Abstract
We consider Zγ production in hadronic collisions and present the first computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS). Spin correlations, interferences and off-shell effects are included by calculating the full process pp → ℓ+ℓ−γ. We extend the recently developed MiNNLOPS method to genuine 2 → 2 hard scattering processes at the LHC, which paves the way for NNLO+PS simulations of all diboson processes. This is the first 2 → 2 NNLO+PS calculation that does not require an a-posteriori multi-differential reweighting. We find that both NNLO corrections and matching to parton showers are crucial for an accurate simulation of the Zγ process. Our predictions are in very good agreement with recent ATLAS data.
Highlights
The theoretical description of fiducial cross sections and kinematic distributions has been greatly improved by the calculation of next-to-next-to-leading order (NNLO) corrections in QCD perturbation theory
We consider Zγ production in hadronic collisions and present the first computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS)
The inclusion of NNLO corrections to Zγ production into the Zγ+jet generator is discussed in section 3, including details on the extraction of the twoloop amplitude, a general discussion on how we extended the MiNNLOPS approach to 2 → 2 processes, and further practical details relevant for the specific case of the Zγ MiNNLOPS generator
Summary
We calculate NLO+PS predictions for Zγ and Zγ+jet using the POWHEG method, which is based on the following master formula [64, 65, 87]: dσ dΦB. For the practical implementation of the Zγ and Zγ+jet generators we exploit the POWHEG-BOX-RES framework [88], which takes into account the different resonance structures of each process. The key idea behind the algorithm used in the POWHEG-BOX-RES framework is to decompose the cross section into contributions associated to a well-defined resonance structure, which are enhanced on that particular cascade chain. As discussed, both Zγ and Zγ+jet production have two different resonance histories, which can be associated to q-type diagrams, where the photon is emitted from the quark/antiquark line, and -type ones, where the final state photon is radiated off one of the two leptons. It is important to note that, for each of the full real flavour structuresR the mappings from the real to the Born configurations preserve the virtualities of the intermediate resonances, which is crucial to guarantee a cancellation of singularities between real corrections and their counterterms
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