Speaker
Description
Coherent vector meson photoproduction in ultraperipheral heavy-ion collisions (UPCs) provides a powerful probe of the nuclear gluon structure at small Bjorken-$x$. Vector mesons with different masses exhibit varying sensitivities to nonlinear QCD dynamics: lighter mesons probe lower energy scales and are therefore more sensitive to gluon saturation and nuclear shadowing effects. Among them, $\rho^0$-the lightest vector meson-serves as one of the most sensitive probes of these nonlinear phenomena in coherent photoproduction, despite the inherent challenges associated with non-perturbative QCD calculations. In this talk, we present the measurement of coherent $\rho^0$ photoproduction in Pb+Pb UPCs at $\sqrt{s_{NN}} = 5.36$ TeV with the CMS detector. The differential cross section is reported as a function of rapidity over a broad kinematic range $|y|<2$. A two-fold ambiguity in the photon direction, intrinsic to symmetric nucleus–nucleus UPCs, has traditionally limited access to the photon–nucleus center-of-mass energy and the corresponding small-$x$ region. We will present the measurement that resolves this ambiguity, enabling the first extraction of the coherent $\rho^0$ photoproduction cross section as a function of the photon–nucleon center-of-mass energy up to about 160 GeV, corresponding to $x$ values down to $\sim 1.9 \times 10^{-5}$. These results provide new constraints on nuclear shadowing and gluon saturation effects in the small-$x$ regime.
