On the excess in the inclusive $W^+W^-\rightarrow l^+l^-ν\barν$ cross section

TL;DR

The paper tackles an apparent excess in the inclusive W+W- cross section reported by LHC experiments by first focusing on fiducial cross sections and their comparison to NLO QCD, finding good agreement within about 1σ. It then analyzes how the fiducial results are extrapolated to inclusive predictions, showing that POWHEG-driven extrapolation overestimates the inclusive rate via jet-veto and shower effects, while NNLO/NNLL corrections to fiducial predictions remain small. The authors estimate that incorporating NNLL+NNLO effects would maintain or improve the agreement with data and advocate for exclusive NNLO+PS tools to reduce extrapolation uncertainties. They also highlight the need to better treat the gg-initiated channel and jet-veto uncertainties to achieve robust high-precision WW predictions.

Abstract

In this note we analyse the excess in the $W^+W^-$ inclusive cross section recently measured by the LHC experiments. We point out that in fact for the ATLAS fiducial cross sections there is no excess in the measurements compared to the NLO QCD predictions. We also argue that higher order effects to the fiducial cross section are small, and tend to cancel each other, hence the inclusion of NNLO and NNLL corrections will not modify this agreement significantly. We find that at 8 TeV a substantial part of the disagreement with the NLO prediction for the total cross section observed by ATLAS is due to the extrapolation carried out with POWHEG.

Figures (5)

• Figure 1: Jet-veto efficiency for $W^+W^-$ production obtained with the POWHEG BOX both at pure NLO and matched to parton shower simulated with Pythia 6.4.28. For comparison, different tunes are shown.
• Figure 2: Jet-veto efficiency for $Z$ production obtained with the POWHEG BOX and showered with Pythia (v. 6.4.28 Perugia tune 350).
• Figure 3: Jet-veto efficiency for single $Z$ (left) and $H$ (right) production. The uncertainty band for the resumed predictions is obtained by varying $\mu_R$, $\mu_F$, and the resummation scale $Q$ by a factor of two in either direction while keeping $1/2<\mu_R/\mu_F<2$. Moreover, the matching scheme for the jet-veto efficiency is also varied as shown in ref. Banfi:2012jm. The dashed black lines denote the $p_{\rm t,veto}$ values relevant for the present analysis (see text for more details).
• Figure 4: Efficiencies with a cut in either leading lepton's $p^{\nu+\bar{\nu}}_{t}$ (left) or $p^{\nu+\bar{\nu}}_{t,{\rm Rel}}$ (right). The plots report the comparison between LHE events and the pure NLO prediction, showing a systematic reduction at the LHA level for the actual fiducial cuts (black dashed lines).
• Figure 5: As in Figure \ref{['fig:eff-res']}, but the jet-veto efficiency for single $Z$ (left) and $H$ (right) production are both obtained with invariant mass $M=222$ GeV. The dashed black line at $p_{\rm t,veto}=25$ GeV denotes the value relevant for the $q\bar{q} \to W^+W^-$ and $gg\to W^+W^-$ in Eq. \ref{['eq:fid-est']}. For the $gg\to H\to W^+W^-$ the result is unchanged.