Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

Marco Chiappini,Satoru Kobayashi,Manuel Meucci,Marco Francesconi,Rina Onda,Patrick Schwendimann

doi:10.3390/universe7120466

Abstract

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

Highlights

For more than half a century, the search for Charged Lepton Flavour Violation (CLFV) processes provided important clues towards our current understanding of the Standard Model (SM) of particle physics [1]
The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7 × 107 μ+/s)
This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years

Summary

Introduction

For more than half a century, the search for Charged Lepton Flavour Violation (CLFV) processes provided important clues towards our current understanding of the Standard Model (SM) of particle physics [1]. Starting as early as 1947, the first upper limit of the muon decay into an electron and a photon (what we call μ → eγ today) was established by Hincks and Pontecorvo [2]; it opened the way for the introduction of the neutrino and later of lepton flavour as conserved quantity. The observation of neutrino oscillations [3] exposed the empirical and approximate nature of the lepton flavour symmetry, but they produce extremely small Branching Ratios (BR) for their charged counterparts in the SM ( 10−50), way beyond experimental sensitivities [4,5]. The μ+ → e+γ process is still very sensitive to new physics, with a current upper limit on the BR of 4.2 × 10−13 at 90% confidence level set by the MEG experiment [8] at the Paul Scherrer Institut (PSI). The μ+ → e+γ search with the MEG experiment upgrade, MEG II, in terms of new physics reach, is competitive with the new generation of CLFV experiments such as Mu3e [10], Mu2e [11], COMET [12] and DeeMe [13]

Methods

Results

Conclusion