Recent results from the STAR experiment on vector meson production in ultra peripheral AuAu collisions at RHIC

Abstract

In 2010, the STAR Collaboration collected a large sample of low-multiplicity triggers for ultraperipheral collision studies. We present preliminary results involving photonuclear interactions in ultraperipheral relativistic heavy-ion collisions (UPCs), based on an analysis of photoproduced pion pairs, from ρ0, ω and direct ππ pair production. The relative amplitudes of the three components have been measured, along with the phase angle between the ρ0 and ω0 components. The ρ0 squared momentum transfer (t) spectrum exhibits a spectrum with both coherent and incoherent production. The coherent component shows two characteristic diffraction minima, with positions that are sensitive to the hadronic radius of the gold nucleus. Using a two-year (2010 and 2011) dataset, we explore higher mass final states, and observe a ππ state roughly compatible with the ρ3(1690).Preliminary results for the J/ψ production cross section as a function of rapidity and transverse momentum pT are also presented; the dominant component at low pT demonstrates the coherent production of J/ψ of the entire Au nucleus, and a significant component at higher pT indicates incoherent production of individual nucleons within the nucleus.Future RHIC runs with polarized protons will provide the opportunity to measure J/ψ production in ultra-peripheral pp and pAu collisions. The STAR Roman Pot system will allow measurement of the final state protons. A non-zero transverse asymmetry of the produced J/ψ’s would be the first measure of the nucleon helicity-flip Generalized Parton Distributions (GPD)-E for gluons, which is connected with the orbital angular momentum of gluons in the nucleon.