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WG1 - VH/VBF subgroup
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40/R-A10 (CERN)
XSWG VBF/VH WG meeting on the 24th of June 2015
Indico agenda of the meeting: https://indico.cern.ch/event/402226/
Luca Perrozzi
* summary of CMS VH analyses and of the past generations (Powheg for signal, with pT reweighting applied + LO for basically all the bkg)
* extensive studies in the past months comparing several choices (Powheg, Madgraph5_aMC@NLO, sherpa)
* Rivet analysis in the signal region, looking at relevant variables (uncertainty is statistical only), will be shared with ATLAS people
* some discrepancies observed, Sherpa far off and also Madgraph5_aMC@NLO different from the various Powheg flavours
* with new samples from CMS we will have the scale factors to account for systematic uncertainties as well
* Mjj distributions show noticeable differences among generators (gen jets matched to a b at parton level)
* the CMS plan is to use Powheg HZj + pythia8 as reference signal for the analyses
* note that orange and green do not share the same Madgraph5_aMC@NLO configuration, therefore differences are not due only to the parton-shower
* in ATLAS similar behaviour in Mjj is observed comparing sherpa and Powheg (even removing additional jet multiplicities, therefore the behaviour seems to come from the PS)
* CMS configurations reported in details
* (not used for the systematics so far)
* alpha_S, pdfs used listed, waiting for META pdf
* negative weights: is it possible to reduce them for Madgraph5_aMC@NLO, similarly to what is done in Powheg with the folding parameters?
Carlo Pandini
* summary of ATLAS VH and VBF analyses Run 1 results
* comparison studies:
* vetoed Powheg shower with “main31” algorithm for the matching scheme
* Madgraph5_aMC@NLO and Powheg in good agreement with basic pre-selections, both coupled to pythia8
* with or w/o main31 prescription plots are very similar
* varying Powheg hdump the from +inf to some scale choices taken from final state particle masses does not change the distributions
* gg > ZH
* k-factors from LO to NLO for gg > ZH don’t change w/ sqrt (s)
* gg / qq fraction increases with sqrt (s) and it’s more important at high pT of the Higgs (the k-factor uncertainty will increase with pT_H, since the heavy top approximation will reach its validity. Q: what’s the scale of the validity breaking? A: hard to say, an estimate of the uncertainty is needed, between 100 - 300 GeV in 20 GeV bins NB: avoid double counting!).
* looking fwd to a NNLO reweighting to NLO Powheg generation
* systematics and corrections list presented
* parton-shower and matching uncertainties systematics could use a more sophisticated recipe than 2-point variation
* EWK corrections: NLO QCD + NLO EWK MC prediction is desired, for this year anyways it’s too late. The best way forward is to use HAWK to calculate the differential EWK corrections.
* VBF modeling based on Run1
* parton-shower and matching uncertainties systematics could use a more sophisticated recipe than 2-point variation
* IS NLO merging of 0- and 1-jet doable?
* with MINLO there’s no showstoppers in principle. Because of manpower, 1 year ± 1 month could be an estimate of the needed time
* for Madgraph5_aMC@NLO we don’t know
* start to address the VBF + gamma channel for Run2 analysis
* ggH contamination is mentioned as an important aspect
* reweighting tool for VBF: REPOLO
Alexander Karlberg
* fully differential NNLO distributions in VBF (beyond the structure function approach)
* NNLO corrections small (∼1%) without cuts and sizeable with VBF cuts (∼ 5%)
* corrections tend to be dominated by the extra real radiation. The effect is to soften jets and hence fewer events pass the cuts
* Code will be made public in the future. Until then cross sections with specific cuts can be provided.
* are EWK corrections orthogonal to the NNLO QCD corrections also in this case?
* we probably can apply the two classes of corrections (EWK NLO and QCD NNLO) independently, at least there’s no other possibilities at the moment
Gionata Luisoni
* H + up to 3 jets @ NLO in gluon-gluon fusion
* 2j/3j ratio does not change going from LO to NLO
* NLO corrections are relevant both for GGF and for VBF: reduction of uncertainties and modification of shapes
* release of the code being prepared
* work in progress!
Jennifer Thompson
* VH and triple-boson production at NLO with SHERPA, in the tri-lepton final state
Eleni Vryonidou
* NNLO HZ cross-section
* Large gluon fusion scale uncertainty (~30%) dominates NNLO uncertainty
* Need for more accurate predictions in this channel
* Gluon-gluon fusion component important for high pT searches at the LHC
* Additional jet ZHj loop amplitudes contributing significantly in the high Higgs pT region
* More accurate description achieved by merging and matching of 0 and 1-jet amplitudes - reduced shower uncertainties
* all the tools for the experimentalists are listed in slide nb 12
Jason Nielsen
* Treatment of gg->ZH in ATLAS Higgs combinations
* gg->ZH dependence on coupling factors (kappa) calculated for each analysis bin in pT(Z), using LoopTools implementation of Englert, McCullough, Spannowsky
* Each bin in pT(Z) is treated as a separate channel with appropriate correlations
* PDF uncertainties are uncorrelated between qq->ZH and gg->ZH channels. (There was some discussion that perhaps these should be partially correlated instead.)
* Would like to have some coupling-dependence comparisons with other calculations, and would like to discuss how to treat correlations, especially scale uncertainties and coupling fits
Conclusions
* We will discuss, maybe not today, what we want to put in the YR4.
* Jason will put the list of topics in a google doc to be circulated among us.
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