Precise Higgs-background predictions: merging NLO QCD and squared quark-loop corrections to four-lepton + 0,1 jet production

Abstract

We present precise predictions for four-lepton plus jets production at the LHC obtained within the fully automated Sherpa + OpenLoops framework. Off-shell intermediate vector bosons and related interferences are consistently included using the complexmass scheme. Four-lepton plus 0-and 1-jet final states are described at NLO accuracy, and the precision of the simulation is further increased by squared quark-loop NNLO contributions in the gg → 4 , gg → 4 + g, gq → 4 + q, and qq → 4 + g channels. These NLO and NNLO contributions are matched to the Sherpa parton shower, and the 0-and 1-jet final states are consistently merged using the Meps@Nlo technique. Thanks to Sudakov resummation, the parton shower provides improved predictions and uncertainty estimates for exclusive observables. This is important when jet vetoes or jet bins are used to separate four-lepton final states arising from Higgs decays, diboson production, and top-pair production. Detailed predictions are presented for the Atlas and Cms H → WW * analyses at 8 TeV in the 0-and 1-jet bins. Assessing renormalisation-, factorisation-and resummationscale uncertainties, which reflect also unknown subleading Sudakov logarithms in jet bins, we find that residual perturbative uncertainties are as small as a few percent.

Next-to-leading-order electroweak corrections to pp → W+W− → 4 leptons at the LHC

“…A comprehensive QCD-based prediction for (off-shell) WW production at the LHC, including the gg channel for both WW and WW+jet production, was presented in ref. [29] in a study of these reactions as background to Higgs-boson production. Very recently, the full NLO prediction for gg → WW with leptonic W-boson decays was presented in the literature [30].…”

The two-loop helicity amplitudes for gg → V 1 V 2 → 4 leptons

“…The projectors themselves can be decomposed in the same 20 tensors as 14) where also B ji are functions of the external invariants and d. Their explicit form can be determined imposing 15) where the polarisation sums are evaluated in d dimensions according to (2.10) and (2.12).…”

“…Their impact on the total cross section was found to range approximately from 5% to more than 10% for different final states at the LHC, and to rise with increasing collider energy [1][2][3][4]. These values can substantially increase up to about 30% when particular sets of cuts, relevant for example for Higgs boson searches, are applied [13,14]. It is therefore clear that the inclusion of gluon channel contributions…”

NLO merging in tt+jets

“…The shown MEPS@NLO calculations are constructed from NLOPS calculations for the individual parton multiplicities, matched using the methods of [19] 1 , according to the algorithm developed in [3,4,23,24]. The MENLOPS technique -the presented results use an implementation according to [4,25,26] -is a special case of a MEPS@NLO calculation where only the lowest multiplicity is calculated at NLOPS accuracy and all higher multiplicities are merged at leading order accuracy.…”

“…3 and 4 show results for MEPS@NLO calculations for pp → + − + / E ⊥ + X production [24], merging pp → + − νν and pp → + − νν j at NLOPS accuracy. Included are also the loop-induced processes gg → + − νν, gg → + − ννg and gq → + − ννq.…”