Abstract
Azimuthally-differential femtoscopic measurements, being sensitive to spatio-temporal characteristics of the source as well as to the collective velocity fields at freeze out, provide very important information on the nature and dynamics of the system evolution. While the HBT radii oscillations relative to the second harmonic event plane measured recently reflect mostly the spatial geometry of the source, model studies have shown that the HBT radii oscillations relative to the third harmonic event plane are predominantly defined by the velocity fields. In this Letter, we present the first results on azimuthally-differential pion femtoscopy relative to the third harmonic event plane as a function of the pion pair transverse momentum kT for different collision centralities in Pb–Pb collisions at sNN=2.76 TeV. We find that the Rside and Rout radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate in phase relative to the third harmonic event plane, similar to the results from 3+1D hydrodynamical calculations. The observed radii oscillations unambiguously signal a collective expansion and anisotropy in the velocity fields. A comparison of the measured radii oscillations with the Blast-Wave model calculations indicate that the initial state triangularity is washed-out at freeze out.
Highlights
Heavy-ion collisions at LHC energies create a hot and dense medium known as the quark–gluon plasma (QGP) [1]
The initial state of a heavy-ion collision is characterized by spatial anisotropies that lead to anisotropies in pressure gradients, and to azimuthal anisotropies in final particle distributions, commonly called anisotropic flow
In particle emission [6], the particles emitted at a particular angle relative to the flow plane carry information about the source as seen from that corresponding direction; these correlations lead to the HBT radii to be sensitive to the collective velocity fields, from which information about the dynamics of the system evolution can be extracted
Summary
Heavy-ion collisions at LHC energies create a hot and dense medium known as the quark–gluon plasma (QGP) [1]. The initial state of a heavy-ion collision is characterized by spatial anisotropies that lead to anisotropies in pressure gradients, and to azimuthal anisotropies in final particle distributions, commonly called anisotropic flow. In particle emission [6], the particles emitted at a particular angle relative to the flow plane carry information about the source as seen from that corresponding direction; these correlations lead to the HBT radii to be sensitive to the collective velocity fields, from which information about the dynamics of the system evolution can be extracted. The HBT radii variations relative to the second harmonic event plane angle ( 2) provide information on the pion source elliptic eccentricity at freeze-out. Wave Model [16] for a quantitative characterization of the final source shape
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
