Event generator tunes obtained from underlying event and multiparton scattering measurements

TL;DR

The paper develops new CMS-era tunes for UE modeling across three event generators (PYTHIA8, PYTHIA6, HERWIG++) by fitting to CMS underlying-event data at 0.3, 0.9, 1.96, and 7 TeV, with Rivet/Professor and eigentunes to quantify uncertainties and extend predictions to 13 TeV. It also introduces DPS-focused tunes by directly fitting DPS-sensitive observables in W+dijet and four-jet final states, deriving sigma_eff for multiple MPI models and channels. The results show improved UE descriptions and robust 13 TeV predictions for UE observables, but highlight tensions between UE-based tuning and DPS observables, indicating current generators struggle to describe soft and hard MPI simultaneously. The work provides a practical framework for tuning and validating MC generators with energy-dependent MPI and diffraction components, and offers a set of tunes with uncertainty estimates to support LHC Run-2 analyses and future collider studies.

Abstract

New sets of parameters ("tunes") for the underlying-event (UE) modeling of the PYTHIA8, PYTHIA6 and HERWIG++ Monte Carlo event generators are constructed using different parton distribution functions. Combined fits to CMS UE proton-proton (pp) data at sqrt(s) = 7 TeV and to UE proton-antiproton (p p-bar) 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 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 of the UE tunes to "minimum bias" (MB) events, multijet, and Drell-Yan (q q-bar to Z / gamma* to lepton-antilepton + jets) observables at 7 and 8 TeV are presented, as well as predictions for MB and UE observables at 13 TeV.

Figures (32)

• Figure 1: Left: Illustration of the azimuthal regions in an event defined by the $\Delta\phi$ angle relative to the direction of the leading object Aaltonen:2015aoa. Right: Illustration of the topology of a hadron-hadron collision in which a hard parton-parton collision has occurred, and the leading object is taken to be the charged particle of largest $p_{\mathrm{T}}$ in the event, $p_{\rm T}^{\rm max}$.
• Figure 2: CDF data at $\sqrt{s}=300\,\text{Ge\spaceV}\xspace$Aaltonen:2015aoa on particle (top) and $p_{\rm T}^{\rm sum}$ densities (bottom) for charged particles with $p_{\rm T}\!>\!0.5\,\text{Ge\spaceV}\xspace$ and $|\eta|\!<\!0.8$ in the TransMIN (left) and TransMAX (right) regions as defined by the leading charged particle, as a function of the transverse momentum of the leading charged-particle $p_{\rm T}^{\rm max}$. The data are compared to pythia8 Tune $4$C, CUETP$8$S$1$-CTEQ$6$L1, CUETP$8$S$1$-HERAPDF1.5LO, and CUETP$8$M$1$. The ratios of MC events to data are given below each panel. The data at $\sqrt{s}=300\,\text{Ge\spaceV}\xspace$ are not used in determining these tunes. The green bands in the ratios represent the total experimental uncertainties.
• Figure 3: CDF data at $\sqrt{s}=900\,\text{Ge\spaceV}\xspace$Aaltonen:2015aoa on particle (top) and $p_{\rm T}^{\rm sum}$ densities (bottom) for charged particles with $p_{\rm T}\!>\!0.5\,\text{Ge\spaceV}\xspace$ and $|\eta|\!<\!0.8$ in the TransMIN (left) and TransMAX (right) regions as defined by the leading charged particle, as a function of the transverse momentum of the leading charged-particle $p_{\rm T}^{\rm max}$. The data are compared to pythia8 Tune $4$C, CUETP$8$S$1$-CTEQ$6$L1, CUETP$8$S$1$-HERAPDF1.5LO, and CUETP$8$M$1$. The ratios of MC events to data are given below each panel. The green bands in the ratios represent the total experimental uncertainties.
• Figure 4: CDF data at $\sqrt{s}=1.96\,\text{Te\spaceV}\xspace$Aaltonen:2015aoa on particle (top) and $p_{\rm T}^{\rm sum}$ densities (bottom) for charged particles with $p_{\rm T}\!>\!0.5\,\text{Ge\spaceV}\xspace$ and $|\eta|\!<\!0.8$ in the TransMIN (left) and TransMAX (right) regions as defined by the leading charged particle, as a function of the transverse momentum of the leading charged-particle $p_{\rm T}^{\rm max}$. The data are compared to pythia8 Tune $4$C, CUETP$8$S$1$-CTEQ$6$L1, CUETP$8$S$1$-HERAPDF1.5LO, and CUETP$8$M$1$. The ratios of MC events to data are given below each panel. The green bands in the ratios represent the total experimental uncertainties.
• Figure 5: CMS data at $\sqrt{s}=7\,\text{Te\spaceV}\xspace$CMS:2012kca on particle (top) and $p_{\rm T}^{\rm sum}$ densities (bottom) for charged particles with $p_{\rm T}\!>\!0.5\,\text{Ge\spaceV}\xspace$ and $|\eta|\!<\!0.8$ in the TransMIN (left) and TransMAX (right) regions as defined by the leading charged particle, as a function of the transverse momentum of the leading charged-particle $p_{\rm T}^{\rm max}$. The data are compared to pythia8 Tune $4$C, and CUETP$8$S$1$-CTEQ$6$L1, CUETP$8$S$1$-HERAPDF1.5LO, and CUETP$8$M$1$. The ratios of MC events to data are given below each panel. The green bands in the ratios represent the total experimental uncertainties.