Ultra-peripheral heavy-ion collisions (UPCs) occur when the impact parameter of the collision is greater than the sum of the radii of the colliding nuclei. UPCs allow one to study photon-induced reactions, such as the photoproduction of a vector meson, which is a well-established tool to probe the gluon structure of the colliding nuclei.
This talk will focus on the measurement of the impact-parameter dependence of the azimuthal anisotropy in the ρ0 meson photoproduction. The anisotropy originates from the linear polarization of the exchanged virtual photons and from quantum interference between amplitudes contributing to the photoproduction process. The interference arises from the ambiguity as to which nucleus emits the photon and which acts as the target. Moreover, the short range of the strong force responsible for the ρ0production means that the meson must be produced within, or very close to, the target nucleus. These facts imply that the process can be seen as a double-slit interferometer, where the impact-parameter is the distance between the slits.
We present the results of the first measurement of this kind at the LHC, using Pb–Pb UPCs and the ALICE detectors. The anisotropy is studied via the distribution of a variable, ϕ, that is, approximately, the angle between the transverse momentum of the ρ0 and that of one of its two decay pions. Models predict a cos(2ϕ) modulation of the ρ0 yield. The amplitude of such a modulation was measured in three impact parameter ranges, defined by event classes with different neutron emission requirements. The amplitude of the modulation is found to increase by about one order of magnitude from large to small impact parameters, in agreement with the available theoretical predictions. This measurement shows quantum mechanics at work at the femtometer scale and opens new possibilities for studying the gluon structure of nuclei.
Refreshments will be served at 10:30