From FRASCATI@axcdfq.fnal.gov Fri Oct 20 12:40:30 2000 Date: Wed, 18 Oct 2000 14:08:47 -0500 From: FRASCATI@axcdfq.fnal.gov To: velev@fnal.gov, v0565@fnal.gov, rebcdf@fnal.gov, frisch@fnal.gov, rlc@fnal.gov, frascati@axcdfq.fnal.gov, goshaw@fnal.gov, bed@fnal.gov Subject: comments on cdf 5450 Dear Authors and GP's, I've read the "no new physics" PRD, CDF5450. It's greatly to the authors' credit that they went from evidence for new physics to no evidence and just limits. However, the paper still needs a lot of editorial work, and contains unexpected prickly surprises in many places. In sections 2 and 3, the reader is bombarded with a lot of generic information, but the basic facts and numbers are missing. It is a long paper, and still has less useful details than a PRL. As examples: 1) there is no mention of how many photon +jet events we select 2) the b-tagger is referenced as JETVTX, but I believe it is SECVTX. It is mentioned its usage in the top discovery; however, is it really the same tagger ? Do we know that this time it is used correctly ? It was not used correctly at the time of the evidence for new physics (CDFNEWS 10131). 3) mistags - it is not clear how they are calculated (the reference is outdated). Actually, mistags are approximated with negative tags, which is not correct. It is not clear which parametrization of the negative tags is used. In section 3.1 is mentioned that the sample has 197 negative and 312 predicted (how?). The top cross section PRD shows that CDF predicts correctly the rate of negative tags in the photon sample. Can one explain how it is possible ? 5) W/gamma and Z/gamma backgrounds are negligible, but it should be explained in more detail how they are evaluated 6) it is surprising that there are tagged photons. To say that they are removed is does not seem good enough 7) no mention of QFL, and its problems, as track degradation or tagging efficiency scale factors. What is it done exactly in this analysis ? 8) the evaluation of the rate of fake photons should be supported by plots to convince the average CDF collaborator first, and then the average reader. After all, this analysis does not use the standard CDF definition of isolated photon. Now, a more substancial point; the background is evaluated using the equivalent of Method II for W+jet events. However, here one uses a simulation of gamma+heavy flavor which is not documented. This is especially bad because the calculation is done by one of the paper's authors. The matrix element has been published ? The implementation of it into a simulation is public ? Who checked it in the collaboration ? In addition the systematic error of the calculation has to be determined correctly and not by handwaving; scaling Q^2 by a factor two is arbitrary. An error of 30% on the Mangano calculation is also arbitrary; for example, this is the error he estimates on its calculation of the bbar production cross section, but the calculation is different from the data by more than a factor of two. I am surprised that the quark mass makes a 10% difference for jet with E_T larger than 30 GeV; ABSOLUTELY SURE ? Finally, one should not add a 20% error for unmeasured effects; why 20%, anyway ? Actually, I believe that the focus of the paper should be just on measuring the production cross section of gamma+ heavy flavor. However, if one insists in putting limits to supersymmetric particles, then one should use another sample (like conversion gammas with E_T > 10 GeV + jets) to calibrate the simulated gamma + heavy flavor production. Without this work, Sections 4 and higher are worthless since they rest on an unchecked calculation with an unknown error. As a detail, the background shown in Figures 1 and 2 is not calculated according to Table 3, and the justification for this in pg. 9 is at least not very clear. I do not understand the reason of singling out events with large missing E_T by run and event number since they are perfectly consistent with the background. Section 4.1 should be dropped. The five events with a large di-jet mass could potentially be more interesting; however, the background calculation is not accurate and, because of this, I see no point in singling out 5 events in Section 4.2. Section 4.3 lists the number of events as a function of the jet-multiplicity, where the additional jets have corrected E_T larger than 10 GeV. Can we show that we can model the data in some other sample ? if not, what is the value of this Section ? Does the Michelangelo calculation predict the jet multiplicity for gamma+ heavy flavor ? As a detail, why some ecpected number in Table 8 are negative ? In section 4.3 what is the point of mentioning the number of double tags, if we do not know the background ? In the same section, what is the purpose of Table 9 ? As said before, Section 5 does not serve any purpose if we don't have a defensible background estimate. Section 6; I do not understand what we gain in this section with respect to the pure list of the rate of observed events. The acceptance may be ease to be evaluated by model builders, epsilon not for sure. At least, the title of the section is not correct: it is a limit on cross section times detector acceptance. Why the uncertainty on the luminosity is quoted to be 8% in Section 6.1 ? Why Section 8 has to be in a CDF publication ? It could easily be a rapid communication of the people who did this work. I was puzzled that in Section 8.1 one efficiency is quoted as a private communication of one of the author of the paper. regards, Paolo Giromini ps: I could not find any comment on the web (where are they ? )