Tilted pion sources from azimuthally sensitive HBT interferometry

Abstract

Intensity interferometry in noncentral heavy ion collisions provides access to novel information on the geometry of the effective pion-emitting source. We demonstrate analytically that, even for vanishing pair momentum, the cross terms R 2 ol and R 2 sl of the HBT correlation function in general show a strong first harmonic in their azimuthal dependence. The strength of this oscillation characterizes the tilt of the major axis of the spatial emission ellipsoid away from the direction of the beam. Event generator studies indicate that this tilt can be large (> 20 • ) at AGS energies which makes it by far the most significant azimuthally sensitive HBT signal at these energies. Moreover, transport models suggest that for pions this spatial tilt is directed opposite to the tilt of the directed flow ellipsoid in momentum space. A measurement of the azimuthal dependence of the HBT cross terms R 2 ol and R 2 sl thus probes directly the physical origin of directed pion flow.

Collision energy dependence of viscous hydrodynamic flow in relativistic heavy-ion collisions

“…beam axis [74,75]. For comparison with such experimentally determined final source eccentricities [76][77][78][79], a more meaningful theoretical quantity would be the spatial eccentricity of the final freeze-out surface Σ,…”

Observable implications of geometrical and dynamical aspects of freeze-out in heavy ion collisions

“…(For the more general case, see References [44,47,48,49,50].) Thus, we are left with four HBT radii, which are related to space-time variances as [6,44] …”

“…Going further, we may recall that in the special case of vanishing space-momentum correlations (ρ = 0 or T = ∞), the transvserse radii oscillate with identical strengths (R 2 os,2 = R 2 s,2 = −R 2 o,2 ), and the in-plane and out-of-plane extents of the source may be directly extracted [44,47,48,49], as the "whole source" is viewed from every angle. In that special case, independent of p T…”

“…The one interesting exception is R 2 ; any φ p dependence is implicit, and thus is generated by spacemomentum correlations [44], which, in this model, arise solely from flow. In the no-flow limit for a boost-invariant source, t 2 , x 2 , ỹ 2 , and z 2 are all φ p -independent constants [44,47,48].…”

What to expect when you're expecting: femtoscopy at the LHC

“…Azimuthally-sensitive pion interferometry-the measurement of spatial scales as a function of emission angle relative to the reaction plane-probes the shape of the freezeout configuration, in addition to its scale [10,14,24]. At finite impact parameter, both the spatial configuration of the entrance channel and the resulting momentum distribution in the exit channel are anisotropic.…”