Vector-boson pair production and electroweak corrections in HERWIG++

TL;DR

This work provides a comprehensive treatment of next-to-leading-order electroweak corrections to vector-boson pair production at the LHC, including mass effects and leptonic decays with spin correlations. It introduces a practical, factorized method to combine EW corrections with state-of-the-art QCD predictions via hat{s}- and hat{t}-dependent K-factors, and implements this approach in the HERWIG++ generator. The results show sizeable EW distortions at high invariant masses and momenta, while a simple a posteriori reweighting scheme maintains accuracy to ~1% for key observables. The methodology enables robust, precision-level predictions and can be extended to V+jet processes, contingent on improved photon PDFs.

Abstract

The detailed study of vector-boson pair production processes at the LHC will lead to a better understanding of electroweak physics. As pointed out before, a consistent inclusion of higher-order electroweak effects in the analysis of corresponding experimental data may be crucial to properly predict the relevant phenomenological features of these important reactions. Those contributions lead to dramatic distortions of invariant-mass and angular distributions at high energies, but may also significantly affect the cross section near threshold, as is the case e.g. for Z-pairs. For this reason, we present an analysis of the next-to-leading-order electroweak corrections to WW, WZ and ZZ production at the LHC, taking into account mass effects as well as leptonic decays. Hence, our predictions are valid in the whole kinematic reach of the LHC and, moreover, respect the spin correlations of the leptonic decay products at NLO accuracy. Starting from these fixed-order results, a simple and straight-forward method is motivated to combine the electroweak corrections with state-of-the-art Monte Carlo predictions, focusing on a meaningful combination of higher-order electroweak and QCD effects. To illustrate our approach, the electroweak corrections are implemented in the HERWIG++ generator, and their phenomenological effects within a QCD environment are studied explicitly.

Figures (15)

• Figure 1: Left: Differential LO cross sections for W-pair production at LHC14. Right: various EW corrections relative to the quark-induced LO process. Top: invariant-mass distribution; Bottom: WW rapidity-gap distribution for $M_{{\rm WW}} > 1$ TeV. The results presented here are obtained in the default setup of Ref. Bierweiler:2012kw.
• Figure 2: Left: Differential LO cross sections for ${\rm W}$ W$^+{\rm Z}$ Z$$ production at LHC14. Right: various EW corrections relative to the quark-induced LO process. Top: invariant-mass distribution; Bottom: WZ rapidity-gap distribution for $M_{{\rm WZ}} > 1$ TeV. The results presented here are obtained in the default setup of Ref. Bierweiler:2013dja.
• Figure 3: Left: Differential LO cross sections for ZZ production at LHC14. Right: various EW corrections relative to the quark-induced LO process. Top: invariant-mass distribution; Bottom: ZZ rapidity-gap distribution for $M_{{\rm ZZ}} > 1$ TeV. The results presented here are obtained in the default setup of Ref. Bierweiler:2013dja.
• Figure 4: Various differential distributions for on-shell Z-pair and $\gamma$-pair production at LHC14. Left: LO predictions; right: relative weak corrections ($\delta^{VV}_{{\rm weak}}$) and the full set of electroweak corrections ($\delta^{VV}_{{\rm EW}}$), including QED contributions. All results are obtained in the default setup defined in Ref. Bierweiler:2013dja.
• Figure 5: Various differential distributions for $e^+e^-\mu^+\mu^-$ production at LHC13. Left: The full LO prediction as well as NWA and DPA are shown; right: relative weak corrections $\delta_{{\rm weak}}^{{\rm full}}$ evaluated in the NWA, including spin correlations; weak corrections evaluated with unpolarized $2 \to 2$$K$-factors ($\delta_{{\rm weak}}^{{\rm unpol}}$); relative deviations of NWA and DPA w.r.t. the full LO are also shown.