The effect of real and virtual photons in the di-lepton channel at the LHC

Elena Accomando,Juri Fiaschi,Francesco Hautmann,Stefano Moretti,Claire H Shepherd-Themistocleous

doi:10.1016/j.physletb.2017.04.025

Elena Accomando, Juri Fiaschi + Show 3 more

Open Access

https://doi.org/10.1016/j.physletb.2017.04.025

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Abstract

We present a study of di-lepton production at the CERN Large Hadron Collider with a particular focus on the contribution resulting from both real and virtual photons in the initial state. We discuss the region of phase space in which the invariant mass of the lepton pair is of the order of several TeV, where searches for new physics phenomena yielding a di-lepton signature are presently carried out. We study both the yield and associated uncertainties for all possible topologies in photon-induced di-lepton production and compare these with what is expected in the standard Drell–Yan channel, where quark–antiquark pairs are responsible for the production of lepton pairs. We analyse the impact of these QED contributions on the expected Standard Model background and on searches for new physics. In this latter case, we use the production of an extra heavy Z′-boson predicted by the Sequential Standard Model (SSM) as a benchmark process.

Highlights

Photon-Initiated (PI) processes in hadron–hadron collisions can be very important at high invariant masses [1], they are relevant to searches for Beyond the Standard Model (BSM) processes
Photons are treated as on-shell partons residing inside the proton and participating in its internal dynamics governed by both Quantum Chromo-Dynamics (QCD) and Quantum Electro-Dynamics (QED) effects
In order to visualise the overall effect due to initial photon interaction, in Fig. 4(a) we plot the complete photon induced contribution to the di-lepton spectrum obtained by summing up the EPA, single dissociative (SD) and double dissociative (DD) terms for the MRST2004QED set and we compare it to the default DY differential cross section and to the inclusive PI predictions obtained with the NNPDF3.0QED, CT14QED_inc and LUXqed sets

Summary

Introduction

Photon-Initiated (PI) processes in hadron–hadron collisions can be very important at high invariant masses [1], they are relevant to searches for Beyond the Standard Model (BSM) processes. Different approaches are taken by different groups and different results are obtained, both in the prediction of the central value of the photon PDF and its error, for a given choice of x (the fraction of proton momentum carried by the photon) and Q 2 (the factorisation/renormalisation scale) An analysis of such results for the hadro-production of di-lepton pairs at the current Large Hadron Collider (LHC) was recently presented in [5,6] The layout of the paper is as follows: section 2 describes the PDF treatment of resolved photons, the EPA approach for lowvirtuality photons and the mixed process where one photon is resolved and the other is virtual; section 3 presents our results for the Standard Model (SM) while in section 4 we discuss the potential impact of PI processes on the search for new heavy Z -bosons.

Photon induced processes

QED effects on the SM di-lepton spectrum

QED effects on Z -boson searches

Findings

Conclusions