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Thoughts to get started on combination of Higgs-like boson properties measurements

Thoughts to get started on combination of Higgs-like boson properties measurements. Marco Zanetti (MIT). Intro and goal. The existence of a new narrow higgs-like resonance at 125 GeV is well established by both experiments.

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Thoughts to get started on combination of Higgs-like boson properties measurements

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  1. Thoughts to get started on combination of Higgs-like boson properties measurements Marco Zanetti (MIT)

  2. Intro and goal • The existence of a new narrow higgs-like resonance at 125 GeV is well established by both experiments. • Combining ATLAS and CMS observations to better constrain its properties makes a lot of sense • We have endorsed a document recommending the framework to explore the coupling structure • Regardless the time scale, we need to implement those recommendations in such a way to enable the ATLAS and CMS datasets combination • Disclaimer: what follows is a (non exhaustive) core dump of items we need to discuss/address

  3. Basics • The aim is to provide estimates for the Mass and the Coupling structure • The latter according to arXiv:1209.0040 • Signal strength k2 • Other quantum numbers (JCP) not addressed at the moment • Test workspaces corresponding to the ICHEP dataset (10/fb): • Hgg+HZZ as a benchmark? • Ok for CMS to use actual workspaces (or simplified/cleaned-up versions) • In specific cases (eventually) we may want to use more channels and combine them individually, e.g. for the signal strength or for the mass

  4. Mass • Mass should be address first, used then as input for the coupling measurement (baseline) • In CMS Hgg and HZZ have a parametric dependency of the signal modifier(s) on the mass. “MH” is a RooRealVar we can fit for • Being implemented for VHbb, Htt and HWW too. Probably not worth considering them for the combination • Any subtle correlation? • Investigate effect of floating mass in the coupling fit, checking for possible biases (e.g. on Hgg)

  5. Variable and nuisances • Agree on the naming convention for the POI and nuisances • Agree on the ranges • Possibly different for different integrated luminosities • Address degeneracies as in the case of CVCF • Thorough review of the theoretical uncertainties • Jet bins migrations, VBF/ggH contamination, mass line-shape, etc • Nuisances PDFs

  6. Fits (CMS case) • Python classes (interfaced to RooFit) to map parameters to signal and background yields (physics models) • We perform multidimensional (profiled) likelihood fits (default) and bayesian fits. • Uncertainties/contours: • Simple Minos for each parameter (works in any dimension, Δχ2=1) • Grid scan: perform a likelihood scan (profiling or not the other POI/nuisances) on a 1D or 2D grid. Errors/contours from the assumption of asymptotic c2 behavior of -2DLogL (robust, split in several jobs)

  7. Fits (CMS case) • Possible to let MH floating in the fit (as additional POI) • Need to study possible biases: • E.g. Hgg is expected to be biased in signal strength -> in other couplings • Toss frequentist toys? • How to treat/communicate correlations between POIs? • In the case of 2D fits, issue the 2D contour (1s and 2s bands) • For n>2 we don’t have yet means to get the n-dim ellipsoid • Ideally we should provide the full covariance matrix..

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