NNLO QCD predictions of the asymmetry probe of the Z γ pair-production process

Abstract

The paper presents for the first time a novel idea of exploiting asymmetry between differential cross sections of the off-shell Zγ pair-production in proton-proton (pp) collisions for the final states of a charged-lepton pair plus a photon pp → Zγ → l+l-γ (leptonic decay) and of a neutrino pair plus a photon pp → Zγ → ννγ (invisible decay). Asymmetry between the leptonic and invisible decays of the Zγ process is investigated by using fixed-order predictions through inclusion of next-to-next-to-leading (NNLO) radiative corrections in quantum chromodynamics (QCD) perturbation theory. NNLO QCD predictions are presented at various pp-collision energies as functions of several key observables including transverse momenta and azimuthal-angle separation, regarding the Zγ decay products. The predicted distributions for the Zγ asymmetry are provided based on realistic fiducial phase-space requirements in line with the related hadron-collider measurements. The predicted distributions are assessed at various pp-collision energies and in different phase-space regions such as with higher lepton-pair invariant mass ml+l- or higher neutrino-pair transverse momentum pT νν requirements. The Zγ asymmetry is shown to be significantly sensitive in different regions of phase space including high-ml+l- and high-pT νν regions. The Zγ asymmetry can therefore be translated into an important quantity for probing deviation from the Standard Model (SM) predictions. In this regard, the asymmetry probe is proposed as a sensitive indicator for indirect searches for physics beyond the SM encompassing high-mass resonances and dark-matter sector.