Observing light-by-light scattering at the Large Hadron Collider

TL;DR

A measurement of elastic light-by-light scattering with no substantial background in Pb-Pb collisions is presented, where one expects about 20 signal events per run, after typical detector acceptance and reconstruction efficiency selections.

Abstract

Elastic light-by-light scattering, $γγ\toγγ$, is open to study at the Large Hadron Collider thanks to the large quasi-real photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The $γγ\toγγ$ cross sections for diphoton masses $m_{γγ} > 5$ GeV amount to 105 fb, 260 pb, and 370 nb in p-p, p-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 14 TeV, 8.8 TeV, and 5.5 TeV respectively. Such a measurement has no substantial backgrounds in Pb-Pb collisions where one expects about 70 signal events per run, after typical detector acceptance and reconstruction efficiency selections.

Figures (2)

• Figure 1: Schematic diagram of elastic $\gamma\,\gamma\to\gamma\,\gamma$ collisions in electromagnetic proton and/or ion interactions at the LHC. The initial-state photons are emitted coherently by the protons and/or nuclei which survive the electromagnetic interaction.
• Figure 2: Left: Cross sections for exclusive $\gamma\,\gamma\to\gamma\,\gamma$, with pair masses above 5 GeV, in ultraperipheral Pb-Pb (top curve), p-Pb (middle) and p-p (bottom) collisions as a function of the nucleon-nucleon c.m. energy in the range $\sqrt{s_{_{\rm NN}}}$ = 1--20 TeV. Right: Stacked diphoton yields as a function of invariant mass for elastic $\gamma\,\gamma$ and backgrounds (CEP $\gamma\,\gamma$ and QED e$^+$e$^-$) expected in 1 nb$^{-1}$ Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 5.5 TeV after analysis cuts (see text). The three superimposed curves indicate the underlying individual LbyL (solid), QED (dashed) and CEP (dotted) distributions after cuts.