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arXiv · 1808.03524

Evidence for light-by-light scattering in ultraperipheral PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV

David d'Enterria
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Abstract

Evidence for light-by-light (LbL) scattering, $γγ\toγγ$, in ultraperipheral PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV is reported. LbL scattering processes are selected in events with just two photons produced, with transverse energy $E_T^γ>2$ GeV, pseudorapidity $|η^γ|<2.4$; and diphoton invariant mass $m^{γγ}>5$ GeV, transverse momentum $p_{T}^{γγ}<1$ GeV, and acoplanarity $(1-Δϕ^{γγ}/π)<0.01$. After all selection criteria, 14 events are observed, compared to $11.1 \pm 1.1$ (theo) and $3.8 \pm 1.3$ (stat) events expected for signal and background processes respectively. The significance of the signal excess over the background-only hypothesis is $4.1σ$. The measured fiducial LbL scattering cross section, $σ_{fid}(γγ\toγγ) = 122 \pm 46$ (stat) $\pm 29$ (syst) $\pm 4$ (theo) nb is consistent with the standard model prediction.

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David d'Enterria. 2018-08-10. Evidence for light-by-light scattering in ultraperipheral PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. https://doi.org/10.1016/j.nuclphysa.2018.10.018

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