The NNLO QCD corrections to Z boson production at large transverse momentum

A Gehrmann-De Ridder,E.W.N Glover,T.A Morgan,T Gehrmann,A Huss

doi:10.1007/jhep07(2016)133

Abstract

The transverse momentum distribution of massive neutral vector bosons can be measured to high accuracy at hadron colliders. The transverse momentum is caused by a partonic recoil, and is determined by QCD dynamics. We compute the single and double-differential transverse momentum distributions for fully inclusive $Z/\gamma^*$ production including leptonic decay to next-to-next-to-leading order (NNLO) in perturbative QCD. We also compute the same distributions normalised to the cross sections for inclusive $Z/\gamma^*$ production, i.e. integrated over the transverse momentum of the lepton pair. We compare our predictions for the fiducial cross sections to the 8 TeV data set from the ATLAS and CMS collaborations, which both observed a tension between data and NLO theory predictions, using the experimental cuts and binning. We find that the inclusion of the NNLO QCD effects does not fully resolve the tension with the data for the unnormalised $p^Z_T$ distribution. However, we observe that normalising the NNLO $Z$-boson transverse momentum distribution by the NNLO Drell-Yan cross section substantially improves the agreement between experimental data and theory, and opens the way for precision QCD studies of this observable.

Highlights

Z pZT = 0 recoil and numerically stable code to compute the transverse momentum distribution of the Z boson at finite transverse momentum at next-to-next-to-leading order (NNLO) precision
In order to reduce the systematic uncertainty on the measurement, the transverse momentum distribution is commonly normalised to the pT -inclusive Z-boson production cross section
The inclusion of the newly derived NNLO corrections to the transverse momentum distribution are crucial for a meaningful comparison between data and theory: (a) they reduce the theory uncertainty to a level that firmly establishes the discrepancy on the absolute cross sections, and (b) they modify the central value and the shape of the theory prediction to better agree with the data on the normalised distributions

Summary

The Z boson transverse momentum distribution

The experimental measurements of Z-boson production at finite transverse momentum are presented in the form of fiducial cross sections over a restricted phase space for the final state leptons, which is fully contained in the detector’s coverage. The NNLO corrections to the transverse momentum distribution can be compared to data by considering the same cuts to the lepton kinematics as presented in the ATLAS [38] and CMS [40] analyses using. We will focus on the ATLAS measurement in the fiducial region defined by a broad dilepton invariant mass window around the Z resonance, 66 GeV < m < 116 GeV, and compare both the absolute and normalised pZT distribution to the experimental data. It can be expected that the inclusion of electroweak corrections will further improve the prediction of the shape in the tail This region of the distribution is still dominated by the statistical uncertainties of the experimental data and a fully consistent inclusion of electroweak corrections is beyond the scope of this work

Double-differential distributions

Findings

Summary and conclusions