Measurement of the underlying event in the Drell-Yan process in proton-proton collisions at sqrt(s) = 7 TeV

TL;DR

This study investigates the underlying event (UE) in proton-proton collisions at $\sqrt{s}=7$ TeV by measuring UE activity in Drell--Yan events with muons, corrected to the particle level and compared to multiple MC tunes. The analysis leverages observables defined in azimuthal regions relative to the dimuon system, examining dependencies on the dimuon mass $M_{\mu\mu}$ and transverse momentum $p_T^{\mu\mu}$ to separate MPI from ISR effects. The results show that UE activity is largely independent of $M_{\mu\mu}$ above 40 GeV/$c^2$ and that the $p_T^{\mu\mu}$ dependence is well described by MADGRAPH, indicating universality of MPI across DY and hadronic processes. These measurements provide stringent constraints for MC tuning and improve the modeling of soft QCD activity in high-energy collisions, with direct implications for precision SM measurements and new physics searches.

Abstract

A measurement of the underlying event (UE) activity in proton-proton collisions at a center-of-mass energy of 7 TeV is performed using Drell--Yan events in a data sample corresponding to an integrated luminosity of 2.2 inverse femtobarns, collected by the CMS experiment at the LHC. The activity measured in the muonic final state (q q-bar to opposite-sign muons) is corrected to the particle level and compared with the predictions of various Monte Carlo generators and hadronization models. The dependence of the UE activity on the dimuon invariant mass is well described by PYTHIA and HERWIG++ tunes derived from the leading jet/track approach, illustrating the universality of the UE activity. The UE activity is observed to be independent of the dimuon invariant mass in the region above 40 GeV, while a slow increase is observed with increasing transverse momentum of the dimuon system. The dependence of the UE activity on the transverse momentum of the dimuon system is accurately described by MADGRAPH, which simulates multiple hard emissions.

Figures (5)

• Figure 1: Top: The UE activity as a function of the dimuon invariant mass ($M_{\mu\mu}$) for events with $p_{T}^{\mu\mu} < 5$${\,\text{Ge\spaceV\space/\space}c}$ for charged particles having $\Delta\phi < 120^{\circ}$: (left) particle density; (centre) energy density; (right) ratio of the energy and particle densities. The predictions of pythia6 Z2, powheg Z2, pythia8 4C, and herwig++ LHC-UE7-2 (with and without MPIs) are also displayed. In the top right plot, the structure around 60--80${\,\text{Ge\spaceV\space/\space}c^\text{2}}$ for herwig++ without MPIs reflects the influence of photon radiation by final-state muons, which is enhanced below the ${Z}$ resonance. Bottom: Ratios of the predictions of various MC models and the measurement. The inner band shows the statistical uncertainity of data whereas the outer band represents the total uncertainty.
• Figure 2: The UE activity in the towards (upper row), transverse (centre row), and away (bottom row) regions as functions of $p_{T}^{\mu\mu}$ for events satisfying $81 < M_{\mu\mu} < 101$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$: (left) particle density; (centre) energy density; (right) the ratio of the energy density and the particle densities. Predictions of MadGraph Z2, powheg Z2, pythia8 4C, and herwig++ LHC-UE7-2 (with and without MPIs) are superimposed.
• Figure 3: Ratios, as functions of $p_{T}^{\mu\mu}$, of the predictions of various MC models to the measurements in the towards (upper row), transverse (centre row), and away (bottom row) regions for events satisfying $81 < M_{\mu\mu} < 101$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$: (left) particle density; (centre) energy density; (right) the ratio of the energy density and particle densities. The inner band shows the statistical uncertainty on the data whereas the outer band represents the total uncertainty.
• Figure 4: Distributions of the charged particle multiplicity (upper row) and transverse momentum (bottom row) of the selected tracks. The left plots show the comparisons of the normalized distributions in the away, transverse, and towards regions for events satisfying $81 < M_{\mu\mu} < 101$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$. Comparisons of the normalized distributions in the transverse region are shown in the centre plots, requiring $81 < M_{\mu\mu} < 101$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$ or $p_{T}^{\mu\mu} < 5{\,\text{Ge\spaceV\space/\space}c}\xspace$ . The right plots show the comparisons of the normalized distributions in the transverse region with the predictions of various simulations for events satisfying $81 < M_{\mu\mu} < 101$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$.
• Figure 5: Comparison of the UE activity measured in hadronic and Drell--Yan events (around the ${Z}$ resonance peak) as a function of $p_{T}^\text{leading~jet}$ and $p_{T}^{\mu\mu}$, respectively: (left) particle density, (centre) energy density, and (right) ratio of energy and particle densities in the transverse region.