LHC-XS-PO kickoff EVO-meeting, 19/2/2010 Michael Duehrssen Martin Gruenewald Sven Heinemeyer Chiara Mariotti Giampiero Passarino Daniela Rebuzzi Michael Spira Reisaburo Tanaka Georg Weiglein Sven chairing, Giampiero minutazing Start at 16:00 -------------------------------------------------------------------------------- Introduction of the list of PO (point 1 in the agenda) -- Generalities on definition of PO, e.g. some preliminar question on what to do with QED+QCD FS radiation. One should allow for that since it is closer to what experiments measure but we have to check on what is done in TH calculations (e.g. in H --> gamma gamma). -- Preliminar discussion on the list of POs. Differences Lep - LHC are emphasized but one proposal is to create a full list (as done at Lep) separating the issue of what theory can compute from the use made in the experimental analysis. -- We seem to agree that the issue of POs does not depend on what LHC will be able to measure and products of POs (e.g. production XS times BRs) are also POs (i.e. POs form a semi-group). -- We agree that the correct TH definition of mass is the real part of the complex pole, i.e. it is this complex quantity we have to deal with. After a first round of opinions the discussion focuses on Higgs mass definition and use: -- We all agree on the fact that there are two different scenarios, light and heavy Higgs, measurements of the properties of a resonance and exclusion of an heavy SM Higgs. Not all problems are equally relevant for the different scenarios, and some can possibly neglected in one or the the other. Nevertheless it seems advisable to adopt one mass definition (which is theoretically clean) for all cases. -- As a consequence, we should check the definition in MC tools (hereafter MCT), e.g. in Pythia, and try to understand the connection with MCT, the interplay between invariant mass measurements and `correct' TH mass definition (complex pole), taking into account background extraction and signal/background interference. -- A proposal is made to select some `simple' bench mark where to study the issue in details, e.g. gamma gamma FS; alternatively ZZ, i.e. gg --> ZZ. Once we are able to make quantitative statements we could propose neglecting interference effects in all cases where it is tiny. -- At the same time make a list of what is available, with the idea of creating a `conversion table' between whatever is used in MCT and (*) the TH definition. -- A long discussion follows, separating the issue of what to do if a peak is observed and measured versus absence of signal and exclusion analysis. To summarize, given a peak there should be freedom enough for computing what we want. One option is also proposed, introducing and using different Pos for observation/exclusion. -- Besides gamma gamma we should look into WW --> llnunu and ZZ --> 4l. At this point a second phase of the discussion opens up, `complexity'. Also Ws and Zs, being unstable, require complex poles. We agree to proceed step-by-step along `complexity avenue' and one example is briefly discussed: in H --Z 4f the on-shell phase space for the WW or ZZ final state introduces an error of the order of 10% near threshold. -- It is clarified that a proper treatment of background/interference is mandatory for consistency of the whole approach, although a TH prediction of POs is, in principle, stand-alone. -- Once we realize that NLO MCs are QCD NLO and that most of what we are discussing refers to the EW component of the calculation, the question is raised on the time-scale for some `complete' MCT. After an attempt to discuss belief and falsification we decide we are no Wittgenstein and compile homework: -- make and overview of `complexities' in order to clarify where to use what, with a bottom-up approach. The top-down approach doesn't receive enough support, at least at the moment. To be more precise: - top-down would require fixing definitions and conventions in the most general context; - bottom-up requires examining the corresponding issues on a case-by-case basis, thereupon judging the numerical impact of approximations. Complexity includes, background, interference, complex poles etc. -- analyze mass definition in MCT, production X decay, Higgs propagators, fixed widths, running widths, etc. (with the goal of a `conversion table', see (*) above). -- start understanding bench marks, in particular WW and ZZ FS. -- Given top priority to clarify `m_H' definition and usage (contrasting ill-usage) we postpone discussions on the remaining of the list but agree in introducing items that include spin measurement of the would-be resonance. We finally agree that people will sign up for the actual work (minutizer's note: the worrisome fact is that we seem to miss young, brilliant people ....) -------------------------------------------------------------------------------- End at 17:45