Nonperturbative effects in triple-differential dijet and Z+jet production at the LHC

TL;DR

This paper addresses nonperturbative corrections in triple-differential dijet and Z+jet production at the LHC to improve precision benchmarks for $\alpha_s$ and proton PDFs. It defines a triple-differential phase space and computes NP corrections using Monte Carlo generators (e.g., Herwig7, Sherpa) by comparing ME+PS+Had+MPI to ME+PS and parameterizing fluctuations with $f(x)=a\ln(x/\text{GeV})^b+c$. The key finding is that NP corrections are not universal across processes: Z+jet exhibits a strong $y^*$ dependence, while dijet corrections are largely insensitive to $y^*$, with part of the effect arising from perturbative modelling (MPI and multi-jet merging) rather than purely NP physics. The authors advocate a triple-differential UE measurement to disentangle NP and perturbative contributions, and they emphasize that accurate Z+jet modelling may require including additional jets via merging at NLO to obtain reliable correction factors for precision QCD studies.

Abstract

In comparisons of precision measurements at colliders to the most accurate predictions available in perturbative quantum chromodynamics (QCD), it is required to correct for nonperturbative effects. By means of Monte Carlo event generators this article investigates the impact of such nonperturbative effects on two processes relevant for precision determinations of the strong coupling constant and the proton structure: triple-differential dijet and Z+jet production. We observe significant differences between the two processes. Whether this non-universal behaviour is realized in nature remains an open question. We therefore propose a triple-differential measurement of the underlying event in Z+jet production.

Figures (16)

• Figure 1: Illustrations of Z+jet (left) Horzela2023 and dijet production (right). In the Z+jet case, an oppositely-charged muon pair and at least one jet are required. For the dijet case, a second jet replaces the muon pair.
• Figure 2: Visualisation of the analysed Z+jet phase space in $y_\text{b}$ and $y^*$ as defined in Eqs. \ref{['eq:yboost']}-\ref{['eq:ystar']}. The bins along the $y_\text{b}$ and $y^*$ axes discussed in the main part of this work are shown with coloured boxes.
• Figure 3: $C_\text{NP}$ vs. dijet $\langle{}p_\text{T}\rangle_\text{1,2}$ using LO (dotted) and NLO MEs (solid lines) in Herwig7 predictions for anti-$k_\text{T}$ jets with $R=0.4$. Bands indicate the statistical uncertainty. The fitted curves are shown for a series of phase space intervals increasing in $y^*$ for $y_\text{b}\xspace < 0.5$ (left) and increasing in $y_\text{b}$ for $y^*\xspace < 0.5$ (right). The factors are shifted with respect to unity by constant offsets.
• Figure 4: Same as Fig. \ref{['fig:dijetAK4-np-herwig-lo-nlo']} but vs. $p_\text{T,Z}$ in Z+jet production.
• Figure 5: Comparison of $C_\text{NP}$ vs. $p_\text{T,Z}$ between Herwig7 (dotted & solid lines) and Sherpa predictions (dash-dotted & dashed lines) using LO (top row) and NLO MEs (bottom row) for anti-$k_\text{T}$ jets with $R=0.4$. Bands indicate the statistical uncertainty. The fitted curves are shown for a series of phase space intervals increasing in $y^*$ for $y_\text{b}\xspace < 0.5$ (left) and increasing in $y_\text{b}$ for $y^*\xspace < 0.5$ (right). The factors are shifted with respect to unity by constant offsets.