The MUonE project

(MUon ON Electron elastic scattering)

The MUonE experiment aims at an independent and precise determination of the leading hadronic contribution to the muon anomalous magnetic moment aμ=(gμ-2)/2, based on an alternative method, complementary to the existing ones.

The method needs the measurement of the effective electromagnetic coupling in the space-like region at low momentum transfer. This can be achieved by measuring with unprecedented precision the shape of the differential cross section of μ-e elastic scattering, using the intense muon beam available at CERN, with energy of 160 GeV, off atomic electrons of a light target, as described in the original publication.

The muon anomalous magnetic moment is one of the most precisely measured quantities as well as one the most precisely calculable in the Standard Model, that’s why it constitutes a stringent test of the theory. For the last twenty years the BNL measurement has been pointing to a significant discrepancy from the theory prediction. Recently the first result from the FNAL g-2 experiment has confirmed the previous measurement, and their combination brings to 4.2σ the deviation from the currently accepted SM prediction.

There are hypotheses trying to explain this deviation as a quantum effect of new exotic particles, however it is still possible that it result from a systematic error in the theoretical calculation. The dominant theory uncertainty is related to the hadronic contribution to the vacuum polarization, which is not calculable in perturbation theory. Its leading order contribution (LO-HVP) is usually determined by a data-driven dispersive approach, using the measurements of low-energy hadron production cross section in e+e annihilation. In contrast, a recent ab initio calculation of the LO-HVP contribution, based on Lattice QCD, weakens the discrepancy with the measurement, and is in tension with the data-driven estimate. More recently a new FNAL measurement from a larger dataset has confirmed the previous experimental result with higher precision. In the next few years the analysis of the full FNAL dataset is expected to further reduce the errors, while another forthcoming experiment at J-PARC should reach a similar precision. To clarify the situation it is then necessary to improve the theory calculations accordingly.

The MUonE project was conceived during the workshop “Physics Beyond Colliders”, held at CERN in late 2016, and the proposal was published in the European Physical Journal C soon after. The proposed apparatus is a sequence of detection stations, each one made of a passive element serving as target and active planes made of silicon microstrip detectors for tracking, with length of 1 meter and transverse dimensions of about 10 cm. The layout foresees an array of 40 such stations, followed by an electromagnetic calorimeter and a muon detector at the end.

Dedicated beam tests were carried out at CERN. In 2017 a first beam test was devoted to the study of multiple scattering on the material, which affects the observable angles of the measured tracks. Its results have been published in the Journal of Instrumentation. In 2018 a second test was carried out aimed to study the selection of elastic events and is also published in Journal of Instrumentation. In June 2019 the MUonE project was submitted for approval to the CERN SPS Committee with the  Letter-of-Intent. The basic unit chosen for the detector is the module developed by the CMS collaboration for the upgrade of their Outer Tracker. These sensors, together with their trigger and readout electronics, are found to have adequate specifics.

CERN has approved a first Test Run, which has been carried out with a couple of tracking stations and a prototype calorimeter in August-September 2023. Continuous readout at 40 MHz has been achieved for long runs, recording 350 TB of raw data, providing about 108 candidate elastic scattering events. The data analysis is on-going.