Missing Energy and Jets for Supersymmetry Searches

TL;DR

This work extends prior Z/γ ratio studies to Z+3-jet and γ+3-jet production at NLO in QCD, enabling improved data-driven estimates of Z+jets backgrounds in MET+jets SUSY searches. By comparing fixed-order NLO results with ME+PS predictions and exploring different CMS-like control and search regions, the authors demonstrate that Z/γ ratios remain robust (around 10% or smaller uncertainty) even with an extra jet. They meticulously assess QCD and electroweak uncertainties, finding strong cancellation of scale dependence in the ratios and only modest EW effects, which supports using γ+jets as a proxy for the Z+jets background. The study also documents methodological adaptations to SHERPA to ensure consistent treatment of Z and γ, reinforcing the viability of photon-based background estimation in LHC analyses.

Abstract

We extend our investigation of backgrounds to new physics signals, following CMS's data-driven search for supersymmetry at the LHC. The aim is to use different sets of cuts in gamma + 3-jet production to predict the irreducible Z + 3-jet background (with the Z boson decaying to neutrinos) to searches with missing transverse energy + 3-jet signal topologies. We compute ratios of Z + 3-jet to gamma + 3-jet production cross sections and kinematic distributions at next-to-leading order (NLO) in alpha_s. We compare these ratios with those obtained using a parton shower matched to leading-order matrix elements (ME+PS). This study extends our previous work [arXiv:1106.1423 [hep-ph]] on the Z + 2-jet to gamma + 2-jet ratio. We find excellent agreement with the ratio determined from the earlier NLO results involving two instead of three jets, and agreement to within 10% between the NLO and ME+PS results for the ratios. We also examine the possibility of large QCD logarithms in these processes. Ratios of Z + n-jet to gamma + n-jet cross sections are plausibly less sensitive to such corrections than the cross sections themselves. Their effect on estimates of Z + 3-jet to gamma + 3-jet ratios can be assessed experimentally by measuring the gamma + 3-jet to gamma + 2-jet production ratio in search regions. We partially address the question of potentially large electroweak logarithms by computing the real-emission part of the electroweak corrections to the ratio using ME+PS, and find that it is 1% or less. Our estimate of the remaining theoretical uncertainties in the Z to gamma ratio is in agreement with our earlier study.

Figures (4)

• Figure 1: Squark pair production illustrates a new-physics process with the signature of three jets plus MET. Here each squark decays to a quark and the lightest neutralino; the escaping neutralinos generate the missing transverse energy.
• Figure 2: Sample virtual diagrams needed for (a) $pp \rightarrow Z(\rightarrow \nu\bar{\nu})+3$-jet production and for (b) $pp \rightarrow \gamma+3$-jet production.
• Figure 3: ptThe $Z\,\!+\,3$-jet to $Z\,\!+\,2$-jet ratio as a function of $H_T^{\rm jet}$ and $H_T^{\rm jet}-|\textrm{MET}|$. The solid line shows where the ratio is roughly 0.5.
• Figure 4: The $H_T^{\rm jet}$ distribution for the ratio of $Z\,\!+\,3$-jet to $\gamma\,\!+\,3$-jet production for the different sets. We omit Sets 2 and 6, as these plots are subsets of those for Sets 3 and 4 respectively.