# $\rho^0$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{S_{NN}}$=200 GeV

Abstract : Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR collaboration presents a measurement of rho^0 and direct pi^+pi^- photoproduction in ultra-peripheral relativistic heavy ion collisions at sqrt(s_{NN})=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of sigma(AuAu) -> Au^*Au^*rho^0 = 530 pm 19 (stat.) pm 57 (syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The rho^0 transverse momentum spectrum (p_{T}^2 ) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find sigma_{inc}/sigma_{coh} = 0.29 pm 0.03 (stat.) pm 0.08 (syst.). The ratio of direct pi^+pi^- to rho^0 production is comparable to that observed in gamma p collisions at HERA, and appears to be independent of photon energy. Finally, the measured rho^0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.
Document type :
Journal articles

http://hal.in2p3.fr/in2p3-00269881
Contributor : Dominique Girod <>
Submitted on : Thursday, April 3, 2008 - 10:33:40 AM
Last modification on : Friday, November 27, 2020 - 10:46:03 PM

### Citation

B.I. Abelev, M.M. Aggarwal, Z. Ahammed, B.D. Anderson, D. Arkhipkin, et al.. $\rho^0$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{S_{NN}}$=200 GeV. Physical Review C, American Physical Society, 2008, 77, pp.034910. ⟨10.1103/PhysRevC.77.034910⟩. ⟨in2p3-00269881⟩

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