Practical improvements and merging of POWHEG simulations for vector boson production

TL;DR

The article develops and validates MENLOPS implementations that combine POWHEG NLOPS accuracy with all-orders Sudakov resummation for vector boson production and vector boson plus jet processes. By constructing two MENLOPS components—one for inclusive vector boson production and one for vector boson plus jet production—and merging them with a phase-space partition defined by a merging scale $p_T^{merge}$, the authors achieve reliable predictions for both inclusive and jet-inclusive observables. They introduce a low-$p_T$ resummation mechanism and carefully constrain the fraction of LO-like events to preserve NLO accuracy, demonstrating improved agreement with Tevatron and LHC data and a reduced dependence on the unphysical merging scale. The work provides a practical, modular approach to exploiting existing NLO+PS tools to obtain robust, versatile event samples that describe a wide range of observables. The methods and merging strategy show promise for broader application to other processes and higher jet multiplicities, with future prospects toward NNLO+PS matching.

Abstract

In this article we generalise POWHEG next-to-leading order parton shower (NLOPS) simulations of vector boson production and vector boson production in association with a single jet, to give matrix element corrected MENLOPS simulations. In so doing we extend and provide, for the first time, an exact and faithful implementation of the MENLOPS formalism in hadronic collisions. We also consider merging the resulting event samples according to a phase space partition defined in terms of an effective jet clustering scale. The merging scale is restricted such that the component generated by the associated production simulation does not impact on the NLO accuracy of inclusive vector boson production observables. The dependence of the predictions on the unphysical merging scale is demonstrated. Comparisons with Tevatron and LHC data are presented.

Figures (19)

• Figure 1: Here we display Powheg (red line) and Menlops$^{\text{$\infty$}}$ (cyan triangles) predictions for inclusive observables. In the upper pair of plots we show the Z boson mass spectrum and the resulting lepton pseudorapidity spectrum assuming a Tevatron collider configuration with a centre-of-mass energy $\sqrt{s}=1.96\,\mathrm{TeV}$, on the left and on the right respectively. The lower plots show the W boson rapidity distribution (left) and the charge asymmetry (right), given $pp$ beams at $\sqrt{s}=7\,\mathrm{TeV}$. The yellow band corresponds to the projection of the Monte Carlo statistical errors on the reference data (the first entry in the legend) onto the ratio plot.
• Figure 2: Powheg (red line) and Menlops$^{\text{$\infty$}}$ (cyan triangles) predictions for the low $p_{{ \mathrm{T}}}$, Sudakov peak region of the vector boson transverse momentum spectrum. On the left the distributions have been obtained for the case of Z boson production in $\sqrt{s}=1.96\,\mathrm{TeV}$$p\bar{p}$ collisions, while on the right they correspond to W production in $\sqrt{s}=7\,\mathrm{TeV}$$pp$ reactions.
• Figure 3: Powheg (red line) and Menlops$^{\text{$\infty$}}$ (cyan triangles) predictions for the integrated $0$-jet cross section as a function of the $k_{{ \mathrm{T}}}$-jet clustering scale. As in Fig. \ref{['fig:menlops_infty_validation_sudakov_peak']} the left plot depicts results for Z boson production in $\sqrt{s}=1.96\,\mathrm{TeV}$$p\bar{p}$ collisions, while those in the right-hand plot correspond to W production in $\sqrt{s}=7\,\mathrm{TeV}$$pp$ reactions.
• Figure 4: Powheg (red line) and Menlops$^{\text{$\infty$}}$ (cyan triangles) predictions for the leading jet transverse momentum (left) and rapidity spectrum (right) in W boson production events, assuming current LHC $pp$ beam energies.
• Figure 5: In this figure we show predictions for observables sensitive to the emission of at least one hard jet in Z production at the Tevatron and W production at the LHC. The colouring is as in the preceding plots in this section, up to the addition of the blue lines, which correspond to predictions made with the vector boson plus jet Powheg simulations.