A new proposed project, MUonE, was described on the EP Newsletter few months ago [2].  It is an experiment aiming at helping in understanding the long-standing discrepancy between the experimental value and the Standard Model (SM) prediction of the anomalous magnetic moment of the muon, aµ, which remains one of fundamental parameters in Quantum Field Theory that still lacks an explanation.

The discrepancy could be due to the presence of new physics, or to a lack of precision in the determination  of the expected SM value or perhaps to a lack of precision in experimental measurements. During 2020, a new experimental result is expected from the new generation of (g-2) experiments at Fermilab (USA).

Since June 2019, the MUonE project made progress : the Letter of Intent [1] was submitted to the concerned committee (the SPSC), which, at their January 2020 meeting,  took into consideration the request of a test run in 2021 with the muon beam M2. The aim of of the test is demonstrating that the proposed measurement is feasible with the planned precision. The challenge is all in the systematic uncertainty. The requested test run was approved by the CERN Research Board.

The MUonE international collaboration is still forming, and at present there are groups from China, Italy, Poland, Russia, UK, and USA. All the efforts of the collaboration are now addressed to prepare this test run, foreseen at the end of 2021 and expected to last  for 3 weeks.

The test must provide answers to several crucial questions, some of the most important are: i) is it possible to operate in an efficient way the setup in order to collect efficiently elastic scattering data μ+e → μ +e at a very high rate (the beam will have ~ 50 MHz);  ii) which systematic uncertainty is achievable with the planned detector; iii) which trigger, acquisition and volume of data will need to be managed.    

In figure 1 the scheme of the layout of the test is shown. It consists of two of the 40 stations foreseen for the final apparatus: each station is  composed of a target  (T1,T2) and 3 planes of Si trackers. The 3 planes before T1 are dedicated to precisely measure the direction of the incoming muon track.

A calorimeter at the end will be used to distinguish muons from electrons when they are emitted at a very close angle, i.e. both at about 2.5 mrad. In this kinematic region, the two particles have about the same energy of around 75 GeV each.

Figure 1: The two foreseen stations followed by a calorimeter and a muon filter (not to scale).

In three weeks of running we expect to collect  statistics of the order of 10⁸ elastic events. Such statistics  will not allow to extract the quantity aµHLO, but we plan to measure the αlep corrections to the running of αem, and that quantity will be taken as a test bench for the feasibility of the final measurement.

Moreover, the collaboration has a strong component of theorists, who are deeply committed to bring the precision of the theoretical calculations to the necessary precision, at NNLO. Beyond NNLO, the contribution to the systematics of the theoretical calculation will be of the order of 10^-6, adequate for the final precision of the MUonE measurement.

Within the theory community involved in the project there is sufficient expertise which has already provided a well definite and necessary solid ground of support for the MUonE experiment. Nevertheless new goals have still to be achieved to further assess the theoretical basis to the experiment and to reach the aimed accuracy.

A first step will be to obtain a fully differential Monte Carlo program containing  fully massive NLO electroweak contributions, fully massive at NNLO as well as the NNLO for hadronic contributions. In addition, the cross-section evaluation with a fixed-order calculation will be matched to a parton shower result by taking into account multiple photon emissions which will be, initially, at leading logarithmic accuracy [3].

Two different implementations of such a code are planned to be developed in order to appropriately check their compatibility. Furthermore a phenomenological analysis, in close collaboration between  experimentalists and theorists will investigate if the theory approach to the measurements  could be considered adequate. In this respect a careful error estimate of the missing terms will be crucial [1].

The MUonE project [2] has entered in a phase where the important demonstration of  the feasibility of a such a precise measurement must be provided, in the conditions we plan to run the final experiment (same beam, same location, same tracking elements).

New collaborators are welcome, and the challenge should be very interesting for young people interested to participate in an experiment through all its different stages, from the planning to the final measurement.

Further Reading     

[1] Letter of Intent, CERN-SPSC-2019-026 / SPSC-I-252 5/06/2019.

[2] G. Abbiendi, C.M. Carloni Calame, U. Marconi, C. Matteuzzi, G. Montagna, O. Nicrosini, M. Passera, F. Piccinini, R. Tenchini, L. Trentadue, G. Venanzoni, Eur. Phys. J. C 77 (2017) no.3, 139

[3] Report of the 2nd WorkStop/ThinkStart, 4-7 February 2019, University of Zurich (in preparation).