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

TL;DR

This study reports the observation of pi+pi−pi+pi− photoproduction in ultra-peripheral heavy ion collisions at 200 GeV, revealing a broad resonance consistent with rho(1700) and providing new insights into photonuclear interactions.

Contribution

First measurement of pi+pi−pi+pi− photoproduction in ultra-peripheral heavy ion collisions at RHIC energies, identifying a broad rho(1700)-like resonance and quantifying its production cross section.

Findings

Observed a broad peak around 1540 MeV/c^2 in the pi+pi−pi+pi− invariant mass spectrum.

Measured the ratio of rho(1700) to rho(770) production cross sections as approximately 13%.

Set an upper limit of 2.5% for the branching ratio of rho(1700) decaying to pi+pi− compared to pi+pi−pi+pi−.

Abstract

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). We do not observe a corresponding peak in the pi^+pi^- final state and measure an upper limit for the ratio of the branching fractions of the rho^0(1700) to pi^+pi^- and pi^+pi^-pi^+pi^- of 2.5 % at 90 % confidence level. The ratio of rho^0(1700) and rho^0(770) coherent production cross sections is measured to be 13.4 pm 0.8 (stat.) pm 4.4 (syst.) %.