Abstract
We present the first azimuthally differential measurements of the pion source size relative to the second harmonic event plane in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of sqrt[s_{NN}]=2.76 TeV. The measurements have been performed in the centrality range 0%-50% and for pion pair transverse momenta 0.2<k_{T}<0.7 GeV/c. We find that the R_{side} and R_{out} radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate out of phase, similar to what was observed at the Relativistic Heavy Ion Collider. The final-state source eccentricity, estimated via R_{side} oscillations, is found to be significantly smaller than the initial-state source eccentricity, but remains positive-indicating that even after a stronger expansion in the in-plane direction, the pion source at the freeze-out is still elongated in the out-of-plane direction. The 3+1D hydrodynamic calculations are in qualitative agreement with observed centrality and transverse momentum R_{side} oscillations, but systematically underestimate the oscillation magnitude.
Highlights
We present the first azimuthally differential measurements of the pion source size relative to the second phaffisrffiffimNffiffiNffioffiffi n1⁄4ic2.e7v6enTteVpl.anTeheinmePabs-uPrbemceonlltissihoanvse at a been center-of-mass performed in energy per nucleon-nucleon the centrality range 0%–50%
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 out of phase, similar to what was observed at the Relativistic Heavy Ion Collider
The final-state source eccentricity, estimated via Rside oscillations, is found to be significantly smaller than the initial-state source eccentricity, but remains positive—indicating that even after a stronger expansion in the in-plane direction, the pion source at the freeze-out is still elongated in the out-of-plane direction
Summary
BðqÞ is the background distribution of uncorrelated particle pairs. Both the AðqÞ and BðqÞ distributions were measured differentially with respect to the second harmonic eventplane angle ΨEP;. The background distribution is built by using the mixed-event technique [4] in which pairs are made out of particles from two different events with similar centrality (less than 2% difference), event-plane angle (less than 10° difference), and event vertex position along the beam direction (less than 4 cm difference). The Bowler-Sinyukov fitting procedure [28,29] was used in which the Coulomb weight is only applied to the fraction of pairs (λ) that participate in the Bose-Einstein correlation. In this approach, the correlation function is fitted to
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