Azimuthally differential pion femtoscopy in Pb-Pb collisions at 2.76 TeV with ALICE at the LHC

TL;DR

This paper presents azimuthally differential femtoscopy measurements of Pb-Pb collisions at 2.76 TeV, revealing oscillations in femtoscopic radii related to the source shape and comparing results with theoretical models.

Contribution

It provides the first detailed azimuthally differential femtoscopy analysis at LHC energies, highlighting the source shape dynamics in non-central heavy ion collisions.

Findings

Radii oscillate as a function of emission angle

Out-of-phase oscillations of R_side and R_out observed

Source remains out-of-plane extended similar to RHIC results

Abstract

Femtoscopy of non-central heavy ion collisions provides access to information on the geometry of the effective pion-emitting source. In particular, the source shape can be studied by measuring femtoscopic radii as a function of the pion emission angle relative to the collision symmetry planes. We present the results of azimuthally differential femtoscopy of Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the LHC relative to the second harmonic event plane. We observe a clear oscillation of the extracted radii as a function of the emission angle. We find that $R_{side}$ and $R_{out}$ oscillate out of phase for all centralities and pion transverse momenta. The relative amplitude of $R_{side}$ oscillation decreases in more central collisions, but remains positive, which indicates that the source remains out-of-plane extended qualitatively similar to what was observed at RHIC energies. We compare our results to existing hydrodynamical and transport model calculations.