  Reaction-plane deblurring is applied to the triple-differential distributions
of protons, deuterons and negatively charged pions from central Ar + KCl
collisions at 1.8 GeV/nucleon, measured with the LBL-GSI Streamer Chamber, to
yield distributions relative to the true reaction plane of the collisions.
Within the reaction plane and in the forward cm rapidity region a side ridge
away from the beam axis is observed: the distributions peak away from the beam
direction for protons and deuterons but not for pions. These findings are
consistent with a source of particles moving at an in-plane transverse velocity
of ~0.1 c. Transport pBUU calculations yield results in semi-quantitative
agreement with those from the data.
