Automation of NLO QCD and EW corrections with Sherpa and Recola

TL;DR

The paper presents a fully automated framework combining Recola for one-loop amplitudes with Sherpa for event generation to deliver NLO QCD and EW predictions for arbitrary SM processes. It details the interface, validation against established codes, and demonstration through fixed-order and shower-merged results for key LHC processes, including Drell-Yan, Z-pair, ttH, and HZ production. The results show excellent agreement with independent tools at NLO QCD and NLO EW, and illustrate the utility of MEPS@NLO merging and combined QCD+EW predictions, including comparisons to experimental data. The work paves the way for publicly available, state-of-the-art SM predictions at NLO accuracy and highlights the framework's readiness for systematic EW corrections across a broad process landscape.

Abstract

This publication presents the combination of the one-loop matrix-element generator Recola with the multipurpose Monte Carlo program Sherpa. Since both programs are highly automated, the resulting Sherpa+Recola framework allows for the computation of -in principle- any Standard Model process at both NLO QCD and EW accuracy. To illustrate this, three representative LHC processes have been computed at NLO QCD and EW: vector-boson production in association with jets, off-shell Z-boson pair production, and the production of a top-quark pair in association with a Higgs boson. In addition to fixed-order computations, when considering QCD corrections, all functionalities of Sherpa, i.e. particle decays, QCD parton showers, hadronisation, underlying events, etc. can be used in combination with Recola. This is demonstrated by the merging and matching of one-loop QCD matrix elements for Drell-Yan production in association with jets to the parton shower. The implementation is fully automatised, thus making it a perfect tool for both experimentalists and theorists who want to use state-of-the-art predictions at NLO accuracy.

Figures (16)

• Figure 1: The transverse-momentum distribution of the reconstructed ${\rm Z}$ boson in Drell--Yan plus zero, one and two-jet production at NLO QCD.
• Figure 2: NLO QCD predictions of the di-muon (left) and 4-lepton invariant-mass (right) distributions in $\text{p}\xspace\text{p}\xspace \to \mu^+\mu^-\text{e}\xspace^+\text{e}\xspace^-$ at $\sqrt{s}=13\,\text{TeV}\xspace$. The plots show a direct comparison of Sherpa + Recola to the Monte Carlo program employed in Ref. Biedermann:2016lvg.
• Figure 3: Differential distributions at a centre-of-mass energy $\sqrt{s} = 13\,\text{TeV}\xspace$ for ${\rm pp} \to {\rm t \bar{t} H}$ at the LHC in a fully inclusive set-up: transverse momentum distribution of the top quark (left) and rapidity distribution of the top quark (right).
• Figure 4: The transverse-momentum distribution of the reconstructed $\text{b}\xspace \bar{\text{b}\xspace}$-decayed $\text{H}\xspace$ boson in the $\text{p}\xspace\text{p}\xspace \to {\rm HZ}$ process at NLO QCD.
• Figure 5: The left-hand figure compares the MEPS@NLO predictions based on Sherpa + Recola for the transverse-momentum distribution of the reconstructed ${\rm Z}$ boson compared to $\sqrt{s}=7\,\text{TeV}\xspace$ LHC data from ATLAS Aad:2013ysa. The right-hand figure shows the inclusive jet multiplicity in comparison to $\sqrt{s}=7\,\text{TeV}\xspace$ CMS data Khachatryan:2014zya. The theoretical-uncertainty band is obtained through 7-point scale variations of $\mu_{\rm R}$ and $\mu_{\rm F}$. The yellow bands in the lower panels indicate the measurements combined statistical and systematic uncertainty.