An event-by-event analysis of the azimuthal angular correlation with respec
t to the reaction plane has been carried out for K+ and pi(+) emission near
mid-rapidity in 158 A . GeV Pb + Pb collisions. In semi-central collisions
, K+ mesons are found to be preferentially emitted out of the reaction plan
e, while pi(+) mesons are emitted in the reaction plane. The results sugges
t that the kaon emission is influenced by in-medium potential effects in ad
dition to collective flow effects. (C) 1999 Published by Elsevier Science B
.V. All rights reserved.