Measurement of the impact-parameter dependent azimuthal anisotropy in coherent $\rho^0$ photoproduction in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

TL;DR

This paper reports the first measurement of impact-parameter dependent azimuthal anisotropy in coherent $ ho^0$ photoproduction in ultraperipheral Pb-Pb collisions at 5.02 TeV, revealing quantum interference effects at femtometer scales.

Contribution

It provides the first experimental measurement of impact-parameter dependent azimuthal anisotropy in coherent $ ho^0$ photoproduction, confirming theoretical predictions of quantum interference effects.

Findings

Anisotropy amplitude increases by about an order of magnitude from large to small impact parameters.

The measured anisotropy matches theoretical models describing quantum interference effects.

Impact-parameter dependence of anisotropy supports the quantum interference interpretation.

Abstract

This Letter presents the first measurement of the impact-parameter dependent angular anisotropy in the decay of coherently photoproduced $\rho^0$ mesons. The $\rho^0$ mesons are reconstructed through their decay into pion pairs. The measured anisotropy corresponds to the amplitude of the $\cos(2\phi)$ modulation, where $\phi$ is the angle between the two vectors formed by the sum and the difference of the transverse momenta of the pions, respectively. The measurement was performed by the ALICE Collaboration at the LHC using data from ultraperipheral Pb$-$Pb collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeV per nucleon pair. Different impact-parameter regions are selected by classifying the events in nuclear-breakup classes. The amplitude of the $\cos(2\phi)$ modulation is found to increase by about one order of magnitude from large to small impact parameters. Theoretical calculations describe the measured $\cos(2\phi)$ anisotropy and its impact-parameter dependence as the result of a quantum interference effect at the femtometer scale, arising from the ambiguity regarding which of the nuclei is the photon source in the interaction.