ResearchThe Symmetry Between Electrons and Muons Looks “Shaky”

7 May 2021

Experimental hints of possible new physical phenomena beyond the Standard Model

Using data acquired on the worldwide biggest and strongest particle accelerator, the Large Hadron Collider (LHC) at the CERN European particle physics laboratory in Geneva (Switzerland), scientists involved in the LHCb experiment have found hints of possible new physical phenomena. There are more and more signs that the standard model of particle physics – the current theory of particle physics that describes the behaviour of all forces and particles in the universe – can possibly no longer explain all measurements. The researchers of the LHCb collaboration presented their measurement regarding muon particles at an international conference for particle physics and a seminar at CERN. Scientists from Heidelberg University, RWTH Aachen University and TU Dortmund University are significantly involved in these analyses.

The Standard Model of particle physics predicts that the muon and tau particles are heavy copies of the electron; accordingly, they should behave just like the electron itself. This identical behaviour of the particles, which all belong to the lepton group, is called lepton universality. With the LHCb detector the scientists are investigating whether the behaviour is really the same or whether there are slight deviations. They are focusing on B mesons, which are produced in large numbers when energy-rich protons collide at the LHC. However, these particles only exist for a few fractions of a second. When they decay, in extremely rare cases this also gives rise to B mesons, which then decay into either electrons or muons. The measurement that has just been presented was about the particle decay of B+ mesons.

The scientists have discovered that B+ mesons decay somewhat more often into electrons than into muons, although, theoretically, with such decay the two final states should come about equally often. The result indicates a violation of Lepton Universality. So far not enough data has accumulated for the researchers to be able to speak of a discovery. Dr Martino Borsato from the LHCb working group at Heidelberg University, who was one of the leaders of the analysis, assumes however – like his colleagues in Aachen and Dortmund – that the symmetry between electrons and muons is “shaky”. If it were possible to confirm the measurement with further data this would, says Dr Borsato, be a strong indication of new physical phenomena beyond the Standard Model. Prof. Dr Stephanie Hansmann-Menzemer, head of the Heidelberg LHCb working group, adds: “This new result is part of a series of measurements that together give a consistent picture. At the moment the data calls for explanations and models that go beyond the Standard Model.”

Meanwhile, physicists, engineers and technicians at Heidelberg University’s Institute for Physics are working to expand the tracking chambers and trigger system for the LHCb experiment, in order to meet the rising challenges in collecting data from the Large Hadron Collider at higher collision rates from 2022 onwards. The new data will make it possible to prove or disprove the violation of Lepton Universality. Besides the LHCb participation, scientists from the Faculty of Physics and Astronomy are collaborating on two other LHC experiments – ALICE and ATLAS. In the framework of ALICE, they are examining the properties of quark-gluon plasma, which possibly filled the whole universe shortly after the big bang. The ATLAS experiment, which in 2012 led to the discovery of the elementary particle known as the Higgs boson, is looking for indications of physical phenomena that do not fit the present models. With its central participation in three of the four big LHC experiments, Heidelberg University has a top position in Germany in exploring the structure of matter and understanding the development of our universe.