This afternoon, March 22, we discussed a number of different physics topics; some very specific, others more general. Here are some rough notes. Anyone should feel to make corrections, add details or references, etc.
  1. Alternatives to symmetric pT cuts. It is well known that there are singularities in fixed-order perturbation theory for dijet rates with equal (or almost equal) cuts on the two jet transverse momenta [Frixione, Ridolfi]. This type of singularity also showed up recently in benchmarking NNLO codes for W/Z production, for use in the W mass determination, in a study Alessandro Vicini and Doreen Wackeroth have been organizing and Stefan Hoeche, Ye Li and Frank Petriello have been contributing to (among others). In this case the cuts are equal pT for the two leptons (one might be a neutrino). If one takes too large a slicing parameter in slicing approaches, the result doesn't quite match results computed with a subtraction formalism. Although the NNLO differences are small, and at this stage are very unlikely to influence selection cuts for the first W mass measurement, there is an issue here --- also for more generic measurements. To avoid this singularity, one could choose asymmetric pT cuts. However, this throws away a sizable portion of the Born term, reducing the accuracy of the theoretical calculation. Gavin Salam pointed out another approach, which they looked at in the LoopSim paper [Rubin, Salam, Sapeta], see Figure 11, the panel labeled HT2, which is basically the sum of the two jet pT's. This sum is less sensitive to IR radiation, because if it is pushes one jet pT up, to first order it pushes the other jet pT down. You can't only cut on the pT sum, because you need some cuts on individual particles to pass triggers; however you could combine a cut on the pT sum with a cut on the ratio of pT's, which effectively includes a cut on each pT, but which still shows less IR sensitivity. E.g. pT1 + pT2 > 60 GeV, 1/2 < pT1/pT2 < 2, implies that each pT is greater than 20 GeV (20,40 is as asymmetric as you are allowed to go). Gavin can show some slides on this while he is here if there is interest.
  2. Loops for BSM. Heidi Rzehak is one of the few of us doing loops in BSM (specifically in SUSY). We can have a discussion of the issues involved in loops for BSM at some point while she is in town.
  3. Anomalies in flavor physics. There are several anomalies in flavor physics at the 2-3.5 sigma level. We had a brief discussion with Uli Haisch about some of them: a) "P5-prime" in B --> K^(*)? mu mu at LHCB. There are possible issues with possible off-shell J/Psi -> mu mu contributions. b) Br(B->K e e)/Br(B->K mu mu) from LHCB. This measurement is 25% off (2.6 sigma) from lepton universality predictions. Actually it is done as a double ratio, normalizing by the J/Psi part of the spectrum, in order to cancel large differences in electron vs. muon efficiency at LHCB. It could be that a careful study of how QED radiation affects this double ratio is called for. c) epsilon-prime/epsilon [direct CP violation in K decays] is now off of the SM prediction, which changed due to a new lattice calculation [see e.g. Buras et al.]. Since Uli is giving a theory talk on flavor next week, the plan is to decide after that talk which topics might warrant follow-up discussion in our program.
  4. The W mass. The measurement of the W mass at ATLAS, CMS, and also LHCB probably warrants more discussion in the 3rd to 5th weeks of our program, when Alessandro Vicini is here. We had a very short discussion of exotic W decays to few body hadronic states, which have no missing transverse energy [see e.g. Mangano, Melia]. It's been proposed [Mangano?] to look for these modes in t tbar events and then reconstruct the W mass from the few hadronic states.
  5. Measuring sin^2theta_W at the LHC and PDF interplay. We should discuss this more after the talks by Arie Bodek and Doreen next Monday.
  6. Issues in alpha_s and PDF interplay. This discussion started as one about getting alpha_s from the top cross section measurement. It then moved to a discussion of alpha_s from non-PDF-dependent observables. For example, there are e+e- event shapes at the Z pole, where the very complete analyses of the thrust distribution by Abbate et al and more recently of the C parameter by Hoang et al. both get values of alpha_s(m_Z) around 0.112-0.113, considerably below the world average around 0.118. Perhaps it is too hard at the Z pole to get far enough away from the shape function region and the "hard 3 jet region" to avoid sensitivity to unknown nonperturbative effects. There are also lattice measurements; one group claims an accuracy about 3 times better than the other groups.
  7. Top mass measurements. How well do we know what mass is being measured by the experiments? Most parton showers miss subleading logs, but (usually) they are at least being matched to NLO cross sections -- which can distinguish the pole mass from the MS-bar mass. (Although it was stated that a recent Run I CMS top-mass analysis using leptonic distributions used only LO+shower simulations -- that should be forbidden for LHC Run II!) Does a 1 GeV systematic uncertainty on a 173 GeV mass really mean that we understand the effects of QCD radiation at the 0.5% level? No, said Gavin, the radiation only moves things by about 10 GeV, so we only need to understand it at the 10% level.
  8. Heavy quarks as partons. Fred Olness mentioned work on the ACOT and simplified-ACOT schemes (in the latter some heavy quark masses can be dropped). This discussion was partly in the context of top as a heavy flavor in LHC production of t tbar Higgs [Han, Westhoff] where a m-ACOT scheme is introduced. We could have more discussion of this later.
  9. Bottom-top interference contributions to the gluon-fusion Higgs cross-section. Although small, they are quite uncertain as they are only known completely to NLO. The Higgs-pT dependence of these terms does not resum like the main top-top contribution does, because the "vertex" is nonlocal until you are below the scale of the b mass, way below the Higgs mass. Some of these issues in large logs of the bottom quark mass have been addressed recently [Melnikov, Penin] and can be discussed starting next week.
  10. Matching high energy and 1/m_top expansions in the Higgs cross section. There was a short discussion of this by Fabrizio Caola and Vittorio del Duca, but a longer one would be useful too for this reporter... A related question from them was whether one could realistically see the finite top mass effects in the SM Higgs production cross sections in the 13 TeV data. It certainly can be at 100 TeV, but at 13 TeV? Does it depend on how easy it is to pick up big branching fractions that are buried in backgrounds a lot pT?