interacting with both quarks and leptons. Finally, the cause may be some as yet unknown manifestation of New Physics. The results obtained today provide an additional stimulus for those conducting experiments, as well as theorists, to pay close to this area of inquiry.