Emission pattern of high-energy pions: A new probe for the early phase of heavy-ion collisions

The emission pattern of charged pions has been measured in Au + Au collisio ns at 1 GeV/nucleon incident energy. in peripheral collisions and at target rapidities, high-energy pions are emitted preferentially towards the targe t spectator matter. in contrast, low-energy pions are emitted predominantly In the opposite direction. The corresponding azimuthal anisotropy is expla ined by the interaction of pions with projectile and target spectator matte r. This interaction with the spectator matter causes an effective shadowing which varies with time during the reaction. Our observations show that hig h-energy pjons stem from the early stage of the collision whereas low-energ y pions freeze out later.