A new approach to modelling elastic and inelastic photon-initiated production at the LHC: SuperChic 4

L. A. Harland-Lang,V. A. Khoze,M. Tasevsky,M. G. Ryskin

doi:10.1140/epjc/s10052-020-08455-0

Abstract

We present the results of the new SuperChic 4 Monte Carlo implementation of photon-initiated production in proton–proton collisions, considering as a first example the case of lepton pair production. This is based on the structure function calculation of the underlying process, and focusses on a complete account of the various contributing channels, including the case where a rapidity gap veto is imposed. We provide a careful treatment of the contributions where either (single dissociation), both (double dissociation) or neither (elastic) proton interacts inelastically and dissociates, and interface our results to Pythia for showering and hadronization. The particle decay distribution from dissociation system, as well the survival probability for no additional proton–proton interactions, are both fully accounted for; these are essential for comparing to data where a rapidity gap veto is applied. We present detailed results for the impact of the veto requirement on the differential cross section, compare to and find good agreement with ATLAS 7 TeV data on semi-exclusive production, and provide a new precise evaluation of the background from semi-exclusive lepton pair production to SUSY particle production in compressed mass scenarios, which is found to be low.

Highlights

A unique feature of the PI channel in proton–proton collisions is that the colour singlet photon exchange naturally leads to exclusive events, where the photons are emitted elastically from the protons, which remain intact
Once the survival factor is included, the fraction of double dissociation (DD) events is further reduced, due to the effect observed in Fig. 1, whereby the survival factor is significantly lower in this case
In this paper we have presented the results of a new MC implementation of PI production in proton–proton collisions

Summary

Introduction

A unique feature of the PI channel in proton–proton collisions is that the colour singlet photon exchange naturally leads to exclusive events, where the photons are emitted elastically from the protons, which remain intact. In all of the above cases, events are selected by imposing a veto on additional tracks associated with the dilepton vertex (with further cuts imposed to reduce non-exclusive backgrounds), which effectively corresponds to the requirement of a rapidity gap in the central detector, for which no additional particle production is present Such measurements are necessarily semi-exclusive in nature: that is, the event sample will contain events with inelastic photon emission from the proton, but where the dissociation products lie outside. This rapidity gap topology is rather far from the standard inclusive case, where no rapidity requirement is imposed The reasons for this are twofold: first, events where decay products from the proton dissociation system enter the veto region must be excluded, and second, there may be additional inelastic proton–proton QCD interactions (in other words, underlying event activity) that fill the gap region.

Objectives

Results

Conclusion