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 the Mu3e experiment aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (phase I) up to an ultimate SES of few ×10-16. Both experiments will be hosted at the Paul Scherrer Institut which 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 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]
The first case (Figure 1, left) refers to the direct searches that can be performed at energy frontiers where the new particles would be directly produced in the final state while the second case (Figure 1, right) indicates the indirect searches, typically performed at the precision and intensity frontiers and at which charged Lepton Flavour Violation (cLFV) searches belong too, where the contribution of the new particles would indirectly appear enhancing the probability of processes that otherwise
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 allows 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 ×10−16 [36] (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 sensitivity of few ×10−17(current upper limit B(μ Au → e Au) < 7 × 10−13 [38])
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