Chapter 1 - Introduction

1. Physics reasons for low trigger thresholds for Run 3 have to be explained. 2. Reasons why GEM Phase II upgrade has to be installed in LS2 have to be summarized. 3. Fig. 1.2: a) Why stand-alone trigger rates matter? In Run 3 CMS will have central tracker trigger which might or might not benefit from extra GEM chambers. b) Add rate of the real muons (mainly from light quark decays, b/c quarks as well as W/Z) to this plot to indicate what are physics driven limits for the trigger rates. c) Why extra chambers provide only ~30% improvements in the trigger rates at ~7 GeV while ~10 times at 25 GeV? d) What is planned L1 rate in Run 3? Total and for the muon triggers. 4. Try to escape CMS jargon in presentations of the TDR, especially abbreviations, as it makes hard for the referees to understand the proposal.

"GEM Chambers" Chapter 2.

1. How optimization of the gas mixture is done? Only two options are presented and it looks as one of them is not viable due to greenhouse gas concerns. 2. What is GEM chamber discharge rate? 3. Are you planning to repeat long term stability and aging tests with final version of the chamber assembly? 4. Selected pitch of the sensitive electrodes provides resolution more than twice better of what is required. Why not to increase pitch (and size of the pick-up electrodes)? Doubling pitch will simplify production and reduce cost of on-chamber electronics by about a factor of two (as well as reduce costs of the power supplies, etc.). 5. Did you consider replacement of CF4 with small amount of CH4 which is a good quencher, fast gas, and in small % amounts not flammable? 6. Did you consider option of using CSC vs GEM? Fluxes after Phase II upgrade in the region of these chambers are reasonable, so even typical wire based detector should work.

Chapter 9 - Organization, responsibilities, planning and costs

1. Fig. 9.1. How "GEM collaboration" is interacting with CMS collaboration, especially with Phase 2 CMS wide upgrade management, resource management, Technical Coordinator, etc.? 2. It is hard to clearly identify roles of the Chairperson and Project Manager. Even charge for them is "mixed" in the paragraph 9.3. Could you split and define responsibilities for each of these two management positions separately? 3. There are a few items in the task matrix (Fig. 9.2) where few groups are involved, like single group in the hardware alignment. What are risks associated with such assignments? 4. What is happening during a year from 6/2017 to 6/2018 after all chambers are ready, but not yet installed (Table 9.1)? 5. Provide detailed cost estimates for a subset of main items to understand how major costs are actually determined. This should include cost estimates from companies, etc. Concentrate on the main items, such as chambers components, electronics, power supplies (including HV), etc. 6. Provide expected needs in FTEs from the collaborating institutions (not accounted in the core cost estimate) vs year required to accomplish the project by main categories: scientist (including students), technician, engineer.

Chapter 3 and 4 o What is the advantage of running the VFAT3 in "lossless" mode compared to running in SPZS mode? o Timing resolution will be critical. What is the expecting timing dispersion per VFAT3 chip and per superchamber? o For a prototype chamber, 97% of the hits are associated with the correct bunch. What does this do to the fake trigger rate? That is, how sensitive is the trigger rate to this fraction? Suppose it were 95%, what would happen? o What inspection will be done on the GEM foils itself, beside tensile strength? o What is the critical path for the production? o Regarding the slice test (appendix A): given that the final VFAT and GBT will not be available by summer 2016, what running information will not be provided by the slice test and how will that information be obtained?

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