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Volume 316 - XXVI International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS2018) - WG2: Small-x and Diffraction
Dipion photoproduction and the $Q^2$ evolution of the shape of gold nuclei
S. Klein* on behalf of the STAR collaboration
*corresponding author
Full text: pdf
Pre-published on: 2018 September 20
Published on: 2018 November 23
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. Furthermore, we find that the depth of the first diffractive minimum varies with pair mass. We perform a two-dimensional Fourier-Bessel transform to see how the effective transverse distribution of the interactions changes with decreasing pair mass. In the lowest mass bin, the nuclear profile is broader, which is consistent with expectations from saturation models.
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