INFN-LNF Laboratori Nazionali di Frascati
Dr. Stefano Miscetti of the Frascati National Laboratories was recently elected new spokesperson of the Mu2e experiment at Fermilab (Batavia, USA) to work alongside Dr. Robert Bernstein of Fermilab and replace prof. Jim Miller of Boston University.
The Mu2e experiment (you can read it as “Mu-to-e”, i.e. transformation of a muon into an electron) seeks the direct conversion of a muon into an electron in the presence of an atomic nucleus, without associated production of neutrinos. This is one of the “Charged Lepton Flavor Violation” (CLFV) processes that is completely prohibited in the Standard Model (SM). Even assuming neutrino oscillation, the SM predicts conversion probabilities of about 10-52, making the event impossible to observe. On the other hand, many of the new physics models, from Supersymmetry to Leptoquarks, to Z’, and so on, predict higher conversion probabilities making these processes observable at a dedicated experiment. Direct observation of a CLFV process is therefore a clear indication of new physics. In this panorama, research in the muon sector plays a golden role both because it is possible to generate very intense muon beams, and because the process identification is simplified by the presence of only leptons in the final state.
Mu2e is currently under construction at the Fermilab’s Muon Campus (Batavia, USA), close to the Muon (g-2) experiment which is currently completing its data collection. Mu2e is part of an international effort for the research of CLFV processes with muons which includes a conversion experiment (COMET) under construction at JPARC (Japan) and experiments for the research of m→eγ decays (MEG-upgrade), in data collection, and m→3e under construction at PSI (Switzerland).
Mu2e aims of improving current discovery sensitivity by about a factor of 10000, by measuring conversion probabilities down to 10-16. This sensitivity would allow, even in the case of failed observation of the signal, to exclude, or put in serious difficulty, many of the new physics models, controlling mass scales that extend up to 1000 or 10000 TeV, well above what can be reached with direct measurements at high energy colliders.
The experimental technique and the Mu2e experiment
To improve the sensibility of the measurement by four orders of magnitude, the Mu2e experiment is based on four main elements: (i) a muon beam of high intensity (10 billion muons per second stopped on the aluminum target) in order to collect 1018 interactions in a few years of data collection, (ii) a pulsed beam with 1.7 msec period to eliminate contributions from fast decays related to pions, (iii) a large solenoid system, 25 meters long, to produce, transport and focus muons on the target and, finally, (iv) a detector dedicated to the observation of the candidate conversion electron which appears as a mono-energetic line (104.7 MeV) very close to the muon rest mass. The detector consists of a tracker with 20000 very light straw-tubes with momentum resolutions of 180 keV, a pure Cesium Iodide crystals calorimeter read with silicon photomultipliers (SiPMs) to distinguish electrons from muons, and a veto system based on scintillation counters to eliminate events produced by cosmic ray interactions that may resemble conversion events.
The cost of the experiment is about 300 M$ with the construction of the detectors started in 2016, after 5-6 years of research and development. The construction phase of the experiment is in a very advanced state, with the large solenoid system nearing completion and the detectors that will start the installation phase in the experimental room in 2024, followed by a subsequent commissioning phase in 2025. The first physics run with muon beams is expected in 2026.
The Mu2e collaboration is made up of approximately 240 people, 38 institutes and 6 participating nations (the United States, Italy, England, Germany, Russia and Cina).
The Italian contribution
The Italian component, made up of 30 members, of which about half of the LNF, plays an important role and constitutes about 15% of the collaboration. INFN has contributed and is still contributing in many parts of the experiment: initially with the construction of the first magnet prototype for the “Transport Solenoid”, created in collaboration between INFN Genoa and ASG Superconducting under the guidance of Dr. Pasquale Fabbricatore; then, with Dr. Miscetti as technical manager, INFN, with the sections of Frascati, Pisa, Lecce and Trieste, led the design, development of prototypes, construction of components and assembly of the electromagnetic calorimeter, for a direct contribution of about 3.5 MEuros. This detector, consisting of 1400 scintillating crystals of pure CsI read with 2800 SiPMs, is being assembled at Fermilab with the first of the two disks practically completed. The work of the LNF group was fundamental for the technological choice, the development of SiPMs on custom design, the test and qualification of crystals and photosensors, the design and construction of many mechanical components, the support mechanics and all the front-end electronics.
“I am truly honored to take on this role in such a delicate and important moment for the experiment”, comments Miscetti, “after more than 10 years of work and 7 years since the start of construction, we are finally seeing the end of the tunnel. The construction of the magnets is well advanced and the calorimeter, of which I am still the Project Leader, has been partially assembled.”
“Over the next two years, my main task will be to collaborate with Dr. Bernstein to guide the experiment into its commissioning phase and prepare it for the upcoming physics data collection. We will have to focus the work of the collaboration on the many details that will remain to be completed at the end of the construction of the experimental apparatus, i.e. make the data acquisition, trigger and data reconstruction programs operational and start optimizing the analysis programs.”
“I will encourage the young people of the experiment, regardless of their race, gender and nationality, to accept visibility roles with the task of making the experiment work well. In the medium and long term, PhD students and young researchers will be the ones who will allow Mu2e to be successful and to obtain physics results of the highest level”.
Stefano Miscetti
Dr. Miscetti is Research Director of the LNF. He began his activity in the CDF experiment at the Fermilab Tevatron, where he worked on hadronic calorimetry and proton-antiproton elastic scattering. He then actively collaborated in the KLOE experiment at DAFNE, where he was one of the leaders on the design, assembly, and calibration of the electromagnetic calorimeter. In KLOE, he carried out many data analysis and coordination roles up to covering the role of Technical Manager of the KLOE-2 experiment from 2006 to 2012. Since 2009 he has been Project Leader for the Mu2e electromagnetic calorimeter and since 2012 national manager of the Italian Mu2e collaboration. He has been a member of important international committees such as LHCC of CERN and the PAC of Fermilab. He was recently asked to join the Detector Advisory Committee for the Electron Ion Collider at BNL.