MEG: Muon to Electron and Gamma

General comments:

The article is very compact and reads very much like a conference report for experts in the field. To fit better
within the planned volume, the article should explain a bit more and provide the reasoning and motivations.

An example is the workshop series out of which MEG was proposed (is is nice to mention these workshops). What is missing to understand the MEG proposal better, is a reference to the earlier MEGA experiment. The reader would like to know the limitations of
earlier measurements to understand better the design concept of MEG.

Also the transition from MEG to MEG II is difficult to follow for an outsider: The text says explicitly (L114-116) that the MEG measurements were background dominated. Thus the naive reader would expect a discussion how the backgrounds could be reduced with MEG II. The detailed MEG II description
however starts with a reasoning on the rate and on the acceptance. (According to the limitations seen with MEG, a higher rate and a larger acceptance are only profitable if the background can be kept under control.)

Important design concepts like e.g. the gradient magnetic field should be explained better. Details which do not help in the conceptual understanding could be dropped (e.g. multi-GHz domino ring sampler chip).

The text makes an extensive and often unnecessary use of acronyms. To improve readability this should be avoided.

In view of these point I suggest that the authors work over the text again.

Figures: The two figures are probably taken from other publications - they are not referenced et al.

Detailed comments:

Title:
Isn't a more generic title more apropriate? e.g.:
"Search for the lepton flavor violating decay \mu->e\gamma"

L9:
Change to: "...existence of the lepton-flavor violating decays \mu->e\gamma..."

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.

L23: indicated -> suggested

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

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 measurment?

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 "continous muon beam" everywhere (makes it more readable).

L62, L63: DC -> continous

L64 Both MEG and MEGII experiments ->
Both, the MEG and the MEG II experiments were

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

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;

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".

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

L117 The dataset -> The MEG dataset

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.

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

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.

L130-149: Some remark must be added that the last 2 bullet point serve for background reduction.

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)