Azimuthal decorrelations and multiple parton interactions in photon+2 jet and photon+3 jet events in ppbar collisions at sqrt{s}=1.96 TeV

TL;DR

The paper probes MPI in hadron collisions by measuring azimuthal decorrelations in γ+2j and γ+3j events at √s = 1.96 TeV, and compares normalized differential cross sections to various MPI models in PYTHIA and SHERPA. It demonstrates a clear DP contribution in γ+2j and, for γ+3j, a DP-dominated component plus a small TP contribution, providing strong evidence against SP-only predictions. The study quantifies DP fractions as functions of the second-jet p_T and, for γ+3j, TP fractions, and uses these results to constrain MPI tunes, with implications for backgrounds in rare-process analyses and precision measurements. Overall, the results favor modern MPI implementations with p_T-ordered showers and underscore the need to model DP and TP contributions accurately in high-p_T jet environments.

Abstract

Samples of inclusive photon+2 jet and photon+3 jet events collected by the D0 experiment with an integrated luminosity of about 1fb^-1 in ppbar collisions at sqrt{s}=1.96 TeV are used to measure cross sections as a function of the angle in the plane transverse to the beam direction between the transverse momentum (pT) of the photon+leading jet system (jets are ordered in pT) and pT of the other jet for photon+2 jet, or pT sum of the two other jets for photon+3 jet events. The results are compared to different models of multiple parton interactions (MPI) in the PYTHIA and SHERPA Monte Carlo (MC) generators. The data indicate a contribution from events with double parton (DP) interactions and are well described by predictions provided by the PYTHIA MPI models with pT-ordered showers and by SHERPA with the default MPI model. The photon+2 jet data are also used to determine the fraction of events with DP interactions as a function of the azimuthal angle and as a function of the second jet pT.

Figures (13)

• Figure 1: Diagram showing the $p_T$ vectors of the $\gamma+{\rm leading~ jet}$ and ${\rm jet2}+{\rm jet3}$ systems in $\gamma+{\rm 3~jet}$ events.
• Figure 2: Diagram showing the $p_T$ vectors of the $\gamma+{\rm leading~ jet}$ system and $\vec{p}_{T}^{\rm~jet 2}$ in $\gamma+{\rm 2~jet}$ events.
• Figure 3: Normalized $\Delta S$ distribution for data and for the reweighted MC sample in the range $15<p_T^{\rm jet 2}<30$ GeV.
• Figure 5: Normalized differential cross section in $\gamma+{\rm 3~jet}$ events, $(1/\sigma_{\gamma 3j})d\sigma_{\gamma 3j}/d\Delta S$, in data compared to MC models and the ratio of data over theory, only for models including MPI, in the range $15 < p_T^{\rm jet 2} < 30$ GeV.
• Figure 6: Normalized differential cross section in $\gamma+{\rm 2~jet}$ events, $(1/\sigma_{\gamma 2j}) d\sigma_{\gamma 2j}/d\Delta\phi$, in data compared to MC models and the ratio of data over theory, only for models including MPI, in the range $15 < p_T^{\rm jet 2} < 20$ GeV.