Physics – Nuclear Physics – Nuclear Experiment
Scientific paper
2007-12-20
Nucl.Phys.Proc.Suppl.184:152-157,2008
Physics
Nuclear Physics
Nuclear Experiment
6 pages, 6 figures, Photon 2007 Proceedings
Scientific paper
10.1016/j.nuclphysbps.2008.09.15
We present a summary of recent photoproduction results in ultra peripheral relativistic heavy ions collisions with STAR. These collisions have impact parameters larger then twice the nuclear radius; the nuclei do not physically collide, but interact via long-range electromagnetic fields. We observe exclusive $\rho^0$ production as well as $AuAu\to Au^*Au^* \rho^0$ with accompanying mutual nuclear excitation at $\sqrt{s_{NN}}=200$ GeV. We report the $\rho^0$ production cross section for both coherent and incoherent coupling accompanied by mutual nuclear excitation. We have studied the cross section as a function of $p_T$, $y_{\rho^0}$ and $M_{\pi\pi}$ and compared it to theoretical models. In addition, we measured the $\rho^0$ helicity matrix elements. They are found to be consistent with s-channel helicity conservation. The ratio of coherent $\rho^0$ and direct $\pi^+\pi^-$ pair photoproduction has been measured and found to be consistent with earlier measurements. The 4-pion final state $AuAu \to \pi^+\pi^-\pi^+\pi^-$ state has also been observed.
for the STAR Collaboration
Seger J. E.
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