Abstract
The STAR Collaboration reports on the photoproduction of pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly real photon emitted by one ion scatters from the other ion.
We fit the invariant-mass spectrum with a combination of and resonances and a direct continuum. This is the first observation of the in ultraperipheral collisions, and the first measurement of interference at energies where photoproduction is dominated by Pomeron exchange. The amplitude is consistent with the measured cross section, a classical Glauber calculation, and the branching ratio. The phase angle is similar to that observed at much lower energies, showing that the phase difference does not depend significantly on photon energy.
The differential cross section exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with two minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.
2 More- Received 18 May 2017
DOI:https://doi.org/10.1103/PhysRevC.96.054904
©2017 American Physical Society