HAL : in2p3-00269881, version 1
 arXiv : 0712.3320
 Physical Review C 77 (2008) 034910
 $\rho^0$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{S_{NN}}$=200 GeV
 STAR Collaboration(s)
 (2008)
 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.
 Thème(s) : Physique/Physique Nucléaire Expérimentale
 Lien vers le texte intégral : http://fr.arXiv.org/abs/0712.3320
 in2p3-00269881, version 1 http://hal.in2p3.fr/in2p3-00269881 oai:hal.in2p3.fr:in2p3-00269881 Contributeur : Dominique Girod <> Soumis le : Jeudi 3 Avril 2008, 10:33:40 Dernière modification le : Jeudi 3 Avril 2008, 15:01:42