Questions:

1). I notice in Table 2 that FastSim and Pythia are used for the ttbar, Z+jets and W+jets backgrounds. Since these are the backgrounds where the fake lepton effects dominate, I think they are the ones that most need to be studied with MadGraph and FullSim. Although general comparisons have been made between FastSim and FullSim, the fake leptons are dependent on the tails of distributions, which is where results will be most sensitive to simulation. I would expect that the current samples will underestimate the background from fakes, which will make the claimed sensitivity overly optimistic and could substantially change the result.

2). How are the error bars determined for figures 3-6? I think they are not calculated correctly, e.g., because one high pT bin in Fig. 3 has efficiency=1 with no visible uncertainty.

3). Section 10 describes how one might measure the lepton identification efficiency, particularly as a function of pT. But, it is not clear how you do that for this particular result. What efficiency is used to calculate the sensitivity, i.e., the limit and significance plots? What efficiency errors are used for that? From line 339, I think that a 1% uncertainty is used. But, Figures 3-6 and the discussion in section 10 lead me to expect a larger uncertainty.

4). The H/E cut in table 3 could be sensitive to pile-up because a cut of H/E<0.016 corresponds to less than a GeV for low pT electrons. How have you studied the effect of pile-up on the sensitivity?

5). I don't see that WW+jets has been included in the background estimate. I'd imagine that it could be important since it could give a real e and mu and leave the second e only requiring loose cuts and the mass window. How have you calculated the WW background?

6). The conclusion of section 8 is that the QCD background is small. However, that relies on the efficiency for the WZ mass cut that is determined from the highest pT QCD MC samples. Those high pT bins will have high HT and therefore I expect them to reconstruct mostly to higher WZ masses. As such, there is likely to be a higher efficiency for the WZ mass cut in the lower pT QCD MC samples. Since that is where most of the, pre-mass cut, QCD background events are, I expect that the quoted QCD background could be a substantial underestimate. How can you predict the WZ mass cut efficiency for the lower pT QCD MC samples?

7). Line 239 states that the Zgamma background "can be determined from data once the FSR Z signal is measured at CMS". But, how do you determine this background for the current estimate?

8). Section 9 states that the ttbar background might be measured in data using the fraction of b-tagged events. I expect that such a method will have limited applicability in this analysis since the fake lepton in a trimuon signature from ttbar will contain b-quarks that fragment mostly into a single lepton. That is a region of the fragmentation function that is not well modeled in the b-tagging efficiency.

9). The data-driven determination of isolation efficiency for fake leptons apparently does not include the WZ, WW, and ttbar samples. Those processes will contribute to actual use of this method in data. Why aren't they included here?

10). I'm puzzled by Tables 10 and 11. First, I would expect that the electron fake rate would be considerably higher than the muon fake rate (despite the 5GeV difference in the pT cut). Why is that not so? Second, I don't understand why B_{TT} is higher for muons than electrons (which is actually negative). Third, the errors quoted on P_{fake} are for 1/fb. Scaling this down to 300/pb, which is relevant for the low mass pT limit, I expect a much larger error on P_{fake}; that could degrade the sensitivity. Are these errors propagated into the limit and sensitivity calculations?

11). I think that the conclusion that "it is possible to discover \rho_T up to masses of about 300 GeV...using ~500/pb" is not correct. That luminosity gives only a 3 sigma significance, which is not sufficient for a discovery. Furthermore, the 313/pb prediction for a 95% CL limit could vary up to 600/pb due to the stated cross-section uncertainty. Taken together, I think the general tone of the conclusion should be that there is only limited sensitivity with 10TeV, i.e., a much weaker conclusion than is stated.

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Topic revision: r1 - 2009-06-18 - TulikaBose
 
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