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MEG: Muon to Electron and Gamma

by A.M. Baldini and T. Mori

Submission summary

As Contributors: Alessandro MAssimo Baldini
Preprint link: scipost_202102_00022v4
Date accepted: 2021-06-08
Date submitted: 2021-05-21 18:25
Submitted by: Baldini, Alessandro MAssimo
Submitted to: SciPost Physics Proceedings
Proceedings issue: Review of Particle Physics at PSI
Academic field: Physics
  • High-Energy Physics - Experiment
Approach: Experimental


The possible existence of the muon to electron and gamma decay predicted by many new physics scenarios is investigated by stopping positive muons in a very thin target and measuring emitted photons and positrons with the best possible resolutions. Photons are measured by a 2.7 ton ultra pure liquid xenon detector while positron trajectories are measured in a specially designed gradient magnetic field by low-mass drift chambers and precisely timed by scintillation counters. A first phase of the experiment (MEG) ended in 2016, and excluded the existence of the decay with branching ratios larger than 4.2x10**(-13) (90% C.L.). This provides approximately 30 times stronger constraints on a variety of new physics models than previous experiments. In the second phase (MEG II), most of the detectors have been upgraded by adopting up-to-date technologies to improve the search sensitivity by another order of magnitude down to O(10**(-19) ). MEG II will pursue new physics beyond the Standard Model complementary to high energy collider experiments with a compatible or even higher sensitivity.

Published as SciPost Phys. Proc. 5, 019 (2021)

List of changes

Dear Editor and referees, here follows a list of answers to
your further suggestions.

Best regards
Alessandro and Toshi

1st referee

1. L45 It might be worth it to quote the BR (<<10^-50) predicted by the SM when the effect of neutrino mixing is considered….

- Ok, added a reference.

2. L47 maybe simply: “making this channel one of the most …” to avoid repeating “mu->eg” and “search”

- Ok, changed.

3. L71- “very good intrinsic energy resolution” and “good resolutions in position and timing”, it might be helpful for the reader if those could be quantified.

- We think that his paper cannot enter so much in detail. If we start quantifying resolutions
we will need to explain conditions etc. and general readers would never be able to appreciate them.

L82 “beam line, were stopped”-> remove comma?

- Ok, removed.

L99 “TC” not defined.

- Ok, changed to "positron spectrometer"

19.3 Toward the discovery: MEG II -> 19.3 Towards the discovery: MEG II
Maybe the authors could consider a more “conservative/explanatory” title :
The MEG upgrade: MEG II

- We prefer the present title

L132-153 again (sorry to insists with this) it might be helpful to quote the values of the improvement in time/energy/momentum resolution also for the reader to appreciate the amazing efforts you did to achieve those….

- As above, given the kind of paper and its limits we prefer to not enter into such issues here.

2nd referee

- These comments are all correct and were introduced

L36 "themselves" -> "itself"
L103 "RMD" - acronym not introduced. Eeither replace by "radiative muon decay" (preferred),
or introduced the acronym e.g. in line 55
L124 "Toward"-> "Towards" (typo)

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