Tilted pion sources from azimuthally sensitive HBT interferometry

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 c ross terms R-ol(2) and R-sl(2) of the HBT correlation function in general s how a strong first harmonic in their azimuthal dependence. The strength of this oscillation characterizes the tilt of the major axis of the spatial em ission ellipsoid away from the direction of the beam. Event generator studi es indicate that this tilt can be large (> 20 degrees) at AGS energies whic h makes it by far the most significant azimuthally sensitive HBT signal at these energies. Moreover, transport models suggest that for pions this spat ial 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-ol(2) and R-sl(2) thus probes directly the physical origin of dire cted pion flow. (C) 2000 Elsevier Science B.V. All rights reserved.