Abstract
The MEG experiment has recently set a new upper limit on the branching ratio of the µ+ → e+γ decay, ℬ(µ+ → e+γ) < 4.2 × 10−13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV µ+ → e+e+e− decay with a completely new apparatus, the Mu3e experiment, aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (Mu3e phase I). An ultimate SES of few ×10−16 is foreseen requiring 109 µ/s (Mu3e phase II). Both experiments will be hosted at the Paul Scherrer Institut which currently delivers the most intense continuous low energy muon beam in the world up to few ×108 µ/s. The status of both the MEGII and Mu3e phase I experiments is given.
Highlights
Lepton flavour violation (LFV) research is currently one of the most exciting branches of particle physics due to its high sensitivity to unveil New Physics (NP) [1]
(i.e. Standard Model (SM) background free) an evidence of such a decays would be a clear signature of NP and charged Lepton Flavour Violation (cLFV) searches turn out to be ideal probes for NP hints
Following the approach of the effective lagrangian and assuming NP natural coupling the current upper limits on muon cLFV processes translates in new energy scale limits Λ > O(100) TeV, independently of the detailed form of the operator responsible for the cLFV process [14, 15]
Summary
Lepton flavour violation (LFV) research is currently one of the most exciting branches of particle physics due to its high sensitivity to unveil New Physics (NP) [1]. Following the approach of the effective lagrangian and assuming NP natural coupling the current upper limits on muon cLFV processes translates in new energy scale limits Λ > O(100) TeV, independently of the detailed form of the operator responsible for the cLFV process [14, 15] 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 ×108 μ/s (continuous, DC) [21, 22], available soon: O(1011) μ/s (pulsed) [23, 24] and understudy: O(1010) 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 allow for discriminating the underlying operators generating a potential signal, given different process sensitivities to the different operators. Following the mentioned complementary approach the Mu3e experiment at PSI will search for the μ+ → e+e+e− decay aiming at a sensitivity of a few ×10−15 [36] (Mu3e phase I) and an ultimate sensitivity of a few ×10−16 (current upper limit B(μ+ → e+e+e−)< 1.0 × 10−12 [37]), and COMET [23] in Japan and Mu2e [24] in US will search for the μ−N → e−N conversion aiming at final sensitivities of few ×10−17(current upper limit B(μ Au → e Au) < 7 × 10−13 [38])
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