Driving Miss Data: Going up a gear to NNLO

TL;DR

The paper delivers NNLO QCD predictions for γ+j production and combines them with NLO electroweak corrections to compare with 8 TeV CMS data, finding improved shape agreement but a normalization tension. It introduces a precise Z/γ ratio at NNLO (with EW effects) as a robust tool to normalize Z→νν+jets backgrounds, significantly reducing theoretical uncertainties. The authors also analyze the γ+2j/γ+j ratio and provide 13 TeV predictions for the Z/γ ratio, emphasizing the practical impact on background estimation for DM and SUSY searches at the LHC.

Abstract

In this paper we present a calculation of the $γ+j$ process at next-to-next-to-leading order (NNLO) in QCD and compare the resulting predictions to 8 TeV CMS data. We find good agreement with the shape of the photon $p_T$ spectrum, particularly after the inclusion of additional electroweak corrections, but there is a tension between the overall normalization of the theoretical prediction and the measurement. We use our results to compute the ratio of $Z(\to \ell^+\ell^-)+j$ to $γ+j$ events as a function of the vector boson transverse momentum at NNLO, a quantity that is used to normalize $Z(\rightarrowν\overlineν) +j$ backgrounds in searches for dark matter and supersymmetry. Our NNLO calculation significantly reduces the theoretical uncertainty on this ratio, thus boosting its power for future searches of new physics.

Figures (11)

• Figure 1: The dependence of the NNLO coefficient on the parameter $\tau^{{\rm{cut}}}_1$ for the processes considered in this paper. The cuts of the CMS analysis Khachatryan:2015ira have been applied. To aid visibility, the values of $\tau^{{\rm{cut}}}_1$ for the $\gamma+j$ calculation have been offset slightly.
• Figure 2: The photon $p_T$ spectrum for $\gamma+j$ at the 8 TeV LHC, at various orders in perturbation theory, compared to CMS data from ref. Khachatryan:2015ira. The lower panel shows the ratio of the data and the NLO prediction to the NNLO one. The bands indicate the scale uncertainty on the NLO and NNLO predictions.
• Figure 3: The ratio of the CMS data from ref. Khachatryan:2015ira to the NNLO prediction (with and without including EW effects) for the photon $p_T$ spectrum. The lower panel normalizes this ratio to the value of the ratio in the $[100,111]$ bin.
• Figure 4: A summary of the theoretical uncertainties discussed in this paper for the photon transverse momentum spectrum. In order from the top, uncertainties from: scales, PDFs, isolation and in the total, as described in the main text. The total uncertainty is obtained by combining linearly those from the sources above. The uncertainty due to the value of $\alpha_{EM}$ is indicated separately by the dashed line in the lower figure.
• Figure 5: The quantities $R_{2/1}^{\rm{NLO}}(p_T^\gamma)$ and $R_{2/1}^{\rm{NNLO}}(p_T^\gamma)$ compared to CMS data from ref. Khachatryan:2015ira. The bands indicate the scale uncertainty on the theoretical predictions.