Observation of pi^+pi^-pi^+pi^- Photoproduction in Ultra-Peripheral Heavy Ion Collisions at sqrt{s_NN} = 200 GeV at the STAR Detector

Understand

We present a measurement of pi^+pi^-pi^+pi^- photonuclear production in ultra-peripheral Au-Au collisions at sqrt(s_{NN}) = 200 GeV from the STAR experiment.

• The pi^+pi^-pi^+pi^- 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^+pi^-pi^+pi^- invariant mass spectrum of the coherent events exhibits a broad peak around 1540 pm 40 MeV/c^2 with a width of 570 pm 60 MeV/c^2, in agreement with the photoproduction data for the rho^0(1700).