Abstract
New sets of parameters (“tunes”) for the underlying-event (UE) modelling of the pythia8, pythia6 and herwig++ Monte Carlo event generators are constructed using different parton distribution functions. Combined fits to CMS UE proton–proton (mathrm {p}mathrm {p}) data at sqrt{s} = 7,text {TeV} and to UE proton–antiproton (mathrm {p}overline{mathrm{p}} ) data from the CDF experiment at lower sqrt{s}, are used to study the UE models and constrain their parameters, providing thereby improved predictions for proton–proton collisions at 13,text {TeV}. In addition, it is investigated whether the values of the parameters obtained from fits to UE observables are consistent with the values determined from fitting observables sensitive to double-parton scattering processes. Finally, comparisons are presented of the UE tunes to “minimum bias” (MB) events, multijet, and Drell–Yan ( mathrm{q} overline{mathrm{q}} rightarrow mathrm{Z}/ gamma ^* rightarrow lepton-antilepton+jets) observables at 7 and 8,text {TeV}, as well as predictions for MB and UE observables at 13,text {TeV}.
Highlights
Monte Carlo (MC) event generators of hadron–hadron collisions based on perturbative quantum chromodynamics (QCD) contain several components
About half of the particles produced in a minimum bias” (MB) collision originate from the hadronization of partons scattered in multiple-parton interactions (MPI), and have their differential cross sections in pT regulated via Eq (1), using the same pT0 cutoff used to tame the hardest 2-to-2 parton-parton scattering in the event
The cross section is calculated using the CTEQ6L1 parton distribution functions (PDF) with a matching scale for ME and parton shower (PS) jets set to 20 GeV. (In Sect. 4, we show that the CMS UE tunes can be interfaced to higher-order ME generators without additional tuning of MPI parameters)
Summary
Monte Carlo (MC) event generators of hadron–hadron collisions based on perturbative quantum chromodynamics (QCD) contain several components. The primary hard 2-to-2 parton-parton scattering process and the MPI are regulated in the same way through a single pT0 parameter This cutoff is expected to have a dependence on√the center-of-mass energy of the hadron–hadron collision s. The MPI are usually much softer than primary hard scatters, occasionally two hard 2-to-2 parton scatters can take place within the same hadron–hadron collision This is referred to as double-parton scattering (DPS) [13,14,15,16], and is typically described in terms of an effective cross section parameter, σeff , defined as: σAB σAσB σeff (3). About half of the particles produced in a MB collision originate from the hadronization of partons scattered in MPI, and have their differential cross sections in pT regulated via Eq (1), using the same pT0 cutoff used to tame the hardest 2-to-2 parton-parton scattering in the event. The appendices contain additional comparisons between the pythia and herwig++ UE tunes and the data, information about the tune uncertainties, and predictions for some MB and DPS observables at 13 TeV
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