Search for Muon-to-Electron Conversion with the COMET Experiment^†

Round 1

Reviewer 1 Report

Dear Authors,

         I found the paper clear, complete and well written. I only have a few questions to be answered and some minor suggestions to be implemented before the publication.

line 6: Please define the quantity the single event sensitivity refers to (i.e. the ratio of mu-e conversion to capture).

line 43-46: The latest developments in EFT, see for instance A. Crivellin et. al, JHEP 05 (2017) 117, show that the sensitivity of mu -> e gamma to contact terms through loops is not negligible and can even give the best limits in some specific scenarios. You should rephrase to take it into account.

line 108-111: The sentence is quite convoluted, please rephrase.

Sec. 2.3.1.: The role of the calorimeter for the muon conversion search in the Phase-II experiment should be better specified.

line 263-266: Please specify the wire diameters. It would be also interesting to know the material and thickness of the inner wall of the CDC.

line 271-279: Depending on the inner wall thickness, I expect a fraction of these X-rays to enter the CDC and produce photoelectrons, although I acknowledge that at 347 keV the cross section is quite small. Do you have an estimate of the x-ray-induced background in the CDC, and how much it could affect the reconstruction performances?

Sec. 3: I suggest to indicate the contribution of the 103.6-106.0 MeV energy cut to the signal efficiency.

line 380: "should be taken care OF"

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper summarizes the current status of the COMET experiment, which will search for the charged lepton flavor violating process of muon-to-electron conversion.

The article gives a very good summary of the COMET experiment including motivation for the experiment, a detailed description of its design and construction, an updated background and sensitivity estimate, a comparison with Mu2e, and future prospects. I look forward to seeing their future results.

I have just two specific comments:

L.332 and Table 2: It is unclear to me why the cosmic rays and delayed antiproton backgrounds will increase so much between Phase-I and Phase-II. For cosmic rays, I would have naivly thought it scaled with the DAQ livetime. which only increases by a factor of 1.7. I think a short sentence briefly explaining why cosmic rays and delayed antiprotons are expected to dominate in Phase-II would be helpful to the reader.

L.346: Should the improved sensitivity be 10^{-18} and not 10^{18}?

Author Response

Dear Reviewer 2,

I would like to thank you for careful reading and precious comments on my paper manuscript. I have carefully considered response and updated the manuscript. Please also find the attached Texdiff file for a comparison with the previous version.

Line 332 and Table 2: 
 That is a good point. For the cosmic-ray background, in addition to increase of the DAQ live time, we have to cover much larger area by veto counters including not only the Detector Solenoid but also the Electron Spectrometer Solenoid and the stopping target solenoid in Phase-II. For the delayed-antiproton background, it is in principle increase as the muon beam increases. We plan to optimize the design of vacuum windows in the Transport Solenoid to suppress the antiproton. 
 I have added short explanations; see in line 340-345 in the updated version or the texdiff file.

Line 346: 
Corrected to 10^{-18}.

 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.