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

Abstract

Evidence for light-by-light (LbL) scattering, $\gamma \gamma \to \gamma \gamma$, 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 $E_T^{\gamma }>2$ GeV, pseudorapidity $|{\eta }^{\gamma }|<2.4$; and diphoton invariant mass $m^{\gamma \gamma }>5$ GeV, transverse momentum $p_T^{\gamma \gamma }<1$ GeV, and acoplanarity $(1-\Delta {\varphi }^{\gamma \gamma }/\pi )<0.01$. 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 $4.1\sigma$. The measured fiducial LbL scattering cross section, ${\sigma }_{fid}(\gamma \gamma \to \gamma \gamma )=122\pm 46$ (stat) $\pm 29$ (syst) $\pm 4$ (theo) nb is consistent with the standard model prediction.

4 pages, 5 figures. Proceedings Quark-Matter 2018 (Venice), NPA to appear

References in corpus (2)

• Particle-flow reconstruction and global event description with the CMS detector
• The Physics of Ultraperipheral Collisions at the LHC

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