Longitudinal momentum shifts, showering and nonperturbative corrections in matched NLO-shower event generators

TL;DR

This work addresses how to incorporate nonperturbative effects and parton showering consistently in NLO–matched event generators for jet-dominated collider processes. It proposes a decomposition of corrections into a nonperturbative component and an all-orders shower component, introducing correction factors $K^{NP}$ and $K^{PS}$ computed with NLO-MC tools to ensure consistency with fixed-order calculations. Through detailed examples, it shows that showering corrections remain significant across the jet $p_T$ spectrum and rapidity, particularly at forward rapidities, and that longitudinal momentum reshuffling from initial-state showering can substantially alter parton momentum fractions $x$—with implications for predictions and PDF determinations. The study highlights the practical impact on jet distributions and PDF fits, and points toward non-collinear frameworks, such as TMD/uPDF approaches, to more faithfully incorporate perturbative and nonperturbative QCD dynamics in event generation.

Abstract

Comparisons of experimental data with theoretical predictions for collider processes containing hadronic jets rely on shower Monte Carlo event generators to include corrections to perturbative calculations from hadronization, parton showering, multiple parton collisions. We examine current treatments of these corrections and propose alternative methods to take into account nonperturbative effects and parton showering in the context of next-to-leading-order (NLO) event generators. We point out sizeable parton-showering corrections to jet transverse energy spectra at high rapidity, and discuss kinematic shifts in longitudinal momentum distributions from initial state showering in the case both of jet production and of heavy mass production at the Large Hadron Collider.

Figures (10)

• Figure 1: The NP correction factors to jet transverse momentum distributions obtained from Eq. (\ref{['npK1']}) and Eq. (\ref{['npK2']}), using Pythia and Powheg respectively, for $|y|<0.5$ and $2 < |y|< 2.5$. Left: $R=0.5$; Right: $R=0.7$.
• Figure 2: The initial and final state parton shower correction factor to jet transverse momentum distributions, obtained from Eq. (\ref{['npK3']}) using Powheg for $|y|<0.5$ and $2 < |y|< 2.5$. Left: $R=0.5$; Right: $R=0.7$.
• Figure 3: The parton shower correction factor to jet transverse momentum distributions, obtained from Eq. (\ref{['npK3']}) using Powheg for $|y|<0.5$ and $2 < |y|< 2.5$. Left: $R=0.5$; Right: $R=0.7$.
• Figure 4: Factorized structure of the jet cross section at high rapidity.
• Figure 5: Distributions in the parton longitudinal momentum fraction $x$ before (POWHEG) and after parton showering (POWHEG+PS), for inclusive jet production at different rapidities for jets with $p_T> 18$ GeV obtained by the anti-kt jet algorithm antiktalgo with $R=0.5$. Shown is the effect of intrinsic $k_t$, initial (IPS) and initial+final state (IFPS) parton shower.