Multijet matrix elements and shower evolution in hadronic collisions: W b bbar + n jets as a case study

TL;DR

This work develops a complete LO framework for multiparton final states in hadronic collisions by merging ALPHA-calculated matrix elements with a coherent parton-shower evolution in HERWIG, focusing on $W\,b\bar{b}+n$ jets (n≤4). It emphasizes assigning colour flows and incorporating heavy-quark mass effects to enable realistic showering and hadronization, and analyzes both parton-level and fully showered results for Tevatron and LHC. The study reveals that higher jet multiplicities contribute substantially and that single-$b$-tag backgrounds can be large, underscoring the need for full matrix-element calculations for accurate background modeling. The proposed framework is extensible to other processes (e.g., $W+n$ jets, $Z+b\bar{b}+n$ jets, $t\bar{t}+n$ jets) and supports more realistic detector-level studies via consistent merging with shower MCs.

Abstract

We study in this paper the production, in hadronic collisions, of final states with W gauge bosons, heavy quark pairs and n extra jets (with n up to 4). The complete partonic tree-level QCD matrix elements are evaluated using the ALPHA algorithm, and the events generated at the parton level are then evolved through the QCD shower and eventually hadronised using the coherent shower evolution provided by the HERWIG Monte Carlo. We discuss the details of our Monte Carlo implementation, and present results of phenomenological interest for the Tevatron Collider and for the LHC. We also comment on the impact of our calculation on the backgrounds to W (H -> b bbar) production, when only one b jet is tagged.

Figures (9)

• Figure 1: Inclusive $\hbox{$p_T$}$ distributions of $b$ jets. The solid lines refer to 2-$b$-jet events (in which case both jets are entered in the histograms); the dashed lines refer to 1-$b$-jet events.
• Figure 2: Dijet invariant mass distribution, in $W$ plus 2 jet events, where at least one of the two jets has a $b$ in it. The solid line refers to 2-$b$-jet events; the dashed line refers to 1-$b$-jet events.
• Figure 3: $b\bar{b}$ angular correlations in events with 2 $b$ jets (plots) and with 1 $b$ jet (histograms). The case of 1 $b$ jet is divided into subsamples defined by the $\hbox{$p_T$}$ of the $b$ jet in the ranges 20-40, 40-100 and $>100~\hbox{$\mathrm{GeV}$}$.
• Figure 4: Inclusive $\hbox{$p_T$}$ distributions of jets at the parton level (solid curves) and after shower evolution (dashed curves). The curves are ordered going from the hardest to the softest jet in the event. Parton level jets are defined by an isolation cut $\Delta R>0.4$, and showered jets by a cone $\hbox{$R_{\mathrm jet}$}=0.4$.
• Figure 5: Inclusive $\hbox{$p_T$}$ distributions of jets at the parton level, with separation cut $\Delta R>0.7$ (solid curves), and of fully-showered $\hbox{$R_{\mathrm jet}$}=0.7$ jets (dashed curves). These last were obtained starting from the full sample of $\Delta R>0.4$ partonic events.