Study of double parton scattering using W+ 2-jet events in p-p collisions at √s = 7 TeV

Abstract

Double parton scattering is investigated in paroton-proton collisions at √s = 7 TeV where the final state includes a W boson, which decays into a muon and a neutrino, and two jets. The data sample corresponds to an integrated luminosity of 5 fb-1, collected with the CMS detector at the LHC. Observables sensitive to double parton scattering are investigated after being corrected for detector effects and selection efficiencies. The fraction of W + 2-jet events due to double parton scattering is measured to be 0.055 0.002 (stat.) 0.014 (syst.). The effective cross section, $\sigma_{\rm{eff}}$, characterizing the effective transverse area of hard partonic interactions in collisions between protons is measured to be 20.7 0.8 (stat.) 6.6 (syst.) mb.

Figures

Figure 1 left Figure 1 right

Figure 1: Feynman diagrams for W + 2-jet production from (left) double parton scattering and (right) single parton scattering.

Figure 2 left top Figure 2 right top
Figure 2 left bottom Figure 2 right bottom

Figure 2: Detector-level comparison of data with MC simulations for the multiplicity (top left) of jets (N$_{\rm j}$) with $p_{T} &amp;gt; 20$ GeV/$c$ and η< 2.0. Data and simulations for the sample with exactly two jets are plotted as a function of the $ p_{T} $ of the leading (top right) and subleading (lower left) jets, as well as of the magnitude of the vector sum of the muon $ p_{T} $ and $ \ensuremath{{E\!\!\!/}_{\mathrm{T}}} $ (lower right). The background distribution represents the sum of the contributions of Drell-Yan, W$\rightarrow~\tau\nu$, diboson, multijet, $ t {\bar t} $, and single-top-quark processes. The bottom panels show the ratio of the data and simulated distributions. The band shows the total uncertainty, with the contributions of the jet energy scale uncertainty and the statistical uncertainties of the MC samples added in quadrature. The error bars on the ratio histogram represent the statistical uncertainty of the data and the simulated samples added in quadrature.

Figure 3 left Figure 3 right

Figure 3: Comparison of data with MC simulations at detector level for the DPS-sensitive observables $ \Delta^{\rm rel}~p_{T} $ (left), and $\Delta S $ (right). The background distribution represents the sum of the contributions of Drell-Yan, W$\rightarrow~\tau\nu$, diboson, multijet, $ t {\bar t} $, and single-top-quark processes. The bottom panels show the ratio of the data and simulated distributions. The band shows the total uncertainty, with the contributions of the jet energy scale uncertainty and the statistical uncertainties of the MC samples added in quadrature. The error bars on the ratio histogram represent the statistical uncertainty of the data and the simulated samples added in quadrature.

Figure 4 left Figure 4 right

Figure 4: Fully corrected data distributions, normalized to unity, for the DPS-sensitive observables $ \Delta^{\rm rel}~p_{T} $ (left), and $\Delta S $ (right). The second panel in both plots shows the ratio of data over MADGRAPH5 + PYTHIA8 with and without MPI, whereas in the third panel the ratio with POWHEG2 + PYTHIA6 is shown. The ratio of the data and PYTHIA8 is shown in the fourth panel of both plots. The band represents the total uncertainty of the data.

Figure 5

Figure 5: The extracted value of the DPS fraction in W + jets events, simulated with MADGRAPH5 + PYTHIA8, using different background templates obtained by varying the transverse momentum cutoff (${ p_{T}^{\rm cut }} $) for the second hard interaction. The DPS fractions obtained by performing both simultaneous and individual fits to the $ \Delta^{\rm rel}~p_{T} $ and $\Delta S $ observables are shown. The DPS fraction, $ {f_{\rm DPS}^{\rm evt}} $, for the simulated W + jets events is shown by a dashed black line. The error bars/bands represent the statistical uncertainty added in quadrature to the systematic uncertainty of the DPS template.

Figure 6 left Figure 6 right

Figure 6: Fit results for the DPS-sensitive observables $ \Delta^{\rm rel}~p_{T} $ (left), and $\Delta S $ (right) using signal and background templates. The distributions of the simulated W + 2-jet events are fitted with signal and background templates. The bottom panels show the ratio of the distributions to the fit results. Here, the term "inclusive" means the simulation also includes the DPS contribution.

Figure 7 left Figure 7 right

Figure 7: Fit results for the DPS-sensitive observables $ \Delta^{\rm rel}~p_{T} $ (left), and $\Delta S $ (right). Corrected data distributions are fitted with signal and background templates.

Figure 8

Figure 8: Centre-of-mass energy dependence of $\sigma_{\rm{eff}}$ measured by different experiments using different processes. These measurements used different approaches for extraction of the DPS fraction and $\sigma_{\rm{eff}}$. The "Corrected CDF" data point indicates the $\sigma_{\rm{eff}}$ value corrected for the exclusive event selection.

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