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 $\eta$, $\eta^{'}(958)$ 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 $\eta$, $\eta^{'}(958)$, $\eta_c(1S)$, $\eta_c(2S)$, $\chi_{c0}(1P)$. 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.
Highlights
The properties of light have at all times fascinated physicists
We extend earlier calculations of this cross section for Wγγ > 5 GeV into the low mass range where photoproduction of the pseudoscalar resonances η, η0ð958Þ contributes to two-photon final states
We present the elementary photon-photon cross section as a function of diphoton mass Mγγ arising from lepton and quark loop diagrams, and the visible cross section obtained with the gamma-gamma decay branching fractions of the preesroipnhaenrcaelsPηb,Pηb0ðc9o5l8liÞs,ioηncsð1aSt Þp, ffisηffiffiNcffiffiNðffiffiffi21⁄4SÞ,5.χ0c20
Summary
The properties of light have at all times fascinated physicists. The development of instruments for measuring light, and the design of experiments using light, have resulted in fundamental contributions to our current knowledge of modern physics. Such measurements in heavy-ion collisions extend photon-photon scattering from a pure QED issue to a subject connecting QED with QCD topics This QED-QCD connection is, for example, an important ingredient in the quantum electrodynamical calculation of the muon anomalous moment (see e.g., [10,11] and references therein). First evidence of diphoton measurements in ultraperipheral heavy-ion collisions have been reported by the ATLAS and CMS Collaborations [13,14] These data are, restricted to photon-photon invariant masses Wγγ > 5 and 6 GeV for the CMS and ATLAS analysis, respectively. The purpose of the study presented here is to examine the possibility of measuring photon-photon scattering in ultraperipheral heavy-ion collisions at LHC energies in the range Wγγ < 5 GeV.
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