Probing color-singlet exchange in $Z+2$-jet events at the LHC

TL;DR

The paper investigates color-singlet exchange in $Z+2$-jet events at the LHC as a proxy for weak-boson scattering. It develops a tree-level Monte Carlo framework to compare electroweak $qq\to qqZ$ signals with QCD $Zjj(j)$ backgrounds, imposes forward-jet tagging and central-lepton criteria to isolate the electroweak contribution, and analyzes minijet radiation patterns using $Zjjj$ final states with two modeling approaches. The findings show distinct radiation patterns: color-singlet exchange yields central rapidity gaps and softer, forward-dominated minijets, while QCD backgrounds produce central, higher-$p_T$ minijets, enabling effective background suppression via a minijet veto. Together, these results provide a practical strategy to study soft-gluon emission in color structures and to tune Monte Carlo models for Higgs searches via weak-boson fusion.

Abstract

The purely electroweak process $qq\to qqZ$ (via $t$-channel $γ/Z$ or $W$ exchange) provides a copious and fairly clean source of color-singlet exchange events in $pp$ collisions at the LHC. A judicious choice of phase-space region allows the suppression of QCD backgrounds to the level of the signal. The color-singlet-exchange signal can be distinguished from QCD backgrounds by the radiation patterns of additional minijets in individual events. A rapidity-gap trigger at the minijet level substantially enhances the signal versus the background. Analogous features of weak boson scattering events make $Z+2$-jet events at the LHC an ideal laboratory for investigation of the soft-jet activity expected in weak-boson scattering events.

Figures (7)

• Figure 1: Feynman graphs for $Zjj$ production via charged-current exchange. The $WW$-fusion graph (e) simulates weak-boson scattering processes.
• Figure 2: Lepton and jet distributions of signal (solid lines) and background (dashed lines) $Zjj$ events within the cuts of Eqs. (\ref{['cut1a']}--\ref{['defjet']}). Shown are normalized distributions of (a) $|\eta_\ell|_{max}$, the maximum lepton pseudorapidity, (b) the pseudorapidity separation $\Delta\eta_{jets}=|\eta(j_1)-\eta(j_2)|$ of the two jets and (c) the differential cross section $d\sigma/dp_{Tj,min}$, where $p_{Tj,min}$ is the smaller of the two jet transverse momenta.
• Figure 3: Lepton and jet distributions of signal (solid lines) and background (dashed lines) $Zjj$ events within the cuts of Eqs. (\ref{['cut1a']}--\ref{['cut2b']}). Shown are (a) the dijet mass distribution of the two tagging jets, and (b) the minimal pseudorapidity separation $\Delta\eta_{\ell j}$ between any of the leptons and tagging jets. Note that the distribution in pseudorapidity separation has been normalized to unit area.
• Figure 4: Characteristics of the third (soft) jet in $Zjjj$ signal (solid lines) and background (dashed lines) events at the LHC. (a) The pseudorapidity $\eta_3^*$ is measured with respect to the center of the two tagging jets, $\bar{\eta} = (\eta_j^{\rm tag1}+\eta_j^{\rm tag2})/2$, and the distributions are normalized to unit area. (b) Integrated transverse-momentum distribution of the third jet, $\sigma(p_{T3}>p_{T,min})$. The acceptance requirements of Eqs. (\ref{['cut1a']}--\ref{['cut2b']}) are imposed on the two tagging jets. The corresponding cross sections at lowest order, with two partons in the final state, are indicated for the signal (dotted line) and for the background (dash-dotted line).
• Figure 5: Probability to find a veto jet with transverse momentum above $p_{T,\rm veto}$ and in the pseudorapidity range of Eq. (\ref{['eq:etaveto']}) in signal and background events within the cuts of Eqs. (\ref{['cut1a']}-\ref{['cut2b']}). The solid (signal) and dashed (background) curves are obtained with the exponentiation ansatz of Eq. (\ref{['eq:expon']}) while the truncated shower approximation yields the dotted curve for the signal and dash-dotted curve for the QCD background.