The first measurement of the impact-parameter dependent angular anisotropy in the decay of coherently photoproduced $\rho^0$ mesons is presented. The $\rho^0$ mesons are reconstructed through their decay into a pion pair. 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, which describe the measurement, explain the $\cos(2\phi)$ anisotropy as the result of a quantum interference effect at the femtometer scale that arises from the ambiguity as to which of the nuclei is the source of the photon in the interaction.
Submitted to: PLB
e-Print: arXiv:2405.14525 | PDF | inSPIRE
CERN-EP-2024-126
Figure group
Figure 1
Invariant-mass distribution of pion pairs, with superimposed Soding (solid line) and Ross-Stodolsky (dotted line) fits, for the range $0<\phi<\pi/7$ in the 0n0n (left) and XnXn (right) neutron classes. The different components of the pion-pair production amplitude in the Soding model are shown: the Breit--Wigner shape that describes the $\rho^0$ (finer dotted line), the continuum process (dash-dotted line), and the interference between the $\rho^0$ and the continuum (dash-dot-dot-dot line). The Breit--Wigner extracted from the Ross-Stodolsky model (finest dotted line) is also shown. In this example, the background contribution from misidentified muons is fixed to zero in the fit. |  |
Figure 3
Amplitudes of the $\cos(2\phi)$ modulation of the $\rho^0$ yield in Pb$-$Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV in all neutron classes. The results are compared with the Xing et al. and W. Zhao et al. model predictions and, for the XnXn class, with the STAR results in Au$-$Au and U$-$U collisions at RHIC. For all the experimental data points, statistical uncertainties are represented with a bar and systematic uncertainties with a box. |  |