Light-by-light scattering in ultra-peripheral heavy-ion collisions at low diphoton masses

Abstract

We present a study of photon-photon scattering in the mass range $W_{\gamma \gamma }<5$ GeV. We extend earlier calculations of this cross section for $W_{\gamma \gamma }>$ 5 GeV into the low mass range where photoproduction of the pseudoscalar resonances $\eta$, ${\eta }^{{}^{\prime }}(958)$ contributes to two-photon final states. We present the elementary photon-photon cross section as a function of diphoton mass $M_{\gamma \gamma }$ arising from lepton and quark loop diagrams, and the visible cross section obtained with the gamma-gamma decay branching fractions of the resonances $\eta$, ${\eta }^{{}^{\prime }}(958)$, ${\eta }_c(1S)$, ${\eta }_c(2S)$, ${\chi }_{c0}(1P)$. We derive the corresponding cross sections in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV by folding the elementary cross section with the heavy-ion photon fluxes. We consider the dominating background of the two-photon final state which arises from gamma decays of photoproduced ${\pi }^0$-pairs. Such ${\pi }^0$-pairs contribute to the background when only two of the four decay photons are within the experimental acceptance, while the other two photons escape undetected. We reduce this background by applying cuts on asymmetries of transverse momenta of the two photons and indicate how the background can be further suppressed using a multivariate sideband analysis. We present the cross section for the signal and the background at midrapidity $|\eta |<$ 0.9, and in the forward rapidity range 2.0 $<\eta <$ 4.5.

15 pages, 8 figures

References in corpus (5)

• Light-by-light scattering sum rules in light of new data
• Exclusive muon-pair productions in ultrarelativistic heavy-ion collisions: Realistic nucleus charge form factor and differential distributions
• High-statistics study of neutral-pion pair production in two-photon collisions
• Multivariate side-band subtraction using probabilistic event weights
• The role of meson exchanges in light-by-light scattering

Cited by in corpus (5)

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• The two-photon decay of X(6900) from light-by-light scattering at the LHC
• New opportunities at the photon energy frontier
• Exclusive photon-fusion production of even-spin resonances and exotic QED atoms in high-energy hadron collisions
• Bremsstrahlung as a probe of baryon stopping in heavy-ion collisions