Violation of lepton universality – first sign of New Physics?

On 23rd March 2021, scientists working at the LHCb experiment conducted by CERN announced that they had observed the violation of lepton universality in the processes of beauty mesons decaying into final states containing pairs of leptons (electrons or muons). Potentially, it is a groundbreaking observation indicating the existence of New Physics processes that cannot be described by the Standard Model of particle physics. Part of the LHCb experiment is a group of scientists and researchers from AGH UST.

Scientists taking part in the LHCb experiment. Source: CERN


The LHCb (Large Hadron Collider beauty) experiment is one of the so-called “large experiments” conducted at the Large Hadron Collider (LHC) by the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. LHCb is a dedicated spectrometer for performing high-precision measurements and tests of the Standard Model in the area of heavy quarks. Its most important constituent parts, which play a major role in observing the violation of lepton universality, are the tracking system (for the reconstruction of the trajectories of charged particles) and the particle identification system.  

Since 2001, part of the experiment has been the LHCb-AGH group of the Faculty of Physics and Applied Computer Science, composed of employees and doctoral students of the faculty. The activity of the 14-person team is connected with all experimental aspects, and involves the designing of readout electronics, the physics of detectors, and developing software for the simulation, reconstruction and operation of detectors, as well as data analysis. The leader of the team is professor Tomasz Szumlak; professor Agnieszka Obłąkowska-Mucha coordinates the analyses of physical data, and professor Marek Idzik is in charge of work connected with the designing of readout electronics. The team makes a significant contribution towards the operation and maintenance of the tracking system of the LHCb spectrometer. The AGH UST physicists also carry out analyses connected with the measurement of the fundamental parameters of the Standard Model.  

The Standard Model, built on the fundamentals of the idea of local gauge symmetry, is believed to be one of the most important theories of modern physics. However, it has some limitations: the lack of a possibility to include gravitational interactions, a relatively large number of free parameters, or problems related to explaining the nature of dark matter and dark energy. These problems can only be described within the framework of another, more comprehensive theory called New Physics. 

For over 10 years, physicists working at the LHC have been intensively searching for new phenomena that cannot be described by the Standard Model. Until recently, these attempts have been unsuccessful. The analysis with the use of this model seems to be now disturbed by the behaviour of leptons in the processes connected with some selected decays of beauty quarks: b sl+ l-, where s denotes a strange quark, and l+ denotes a charged lepton. A diagram representing this type of process is shown below:  


A fundamental assumption of the Standard Model is the universality of interactions between leptons and intermediate bosons (carriers of electroweak interactions). If lepton universality is observed, any processes leading to the production of final states that differ in lepton flavour (or type, in other words) should happen in exactly the same way. The LHCb experiment carried out an analysis of two decay channels of a beauty meson B+B+  K+ e+ e- and B+  K+ μ+ μ-, and the rate of both decays was determined. In the case where lepton universality is observed, the rate should equal 1. Measurements performed by the team working at the LHCb experiment obtained the following value:  



It means that decays into final states containing electrons are favoured over final states containing muons. It is the first observation of this kind that undermines the Standard Model. The significance of this observation is at the level of 3.1 σ, which means that it is not yet possible to announce a definite discovery, although the observed probability of consistency with the Standard Model is about 0.1%. New data, which will unambiguously determine the discovery, will start to be collected by the experiment in 2022. 

More information can be found on the website of CERN’s LHCb experiment.  

Professor Tomasz Szumlak, 
Head of the Department of Particle Interactions and Detection Techniques at the Faculty of Physics and Applied Computer Science