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\title{Observation of $\pi^+\pi^-\pi^+\pi^-$ photoproduction in ultraperipheral heavy-ion collisions at $\sqrt{s_{NN}} = 200$ GeV at the STAR detector}
\author{David Tlusty (for the STAR Collaboration)}
\affil{Creighton University, Omaha, NE USA}
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\begin{abstract}
    One of the most pressing questions in both hot and cold QCD communities is what the physics mechanism responsible for modified parton densities
    in heavy nuclei is. One promising channel to address this question is the photoproduction of vector mesons, which is considered a clean probe
    to the nuclear parton structures.   
    We present a measurement of $\pi^+\pi^-\pi^+\pi^-$ photonuclear production in ultraperipheral Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV.
    The data were collected in 2014 by the STAR experiment. The $\pi^+\pi^-\pi^+\pi^-$ final states, accompanied by mutual excitation of projectile
    and target, are observed to be greatly enhanced at low transverse momentum, which is consistent with coherent photoproduction.
    The $\pi^+\pi^-\pi^+\pi^-$ invariant mass spectrum in coherent events exhibits a two-resonance structure around $\sim 1370$ and $\sim 1610$ MeV/$c^2$
    with widths of 200 and 330 MeV/$c^2$, likely corresponding to $\rho(1450)$ and $\rho(1700)$. Furthermore, a peak corresponding to $\rho(2150)$ is
    observed. We also observe corresponding peaks to  $\rho(1450)$ and $\rho(1700)$ in the $\pi^+\pi^-$ final state and report the ratio of the branching
    fractions of the $\rho(1450)$, $\rho(1700)$ to $\pi^+\pi^-$ and $\pi^+\pi^-\pi^+\pi^-$. We further present the ratios of $\rho(1450)$, $\rho(1700)$,
    and $\rho(2150)$ to $\rho_0(770)$ coherent production cross sections.
\end{abstract}
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