Tension in lepton universality builds up

In the heart of the LHCb detector: view of a beam-pipe going though the LHCb magnet © CERN

In the heart of the LHCb detector: view of a beam-pipe going though the LHCb magnet © CERN

The LHCb collaboration, which includes EPFL scientists, has just released an update of a measurement which is one of the very few at the Large Hadron Collider (LHC) consistently hinting at a deviation from the theoretical expectations. This result was presented on Tuesday March 23th, simultaneously at the 55th Rencontres de Moriond conference and at a dedicated LHC seminar at CERN, followed by a CERN press release.

The measurement compares the decays of a B meson (containing a beauty quark, b) to a K meson and a pair of electrons with those having a pair of muons instead. A muon is an elementary particle identical to an electron apart from its mass, which is about 200 times larger. The Standard Model of particle physics treats all leptons (i.e. the electron, the muon, and even the heavier tau lepton) in the same way, except for differences due to their masses. This property of the Standard Model is called lepton universality. Since the b quark is heavy compared to electrons and muons, it is expected to decay with the same rate to these two lepton types, and the ratio, called RK, between the two decay probabilities is predicted to be almost equal to one. In the updated analysis, which uses all the data collected by the LHCb detector so far, this ratio was measured with much higher precision compared to previous results. The new result deviates from one with a statistical significance of 3.1 standard deviations, hence indicates evidence for violation of the lepton universality in beauty meson decays. This corresponds to a probability of about 0.1% that the data are compatible with the Standard Model expectation. 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. However, more data and complementary measurements are needed to confirm this conclusion.

Comparison between RK measurements. The measurements by the BaBar and Belle collaborations combine B+→K++ and B0→KS0+ decays. The previous LHCb measurements and the new result, which supersedes them, are also shown. (Credit: LHCb collaboration)

The LHCb experiment at the LHC is specifically designed to study with high precision the decays of particles containing heavy beauty or charm quarks. The resulting measurements of matter-antimatter differences and rare decays of beauty and charm hadrons, which often cannot be performed elsewhere, provide sensitive tests of the Standard Model of particle physics.

The High Energy Physics Laboratory (LPHE) at EPFL is one of the founding members of LHCb, with Professor Emeritus Tatsuya Nakada having been the first spokesperson of the LHCb collaboration for 14 years encompassing detector design, development, construction and commissioning leading to the first data taking in 2009. LPHE, led by Prof. Aurelio Bay, Prof. Tatsuya Nakada, both now Professors Emeriti, and by Prof. Olivier Schneider, has carried out numerous tests of the Standard Model with the data collected by LHCb, and has made key contributions to the initial detector construction and operation, as well as to the currently ongoing major detector upgrade. A few years after the Lausanne group, the University of Zurich joined the LHCb collaboration, reinforcing the impact of Switzerland in the project. The Zurich group, currently led by Prof. Nicola Serra, has made major contributions to studies of rare decays of beauty mesons, including the result recently released by the collaboration.

In 2019, Prof. Lesya Shchutska joined EPFL and LHCb. Together with Prof. Olivier Schneider, she will bear EPFL’s responsibility to bring the experiment to the next level of precision and exploit its data to search for physics beyond the Standard Model. New tracking stations, based on scintillating fibre technology, dubbed SciFi, have been developed thanks to the R&D efforts at EPFL. Crucial parts of the SciFi detector have been built and tested at EPFL. The SciFi tracker will be at the core of the new LHCb detector set out to record data at a rate five times higher than previously, already in 2022. Such high volume of data is indispensable to confirm or disprove the hints of possible lepton flavour violation with certainty. New phenomena, responsible for lepton universality violation, could manifest themselves in even more striking transitions involving leptons of different types simultaneously, so-called lepton flavour violating processes. LPHE members are actively pursuing searches for such processes, claiming the world best sensitivity in the ones already studied.

In particle physics, the gold standard for discovery is five standard deviations, corresponding to a probability of less than 0.00003%, hence it is too early to conclude anything so far. However, the new evidence of a deviation agrees with a pattern of anomalies which have manifested themselves over the last decade. Fortunately, LHCb is well placed to clarify the potential existence of new physics effects in these decays, with many related measurements upcoming soon.