Measurement of the hadronic contribution to the muon g-2 with the MUonE experiment at CERN

The muon anomalous magnetic moment (muon g-2) represents one of the most intriguing observables to test the validity of the Standard Model (SM), as a long standing discrepancy between theoretical and experimental values persists since more than 20 years. The precision on the muon g-2 SM prediction is limited by the knowledge of the hadronic vacuum polarization (HVP) contribution, which has been computed so far using lattice QCD techniques or a data driven approach based on e+e- data taken at colliders. At present, these two methods lead to different results, and tensions are preventing from a solid comparison between theory and experiment.

The MUonE project aims at determining the muon g-2 HVP using an innovative approach, thus contributing to shed light on the muon g-2 puzzle. The new method proposed by MUonE is based on the measurement of the hadronic running of the electromagnetic coupling in the space-like region, which will be extracted from the shape of the differential cross section of the muon-electron elastic scattering. The experiment is carried out at the M2 beam line at CERN North Area, which provides 160 GeV muons with a maximum in-spill rate of 50 MHz. Muons will collide on a Be (or C) fixed target, and interactions with the atomic electrons will be detected by a tracking system instrumented with Si strip sensors. The final apparatus will be composed of a repetition of 40 target+tracker units, called stations. The equipment is completed by a calorimeter and a muon filter placed downstream of the stations, and a spectrometer (Beam Momentum Spectrometer, BMS) placed upstream. The experiment poses several challenges, both on the hardware and DAQ system, given the high rate beam conditions, as well as on the data analysis: in order to be competitive with the current evaluations, the systematic uncertainty on the measurement of the differential cross section must be kept at the level of 10ppm, an unprecedented level for a scattering experiment.

 

MUonE has recently submitted an experiment proposal to the CERN SPS Committee to run with a small scale version of the apparatus. Data taking will occur in the first half of 2025, and will be of great importance to study systematic errors and background events. 2025 data will also serve as a basis for the full scale experiment proposal, to be prepared during the CERN Long Shutdown 3 (2027-2029), aimed at taking data after 2030.

 

The University of Liverpool is deeply involved in many aspects of data analysis, and is in charge of the BMS mechanics and measurement of the beam momentum, which is one of the main sources of systematic uncertainty. The student will contribute to the analysis of elastic scattering events in 2025 data, as well as to the event-by-event measurement of the beam momentum and its further development in view of the final experiment.

Data analysis will be accompanied by a simulation work aimed at optimizing the detector design for the final version of the experiment, as well as participation in dedicated beam tests at CERN and/or other facilities to prove simulations outcome in realistic conditions. Student’s involvement in the different topics can be balanced based on their interests. The research activity will be included in the full scale experiment proposal.

 

 

Back to: Department of Physics