Photon-initiated production of a di-lepton final state at the LHC: cross section versus forward-backward asymmetry studies

TL;DR

This study investigates photon-induced dilepton production at the LHC, analyzing cross sections and forward-backward asymmetry to understand their roles in Z' searches, highlighting the importance of asymmetry due to its reduced systematic errors.

Contribution

It provides a detailed error analysis of photon-induced versus Drell-Yan dilepton production, emphasizing the robustness of forward-backward asymmetry in Z' searches despite large PDF uncertainties.

Findings

Photon-induced processes dominate above 3 TeV dilepton mass.

Large QED PDF uncertainties affect PI predictions significantly.

Forward-backward asymmetry remains a reliable observable despite PI uncertainties.

Abstract

We explore the effects of Photon Induced (PI) production of a dilepton final state in the Large Hadron Collider environment. Using QED Parton Distribution Function (PDF) sets we can treat the photons as real partons inside the protons and compare their yield directly to that of the Drell-Yan (DY) process. In particular, we concentrate on an error analysis of the two mechanisms. In order to do so, we use the NNPDF set, which comes with a set of replicas to estimate the systematic PDF error. On the one hand, we find that the PI contribution becomes dominant over DY above a dilepton invariant mass of 3 TeV. On the other hand, the PI predictions are affected by a large error coming from the QED PDFs, well above the one affecting the DY mode. We assess the impact of these uncertainties in the context of resonant and non-resonant searches for a neutral massive vector boson (Z') through the differential cross section and Forward-Backward Asymmetry (AFB) observables as a function of the dilepton invariant mass. While the former is subject to the aforementioned significant residual errors the latter shows the systematic error cancellation expected (recall that AFB is a ratio of cross sections) even in presence of PI contributions, so that the recently emphasized key role played by AFB as a valid tool for both Z' discovery and interpretation in both resonant and non-resonant mode is further consolidated.