Observation of pi{sup +}pi{sup -}pi{sup +}pi{sup -} photoproduction in ultraperipheral heavy-ion collisions at sq root(s{sub NN})=200 GeV at the STAR detector

Abstract

We present a measurement of pi{sup +}pi{sup -}pi{sup +}pi{sup -} photonuclear production in ultraperipheral Au-Au collisions at sq root(s{sub N{sub N}})=200 GeV from the STAR experiment. The pi{sup +}pi{sup -}pi{sup +}pi{sup -} 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 pi{sup +}pi{sup -}pi{sup +}pi{sup -} invariant mass spectrum of the coherent events exhibits a broad peak around 1540+-40 MeV/c{sup 2} with a width of 570+-60 MeV/c{sup 2}, in agreement with the photoproduction data for the rho{sup 0}(1700). We do not observe a corresponding peak in the pi{sup +}pi{sup -} final state and measure an upper limit for the ratio of the branching fractions of the rho{sup 0}(1700) to pi{sup +}pi{sup -} and pi{sup +}pi{sup -}pi{sup +}pi{sup -} of 2.5% at 90% confidence level. The ratio of rho{sup 0}(1700) and rho{sup 0}(770) coherent production cross sections is measured to be 13.4+-0.8{sub stat.}+-4.4{sub syst.}%.