The quest for $$\mu \rightarrow e \gamma $$ μ → e γ and its experimental limiting factors at future high intensity muon beams

Abstract

The search for the lepton flavor violating decay μ + → e + γ will reach an unprecedented level of sensitivity within the next five years thanks to the MEG-II experiment. This experiment will take data at the Paul Scherrer Institut where continuous muon beams are delivered at a rate of about 10 8 muons per second. On the same time scale, accelerator upgrades are expected in various facilities, making it feasible to have continuous beams with an intensity of 10 9 or even 10 10 muons per second. We investigate the experimental limiting factors that will define the ultimate performances, and hence the sensitivity, in the search for μ + → e + γ with a continuous beam at these extremely high rates. We then consider some conceptual detector designs and evaluate the corresponding sensitivity as a function of the beam intensity.

Lepton flavour violation and neutrino masses

“…Radiative cLFV muon decays, µ + → e + γ, have been searched since the 1940's; the current bound on these decays is BR(µ → eγ)< 4.2 × 10 −13 , obtained by the MEG Collaboration at PSI [6]. In the future, MEG II is expected to improve the sensitivity to 6 × 10 −14 [7] (see also [8]). The three-body muon decay, µ + → e + e − e + also offers excellent prospects to look for cLFV.…”

Status and prospects of charged lepton flavor violation searches with the MEG-II experiment

“…Since a thick layer of plastic scintillator would deteriorate unacceptably the photon energy resolution and thin scintillating fibers cannot provide adequate timing resolution, a technological breakthrough is needed here. A possibile solution is given by a new generation of silicon detectors [7]. An estimate of the sensitivity reach has been made using Monte Carlo simulations [7] assuming as benchmark performances those summarized in Tab.…”

“…A possibile solution is given by a new generation of silicon detectors [7]. An estimate of the sensitivity reach has been made using Monte Carlo simulations [7] assuming as benchmark performances those summarized in Tab. 2.…”

Charged lepton flavour violation searches at the Paul Scherrer Institut: Status of the MEGII and Mu3e experiments

“…Muonic rare channels such as the µ + → e + γ decay, the µ + → e + e + e − decay and µ − N → e − N conversion in nuclei are the most promising and complementary cLFV processes (often referred to as "golden muonic channels" [1,[16][17][18][19][20]): (a) The tremendous muon beam intensities (already available: up to few ×10 8 µ/s (continuous, DC) [21,22], available soon: O(10 11 ) µ/s (pulsed) [23,24] and understudy: O(10 10 ) µ/s (DC) [25,26]), implying for huge statistical samples, together with ultimate performing detectors allow for astonishing muonic cLFV SES; (b) The combined phenomenological analysis of these three processes allows for discriminating the underlying operators generating a potential signal, given different process sensitivities to the different operators. Figure 2 shows the history of cLFV experiments based on the golden muonic channels.…”