Dipion photoproduction and the $Q^2$ evolution of the shape of the gold nucleus

Understand

Coherent photoproduction of vector mesons is sensitive to the shape of the target nucleus, as probed at four-momentum scale $Q^2 \propto (M_V/2)^2$.

• Previously STAR presented a high-statistics measurement of $d\sigma_c/dt$ for coherent $\pi^+\pi^-$ photoproduction in ultra-peripheral gold-gold collisions, and made a two-dimensional Fourier-Bessel (Hanckel) transformation to give the transverse distribution of interactions in the nucleus.
• Here, we study how $d\sigma_c/dt$ evolves with $Q^2$.
• We divide the $\pi^+\pi^-$ signal into three different mass ($Q^2$) bins to measure how $d\sigma_c/dt$ evolves with dipion mass.