Re-organization proposal:

I believe it is better to describe each of the two detector concepts as complete entities in individual chapters. Based on what I have heard, there is consensus that this is the preferred organization from the technical point of view. The alternative is to collect all trackers from the two concepts into one chapter; collect all the calorimeters from the two concepts into another chapter; and so on. This appears to be motivated more by sociological worries than technical.
 
Thinking about the two concepts, I see their fundamental difference is the calorimeter. Once the calorimeter choice is made, the substantive differences follow. [And non-substantive decisions can end up being the same or different.] PFA wants large field to spread out particles and nothing between tracker and calorimeter for linkage. Dual readout calorimeter is more tolerant of additional dead material and allows the tradeoff of having the coil inside the calorimeter to save cost. The drift chamber tracker can function with a 2-T field but does not require it – this is one I would call non-substantive choice. In the attached document organization proposal, I also included some text that one might want to consider for the document to explain these technical as well as sociological points. Blue and indented text is this text, and black is comments.

Some general comments.

(1) We need a chapter on detector requirements. This should be at the level of resolution goals rather than how they are achieved. For example, talk about X% momentum resolution rather than magnet field strength and dimensions or measurement point resolution. Similarly, talk about energy resolution of Y% rather than the number of sampling layers. It is up to the detector concept chapters to make that translation. 

(2) Make sure that numbers within the document are internally consistent from one chapter to another, e.g. 3 T vs 3.5 T. And in cases where we offer an option to improve something, also include any negative impacts. For example, we say in one place to go from 10 x 10 mm2 to 20 x 20 mm2 calorimeter cells to reduce power. I believe that would increase the necessary dynamic range (which we have not quite satisfied even before the change). It is not that we need to have solved the problem already, but we need to include it so the pluses and minuses are all on the table. 

(3) Many of the sub-detector sections look like they are articles for NIM rather than part of the CEPC CDR. They sometimes trace the development path of the particular detector technology. I suggest that we focus on the one version that we think will go into the CEPC detector. They sometimes describe the test-beam DAQ system — this is irrelevant to the CDR and can be dropped. Some talk about performance improvements when the technology is already a large factor better than required for CEPC. Again, it should be dropped. 

(4) I think that in general offering two technology options for a detector sub-system is fine. There is also nothing wrong to offer one when there is nothing else comparable. In particular, the muon detector section has way too many options. I strongly prefer to keep only the RPC here because RPC is cheap and easily satisfies all the requirements while the other technology options solve problems we do not have, e.g. rate capability or position resolution. They are more expensive and some are unproven for the scale needed for CEPC. Including obviously non-competitive options in the CDR tells the reader we are incapable of simple decisions. Note that this is a Conceptual Design Report. It does not lock us into only those options that are listed in it. If another technology becomes more appealing in cost vs performance, we are free to adopt it in the eventual detector.

(5) Performance numbers should be based on simulation or even better on a realistic measurement, and not on speculation that it ought to be some number or other. One example is the scintillator bar readout of imaging calorimeter. These bars are 5 mm x 45 mm, with two orthogonal orientations. The “cell size” of 5 mm x 5 mm is applicable for MIP, but it does not apply for a shower. We need to evaluate properly the performance with 5 mm x 45 mm strips.