10-12 April 2019
Denver, CO
US/Mountain timezone
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Contribution invited talk

Denver, CO - Director's Row I

Ultra-peripheral Collisions in STAR


  • Prof. Janet SEGER

Primary authors


In ultra-peripheral relativistic nuclear collisions, the impact parameter is greater than the sum of the radii of the colliding nuclei. Hadronic interactions are suppressed in these collisions, but the intense flux of photons allows the study of photonuclear and two-photon interactions, providing information about the initial state of the nuclei. The STAR experiment has studied Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV at central rapidity. High-statistics samples of coherently produced $\rho$ mesons and $\pi^+\pi^-$ pairs allowed the observation of a diffractive pattern in $d\sigma/dt$. The diffractive dips have been studied as a function of $\pi^+\pi^-$ pair mass and compared with model expectations. Preliminary results of the coherent photoproduction of $J/\Psi$ mesons and $e^+e^-$ pairs will also be presented. The experimental cross section for coherent $J/\Psi$ photoproduction is of interest as a probe of gluon saturation and nuclear gluon shadowing. The $J/\Psi$ mesons are heavy enough to be described by perturbative QCD, where at the first order the coherent cross section is proportional to the square of the nuclear gluon distribution. The coherent production of $e^+e^-$ pairs is a background process for the $J/\Psi$ measurement, but also interesting in its own right. STAR has observed an enhancement, relative to known hadronic interactions, in $e^+e^-$ production at low transverse momentum in peripheral Au+Au and U+U collisions. This enhancement can be explained by photoproduction, and a precision measurement of $e^+e^-$ production in ultra-peripheral collisions provides a baseline for comparison with these results.