Using gamma+jets Production to Calibrate the Standard Model Z(nunu)+jets Background to New Physics Processes at the LHC

TL;DR

This paper proposes a method to calibrate the Z(nunu)+jets background in LHC new physics searches using gamma+jets data, relying on theoretical predictions of the Z/gamma cross section ratio at high pT.

Contribution

It provides a detailed analysis of the Z/gamma ratio at high pT using different Monte Carlo tools, quantifies theoretical uncertainties, and demonstrates the method's potential accuracy.

Findings

Theoretical uncertainty in the ratio is about 7%.

Full event simulation uncertainties are small.

Overall method accuracy is estimated at around 10%.

Abstract

The irreducible background from Z(nunu)+jets, to beyond the Standard Model searches at the LHC, can be calibrated using gamma+jets data. The method utilises the fact that at high vector boson pT, the event kinematics are the same for the two processes and the cross sections differ mainly due to the boson-quark couplings. The method relies on a precise prediction from theory of the Z/gamma cross section ratio at high pT, which should be insensitive to effects from full event simulation. We study the Z/gamma ratio for final states involving 1, 2 and 3 hadronic jets, using both the leading-order parton shower Monte Carlo program Pythia8 and a leading-order matrix element program Gambos. This enables us both to understand the underlying parton dynamics in both processes, and to quantify the theoretical systematic uncertainties in the ratio predictions. Using a typical set of experimental cuts, we estimate the net theoretical uncertainty in the ratio to be of order 7%, when obtained from a Monte Carlo program using multiparton matrix-elements for the hard process. Uncertainties associated with full event simulation are found to be small. The results indicate that an overall accuracy of the method, excluding statistical errors, of order 10% should be possible.