Polarized Deep-Inelastic Scattering with Spin Correlations in Herwig 7

Abstract

We investigate polarized deep-inelastic scattering (DIS) in the context of fully exclusive Monte Carlo simulations for the Electron-Ion Collider (EIC). We present a next-to-leading-order (NLO) treatment of polarized DIS in HERWIG 7 using the POWHEG matching scheme, including neutral-current $\gamma /Z$ exchange and charged-current scattering. We also construct a spin-correlation treatment for NLO-matched events, first by initializing the shower from the spin-density matrix of the polarized DIS Born process and then by propagating the spin-density matrix of the accepted real-emission configuration without changing the POWHEG event weight or hardest-emission choice. We validate the integrated cross sections against fixed-order calculations, use parton-level comparisons without subsequent shower evolution to isolate the accepted POWHEG real-emission kinematics, and employ shower-level observables to test the numerical importance of the Born-level and real-emission spin information. We find that the Born-level spin-density initialization can have a visible impact on shower-sensitive observables, while the additional spin information from the accepted real-emission configuration is not resolved as a robust separate effect within the independent-sample Monte Carlo precision for the longitudinally polarized observables considered here.

55 pages, 17 figures, 6 tables. Updated shower-spin study using independent physical-helicity samples and covariance-aware uncertainties; replaced the spin-correlation figures and clarified the associated conclusions. Corrected the report number and NNLO references