The Associated Production of Weak Bosons and Jets by Multiple Parton Interactions

TL;DR

The paper investigates how multiple-parton interactions (MPI) contribute to W/Z + n-jet events at the Tevatron, showing that MPI can rival or dominate higher-order QCD processes at low jet p_T and can bias α_s determinations and low-p_T QCD/BFKL studies. Using MADGRAPH, MPI is modeled with an effective area πR^2 and studied across W+2, W+3, and Z+n jet final states under Tevatron cuts, revealing that MPI signals are observable with modest luminosity and measurable via jet-p_T distributions and cross-section ratios. The findings indicate MPI contributions persist at the 10–20% level even for p_T,min around 20 GeV and emphasize the potential to experimentally determine the effective quark/gluon areas in the nucleon, possibly differing between parton types. Overall, the work underscores the necessity of accounting for MPI in precision QCD analyses and offers concrete strategies to isolate or quantify these effects in W/Z+n-jet channels.

Abstract

The sources of $W + n$-jet events in hadron collisions are higher-order QCD processes, but also multiple-parton interactions. A subprocess producing a $W + k$-jet final state, followed by one producing $l$ jets in the same nucleon-nucleon interaction, will result in a $W + n$-jet event if $k + l = n$. In the simplest case a $W + 2$-jet event can be produced by a quark-antiquark annihilation into $W$ and a 2-jet event occurring in the same proton-antiproton interaction. We compute that this happens at the 10% level of the higher-order QCD processes for the type of cuts made by the Tevatron experiments. For jet $p_T$ values of order $5{\sim}10$ GeV, multiple-parton interactions dominate higher-order QCD-processes. The emergence of this new source of $W + n$-jet events towards lower $p_T $ simulates the running of $α_s$; it is imperative to remove these processes from the event sample in order to extract information on the strong coupling constant. Also, BFKL studies of low-$p_T$ jet cross sections are held hostage to a detailed understanding of the multiple-parton interactions. We perform the calculations required to achieve these goals. A detailed experimental analysis of the data may, for the first time, determine the effective areas occupied by quarks and gluons in the nucleon. These are not necessarily identical. We also compute the multiple-parton contribution to $Z + n$-jet events.

Figures (9)

• Figure 1: $W,Z + 2$-jet multiple-parton scattering.
• Figure 2: Total cross section for the production of a $W$ accompanied by 2 jets, as a function of the minimum-$p_T$ cut, for: QCD processes (open circles), double-parton interactions (triangles), multiple interactions (squares).
• Figure 3: $p_T$ distribution of the jets in mb/GeV, for: (a) QCD 2-jets production, (b) $W+2$-jets production.
• Figure 4: Statistical significance (Signal/$\sqrt{\rm background}$) of the double-parton interactions for the production of a $W+2$-jets (squares) and $W+3$-jets (triangles) as a function of the minimum transverse momenta of the jets.
• Figure 5: Total cross section for the production of a $W$ accompanied by 3 jets, as a function of the minimum-$p_T$ cut, for: higher-order QCD processes (open circles), double-parton interactions (triangles), multiple interactions (squares).