Novel Cross Section Ratios as Possible Signals of Saturation in UPCs

Abstract

We propose new cross section ratios in ultra-peripheral A$+$A and $p+$A collisions (UPCs) as a potential experimental signal of saturation physics. We consider the ratio $R_1$ of elastic vector meson photoproduction cross section to the inclusive hadron or jet photoproduction cross section. The ratio can be measured in the $\gamma +$A and $\gamma +p$ collisions taking place in the UPCs. We label the ratios $R_1(\mathrm{A})$ and $R_1(p)$, respectively. Constructing the double ratio $R_{\mathrm{UPC}}(\mathrm{A})=R_1(\mathrm{A})/R_1(p)$, and performing a small-$x$ calculation both in the quasi-classical approximation and by including small-$x$ evolution, we observe that $R_{\mathrm{UPC}}(\mathrm{A})$ exhibits a markedly different dependence on the nuclear atomic number inside and outside the saturation region. This result indicates that $R_{\mathrm{UPC}}(\mathrm{A})$ and $R_1(\mathrm{A})$ measurements in UPCs may help in the experimental searches for the evidence of saturation physics.

arXiv commentFinal version published in Journal

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