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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_00022v3
Date submitted: 2021-04-15 14:43
Submitted by: Baldini, Alessandro MAssimo
Submitted to: SciPost Physics Proceedings
Proceedings issue: PSI Particle Physics
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.

Current status:
Editor-in-charge assigned

List of changes

Here is a list of the changes we made in response to the referee's comments:

1---No, this is just a matter of taste


Isn't a more generic title more appropriate? e.g.:

"Search for the lepton flavor violating decay \mu->e\gamma"

2--Again No, just a matter of taste


Change to: "...existence of the lepton-flavor violating decays \mu->e\gamma..."



Sentence is grammatically incorrect. Moreover it is from the phyisics point of view also not fully correct:

complementary measurements cannot have compatible sensitivities as they are looking for different effects.

I therefore suggest: MEG II will perform a search for physics beyond the Standard Model, complementary

to the high-energy collider experiments and with compatible or even higher mass



L23: indicated -> suggested


L33: This -> This workshop series; Letter of Intent -> Letter of Intent for a new experiment

6--Yes, we would add at the end: "At the time the best upper limit was 1.2*10**-11[reference]"
But we do NOT need to guess what was the limitation of that experiment. (We don't have to point to any of their shortcomings.)

L37: The reader is interested to know the pre-MEG limit. Can you provide this number with a reference? What was the limitation of this measurement?

7--yes (LFV couplings are generally expected, but they may not be sizable in some scenarios.)

L41 induce -> could induce (I don't think that there MUST be LFV couplings, e.g in minimal flavor violating SUSY scenarios have Standard Model like couplings)


L47 "the" \mu-> e\gamma


L48 access new physics -> to search for new physics


L54 background in a -> background for the


L55 and a photon "either" from "a" radiative muon decay or "from" the


L60 and throughout the paper:

I suggest to avoid the expression "DC muon beam" and instead use "continuous muon beam" everywhere (makes it more readable).


L62, L63: DC -> continuous


L64 Both MEG and MEGII experiments ->

Both, the MEG and the MEG II experiments were

15-Yes: We would modify the sentence to: "with a gradient magnetic field, called COBRA (COnstant Bending RAdius), was designed
to avoid positrons piling up in the central part of the tracker as well as to discriminate absolute momenta of positrons,"

L76-77: gradient magnetic field / COBRA: explain better the advantage of this design.

18--No. We want to show that having various ways of constantly monitoring and calibrating the detectors is essential to the experiment.

L91-99: Gamma calibration: paragraph is difficult to follow. Are the details really necessary in the context of this article?


L99 decay-> decays


L101 relative direction -> opening angle; emission time -> difference of their emission times;

21--No. The blind area is defined in two dimensions only, covering almost all the areas in Fig 19.1, and we do not think it would add anything to the Fig.

L109 and Fig 19.1: can the blind area be indicated ?


L114-116: could you be a bit more specific? e.g.: "limits would have improved only with square root of time".

23--Yes. We would add such a sentence there.

L116: with an upgraded detector -> with an upgraded detector able to reduce the background further.


L117 The dataset -> The MEG dataset

25--No. This is a document NOT about LFV searches BUT about the MEG & MEG II experiments. It is important to show these experiments have capabilities to
look for new light particles like axions as well.

L117-122: A complete new topic (axion search) is addressed here. It might be better to drop this paragraph to have more space to explain MEG MEG II a bit more in detail.

26--No, we think the text is clear with the reference [10].

L119-120: "mass range of the axion-like particle X" - the text refers here w/o any introduction to ALPs.

27--No. The background in MEG was adequate, not a problem. We just run the MEG at the optimized beam rate matched to the detector resolutions.
For MEG II, we improved the detectors so that we can use the highest beam rate available at PSI.

Perhaps we may rephrase the sentences to be more understandable - something like:
"The basic concept of the upgraded MEG experiment - MEG II - is to improve the detector resolutions everywhere so that it can run at the highest muon intensity
available at PSI without suffering a high rate of the accidental background: MEG had to reduce the muon intensity for stable detector operation, and
to keep the accidental background at a sufficiently low level. A significant improvement of the detector resolutions enables the higher muon stopping rate
with a similar level of the background as MEG, and, together with the improved detector efficiency, can achieve an order of magnitude higher sensitivity
than MEG."

L124-125: "Basic PSI"

-> as it was pointed out, the problem of MEG was the high background, thus the main target of MEG II is to reduce the backgrounds to make the high muon rates at PSI usable for the measurement.

28--The last bullet "Background suppression" really refers to things that suppress sources of background. But the bullet "Improved detector resolutions"
do not just reduce background. Obviously we need to increase the beam rate to reach higher sensitivity - we need to observe more muons. For a given
background rate that you can survive, you can increase the beam rate by improving your detector resolutions. So "improved detector resolutions"
actually help to increase the beam rate with the same background rate. We think this point is well addressed in 27 above.
L130-149: Some remark must be added that the last 2 bullet point serve for background reduction.

29--Non-uniformity means non-uniform resolutions, i.e. worse resolutions. A well known fact for calorimetry.

Maybe: "better photon resolution with more uniform calorimeter responses by using SiPMs instead of PMTs"

L143: "better photon resolution with more uniform light collection by SiPMs" -> can you work out the connection between the two parts of the sentence.


L172 new physics -> physics; (physics beyond the SM is always new)

Submission & Refereeing History

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Resubmission scipost_202102_00022v3 on 15 April 2021
Resubmission scipost_202102_00022v2 on 31 March 2021

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