Observation of pi+ pi- pi+ pi- photoproduction in ultra-peripheral heavy ion collisions at sqrt(s)NN=200 GeV at the STAR detector

We present a measurement of π+π-π+π- photonuclear production in ultraperipheral Au-Au collisions at √sNN=200 GeV from the STAR experiment. The π+π-π+π- final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The π+π-π+π- invariant mass spectrum of the coherent events exhibits a broad peak around 1540±40 MeV/c2 with a width of 570±60 MeV/c2, in agreement with the photoproduction data for the ρ0(1700). We do not observe a corresponding peak in the π+π- final state and measure an upper limit for the ratio of the branching fractions of the ρ0(1700) to π+π- and π+π-π+π- of 2.5% at 90% confidence level. The ratio of ρ0(1700) and ρ0(770) coherent production cross sections is measured to be 13.4±0.8stat.±4.4syst.%.