Measurement of the Underlying Event Activity in the Drell-Yan process in proton-proton collisions at √s = 7 TeV

A measurement of the underlying event activity using Drell-Yan events is performed in proton-proton collisions at a centre-of-mass energy √s = 7 TeV using data, corresponding to an integrated luminosity of 2.2 fb^-1 collected in CMS experiment during 2011. The Drell-Yan process with muonic final state, q q^- → μ+ μ-, provides an excellent way to study the underlying event activity by separating the hard interaction from the soft component. The UE activity is observed to be independent of the di-muon invariant mass in a region above 40 GeV/c², implying a saturation of the multiple parton interactions at these energy scales. The boost of the dimuon system considered has been constrained in this measurement to reduce the contribution of radiation balancing the boost. A slow growth of the activity is observed with increasing transverse momentum of the di-muon system which can be attributed to the increase in radiation. The data are corrected to the particle level and compared with the predictions of various Monte-Carlo generators and hadronization models.

Approved Plots

• Table 1:
  Summary of the systematic uncertainties (%) from different sources. The first three rows show the systematic uncertainties for the particle density in the towards, transverse and away regions, whereas the last three rows report the systematic uncertainties for the energy density in three regions. The numbers out of parentheses refer to the densities as a function of M_μμ and those in parentheses correspond to the study as a function of p_T^μμ.

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• Figure 1:
  The UE activity as function of invariant mass (M_μμ) for events with requirement of p_T^μμ < 5 GeV/c for the charged particles having Δφ < 120^o: (left) particle density; (centre) energy density; (right) ratio of the energy and particle density together with the predictions of PYTHIA-6 Z2, Powheg Z2, PYTHIA-8 4C and Herwig++ LHC-UE7-2 (with and without MPI). Plots in bottom row show the ratio of MC prediction and the measurement for the corresponding observables. The inner band shows the statistical uncertainity of data whereas the outer band represents the total uncertainty (statistical and systematic uncertainties added in quadrature).

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• Figure 2:
  The UE activity in the towards (upper row), transverse (central row) and away (bottom row) regions as function of p_T^μμ for events with requirement of 81 < M_μμ < 101 GeV/c²: (left) particle density; (centre) energy density; (right) the ratio of the energy density and the particle density together with the predictions of MadGraph Z2, Powheg Z2, PYTHIA-8 4C and Herwig++ LHC-UE7-2 (with and without MPI).

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• Figure 3:
  Ratios, as a function of the p_T^μμ, of MC predictions to the measurements in the towards (upper row), transverse (bottom row) and away (bottom row) regions for events with the requirement of 81 < M_μμ < 101 GeV/c²: (left) particle density; (centre) energy density; (right) the ratio of the energy density and particle density. The inner band shows the statistical uncertainity of data whereas the outer band represents the total uncertainty (statistical and systematic uncertainties added in quadrature).

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• Figure 4:
  The charge multiplicity (upper row) and p_T spectrum (bottom row) of the selected tracks: (left) comparison of the normalized distribution in the away, transverse and towards region for events with requirement of 81 < M_μμ < 101 GeV/c²; (centre) comparisons of the normalized distribution in transverse region for events with the requirement of 81 < M_μμ < 101 GeV/c² and p_T^μμ < 5 GeV/c; and (right) comparison of the normalized distribution with the prediction of various simulations in transverse region for events with the requirement of 81 < M_μμ < 101 GeV/c²and bottom panel shows the ratio of monte-carlo prediction and the measurements.

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• Figure 5:
  Comparison of the UE activity measured in hadronic and Drell-Yan events as a function of p_T^{leading jet} and p_T^μμ respectively; (left) particle density, (centre) energy density and (right) ratio of energy and particle density in transverse region.

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-- SunilBansal - 12-Dec-2011