Observation of Strong Nuclear Suppression in Exclusive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> Photoproduction in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Au</mml:mi><mml:mo stretchy="false">+</mml:mo><mml:mi>Au</mml:mi></mml:mrow></mml:math> Ultraperipheral Collisions at RHIC

We report a measurement of exclusive $J/\ensuremath{\psi}$ and $\ensuremath{\psi}(2s)$ photoproduction in $\mathrm{Au}+\mathrm{Au}$ ultraperipheral collisions at $\sqrt{{s}_{\mathrm{NN}}}=200\text{ }\text{ }\mathrm{GeV}$ using the STAR detector. For the first time, (i) the $\ensuremath{\psi}(2s)$ photoproduction in midrapidity at the Relativistic Heavy-Ion Collider has been experimentally measured; (ii) nuclear suppression factors are measured for both the coherent and incoherent $J/\ensuremath{\psi}$ production. At average photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent $J/\ensuremath{\psi}$ cross sections of Au nuclei are found to be $71\ifmmode\pm\else\textpm\fi{}10%$ and $36\ifmmode\pm\else\textpm\fi{}7%$, respectively, of that of free protons. The stronger suppression observed in the incoherent production provides a new experimental handle to study the initial-state parton density in heavy nuclei. Data are compared with theoretical models quantitatively.

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