Driving Missing Data at Next-to-Leading Order

TL;DR

The paper tackles the challenge of estimating irreducible Z(νν)+jets backgrounds in MET+jets searches at the LHC by evaluating gamma+2j as a proxy. It performs NLO QCD calculations for both γ+2j and Z+2j (and a LO ME+PS counterpart), using Catani-Seymour subtraction and BlackHat/SHERPA tools, while carefully treating photon isolation with a Frixione-like criterion. The key finding is that the photon-to-Z ratios are robust across calculational schemes and uncertainties, with conversion uncertainties below ~10%, making γ+jets a reliable data-driven estimator for Z(νν)+jets backgrounds. This has practical impact for CMS and similar experiments, enabling more accurate background estimates and extending to higher jet multiplicities in future work.

Abstract

The prediction of backgrounds to new physics signals in topologies with large missing transverse energy and jets is important to new physics searches at the LHC. Following a CMS study, we investigate theoretical issues in using measurements of gamma + 2-jet production to predict the irreducible background to searches for missing energy plus two jets that originates from Z + 2-jet production where the Z boson decays to neutrinos. We compute ratios of gamma + 2-jet to Z + 2-jet production cross sections and kinematic distributions at next-to-leading order in alpha_s, as well as using a parton shower matched to leading-order matrix elements. We find that the ratios obtained in the two approximations are quite similar, making gamma + 2-jet production a theoretically reliable estimator for the missing energy plus two jets background. We employ a Frixione-style photon isolation, but we also show that for isolated prompt photon production at high transverse momentum the difference between this criterion and the standard cone isolation used by CMS is small.

Figures (9)

• Figure 1: Squark pair production illustrates a new physics process with the signature of two jets plus MET. Here each squarks decays to a quark and the lightest neutralino; the escaping neutralinos generate the missing transverse energy.
• Figure 2:
• Figure 3: A comparison of different NLO theoretical predictions for $\gamma+X$ production at the LHC at 7 TeV. The CMS data points CMSInclusivePhoton are shown (red) with combined experimental uncertainties; the prediction using using the Gordon-Vogelsang (GV) code and the Frixione isolation criterion is given by the dashed (blue) line; the JetPhoX prediction using a standard cone isolation is given by the solid (black) line. The lower panel shows the two theoretical predictions normalized to the CMS data along with the scale dependence (shaded gray) as determined using JetPhoX.
• Figure 4: A comparison of four different NLO predictions for $\gamma + X$ production at the LHC at 7 TeV. All predictions are normalized to the JetPhoX predictions shown in fig. \ref{['fig:gammaIncsv_CMS_jetphox_GV']}. The prediction for a standard cone isolation using the Gordon-Vogelsang (GV) code is given by the dot-dashed (red) line; that for the Frixione isolation criterion using the GV code, by the dashed (blue) line; and that for the Frixione isolation using the BlackHat+Sherpa code, by the solid (black) line. The JetPhoX scale dependence band is shown in gray.
• Figure 5: ptThe $p_T$ distribution of the first jet. The left column shows distributions for the Set 1 cuts, and the right column for the Set 2 cuts. Each column displays the differential cross section for $Z\,\!+\,2$-jet production (top), $\gamma\,\!+\,2$-jet production (middle), and their ratio (bottom). In the top and middle plots, the upper panel shows the LO, NLO, and ME+PS results for the distribution, and the lower panel shows the ratio to the central NLO prediction, along with the LO and NLO scale-dependence bands. The numerical integration uncertainties are indicated by thin vertical lines.