Azimuthally differential femtoscopic measurements, being sensitive to spatiotemporal characteristics of the source as well as collective velocity fields at freeze-out, provide very important information on the nature and dynamics of
the system evolution. While the radii modulations with respect to the second
harmonic event plane reflect mostly the spatial geometry of the source, the
third harmonic results are mostly defined by the system dynamics. In this talk,
we present the azimuthally differential measurements of the pion source in Pb-
Pb collisions relative to the second and the third harmonic event planes as a
function of the pion-pair transverse momentum (kT ) for different centralities of
the collision. The dependence of the side-, out-, and long-radii on the pion-pair
emission angle with respect to the second harmonic event plane qualitatively
agrees with theoretical calculations, but the details show significant deviations.
The final-state source eccentricity, estimated via side radius oscillations, is
found to be significantly smaller than the initial-state source eccentricity, but remains positive at all measured kT – an indication of the out-of-plane extended
source even after strong in-plane expansion. The observation of the radii modulations with respect to the third harmonic event plane unambiguously signal a
collective expansion and anisotropy in the flow field. We compare our results
to the existing model predictions.
|List of tracks||Femtoscopy at RHIC and LHC: links to QGP physics|