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

TL;DR

This work tackles the challenge of accurately predicting the Z(νν)+jets background for LHC new-physics searches by exploiting γ+jets data. It evaluates the high-pT Z/γ cross-section ratio using exact multijet LO matrix elements (Gambos) and LO parton-shower results (Pythia8) for V+jets with up to 3 jets, and quantifies the dominant theoretical uncertainties from PDFs and scales. The study finds a total ratio uncertainty around 7% (roughly 10% when excluding statistics) and shows full-event simulation effects are small, supporting a data-driven background estimate via Z/γ. It also demonstrates a practical SUSY-zero-lepton background estimation method by translating γ events into Z→νν predictions, and discusses the potential impact of non-cancelling electroweak Sudakov corrections at very high pT requiring further study.

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.

Figures (14)

• Figure 1: Sample Feynman diagrams for $V + 1,2$ jets production, where $V=\gamma, Z$.
• Figure 2: \ref{['fig:ud:ud']} Dependence of $R = \sigma(Z)/\sigma(\gamma)$ at the coupling constant level on the ratio of average $d$ and $u$ parton distribution values, see eq. \ref{['eq:ud']}. \ref{['fig:ud:loud']}$u$ and $d$ quark MSTW2008LO PDFS Martin:2009iq at $Q^2 = 10^4$ GeV$^2$.
• Figure 3: \ref{['fig:gam:pt']}$Z$ and $\gamma$$p_T$ distributions at $\sqrt{s}= 7$ TeV and 14 TeV, with parameters and cuts as described in the text. \ref{['fig:gam:rat']} Ratio of the $Z$ and $\gamma$$p_T$ distributions.
• Figure 4: \ref{['fig:pyt:pteCM']} Differential cross section as a function of the vector boson $p_T$ for the process $pp\to V+1$ parton with $V=\gamma, Z$ from Pythia8, and \ref{['fig:pyt:rateCM']} the ratio of these.
• Figure 5: Effect on the boson $p_T$ and the ratio from using a matrix--element generator (Gambos) and Pythia8 at 7 and 14 TeV.