A POWHEG generator for di-jet production in polarized proton-proton collisions

TL;DR

This work delivers a first fully developed NLO+PS Monte Carlo generator for di-jet production in longitudinally polarized proton–proton collisions, implemented within the POWHEG BOX V2 framework. By extending the POWHEG formalism to polarized initial states, it introduces a polarized Sudakov and the appropriate polarized counterparts of fixed-order ingredients, enabling realistic event generation and showering for spin-sensitive observables. The authors validate the implementation against existing fixed-order results and cross-check with MadGraph-based amplitudes, demonstrating consistent behavior across fixed-order, hardest-emission, and showered predictions. Phenomenological studies for STAR measurements at RHIC energies show overall good agreement at NLO, with parton-shower effects providing improvements in certain kinematic regions and the ability to access spin-dependent parton distributions more reliably. This polarized di-jet generator thus offers a robust, publicly available tool for RHIC spin analyses and future global PDF studies of the proton’s spin structure.

Abstract

We present a new Monte-Carlo generator for the simulation of di-jet production in polarized proton-proton collisions at the next-to-leading order in QCD matched to parton showers using the framework of the POWHEG BOX. With this program we compute a variety of observables of immediate relevance for the spin program of the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. While parton-shower effects are generally small, we find that in some search regions their inclusion improves agreement of predictions with data. Moreover, we provide a critical assessment of selection criteria applied in experiment in the light of perturbative stability.

Figures (9)

• Figure 1: NLO differential cross section as a function of the inclusive-jet transverse momentum $p_T^{\text{inc.\,jet}}$ (top left), pseudo-rapidity $\eta^{\text{inc.\,jet}}$ (top right), and the transverse-momenta of the two hardest jets $p_T^{\mathrm{jet\,1}}$, $p_T^{\mathrm{jet\,2}}$ (bottom left and right, respectively) for di-jet production at $\sqrt{s}=510~\text{GeV}$, for the cuts indicated in the respective panels, as obtained with our default POWHEG BOX (solid green) implementation, dFFSV code (dotted pink) and a MadGraph-based POWHEG BOX implementation (dot-dashed blue). In each case, the lower panels depict the ratio to the result from the standard POWHEG BOX implementation. Error bars indicate the numerical uncertainty.
• Figure 2: Differential cross section for di-jet production at $\sqrt{s}=510~\text{GeV}$ as a function of the inclusive-jet transverse momentum $p_T^{\text{inc.\,jet}}$ (left) and pseudo-rapidity $\eta^{\text{inc.\,jet}}$ (right) at NLO (green), LHE (yellow) and NLO+PS (pink) level, as obtained with our POWHEG BOX implementation. The lower panels display the ratio to the fixed-order NLO result. Error-bars indicate the numerical uncertainty from the Monte Carlo integration, while the bands indicate the theoretical uncertainty from the 7-point scale variation.
• Figure 3: Similar to Fig. \ref{['fig:LHEcomp_inc']} for the invariant mass of the di-jet system $M_{jj}$ (left) and the transverse momentum of the third hardest jet $p_T^{\mathrm{jet\,3}}$ (right). The cuts of Eq. \ref{['eq:star-cuts']} were applied.
• Figure 4: Similar to Fig. \ref{['fig:LHEcomp_Mjj']} for the transverse momenta of the two hardest jets.
• Figure 5: Inclusive jet cross section as a function of $\Delta p_T=(p_T^{\mathrm{jet\,1}}-p_T^{\mathrm{jet\,2}})$ for jets satisfying the same cuts as in Fig. \ref{['fig:LHEcomp_pt']} at the NLO (green), LHE (yellow), and NLO+PS (pink) levels.