Observation of π+π−π+π− photoproduction in ultraperipheral heavy-ion collisions at sNN=200 GeV at the STAR detector

Le résumé fourni par la source

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

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.