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MEG: Muon to Electron and Gamma

by A.M. Baldini and T. Mori

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Submission summary

Authors (as registered SciPost users): Alessandro MAssimo Baldini
Submission information
Preprint Link: scipost_202102_00022v3  (pdf)
Date submitted: 2021-04-15 14:43
Submitted by: Baldini, Alessandro MAssimo
Submitted to: SciPost Physics Proceedings
Proceedings issue: Review of Particle Physics at PSI (PSI2020)
Ontological classification
Academic field: Physics
Specialties:
  • High-Energy Physics - Experiment
Approach: Experimental

Abstract

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.

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

Title:

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

L9:

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

3--Yes

L19-21:

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

sensitivity.

4--Yes

L23: indicated -> suggested

5--Yes

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)

8--yes

L47 "the" \mu-> e\gamma

9--yes

L48 access new physics -> to search for new physics

10--yes

L54 background in a -> background for the

11--yes

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

12-OK

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).

13--Yes

L62, L63: DC -> continuous

14-Yes

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?

19--Yes

L99 decay-> decays

20--Yes

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 ?

22--Yes

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.

24--Yes

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 concept.....at 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.

30--Yes

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

Current status:
Has been resubmitted

Reports on this Submission

Anonymous Report 2 on 2021-5-15 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:scipost_202102_00022v3, delivered 2021-05-15, doi: 10.21468/SciPost.Report.2923

Report

The very good comments given by reviewer 1 were properly implemented by the authors and the article is ready for publication.
Below few suggestions the authors might want to consider prior to publication.

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

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

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.

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

L99 “TC” not defined.

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

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….

  • validity: good
  • significance: good
  • originality: ok
  • clarity: good
  • formatting: good
  • grammar: good

Author:  Alessandro MAssimo Baldini  on 2021-05-21  [id 1453]

(in reply to Report 2 on 2021-05-15)
Category:
answer to question

Dear referee, here follows a list of answers to your further suggestions.

Best regards Alessandro and Toshi

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.

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

  • Ok, changed.

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.

Author:  Alessandro MAssimo Baldini  on 2021-05-21  [id 1452]

(in reply to Report 2 on 2021-05-15)
Category:
answer to question

Dear referee, here follows a list of answers to your further suggestions.

Best regards Alessandro and Toshi

1st referee

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.

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

  • Ok, changed.

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.

Author:  Alessandro MAssimo Baldini  on 2021-05-21  [id 1451]

(in reply to Report 2 on 2021-05-15)
Category:
answer to question

Dear referee, here follows a list of answers to your suggestions.

Best regards

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.

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

  • Ok, changed.

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.

Anonymous Report 1 on 2021-5-13 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:scipost_202102_00022v3, delivered 2021-05-13, doi: 10.21468/SciPost.Report.2912

Report

The article is ready for publication. However there are very few typos which should be removed;

Maine text:
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)

  • validity: -
  • significance: -
  • originality: -
  • clarity: -
  • formatting: -
  • grammar: -

Author:  Alessandro MAssimo Baldini  on 2021-05-21  [id 1454]

(in reply to Report 1 on 2021-05-13)
Category:
answer to question

Dear referee,

your 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)

Best regards
Alessandro and Toshi

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