Subthreshold K+ production in deuteron and alpha induced nuclear reactions

Double differential cross sections d2?/dpd? have been measured for ?+ and K+ emitted around midrapidity in d+A and ?+A collisions at a beam kinetic energy of 1.15 GeV/nucleon. The total ?+ yield increases by a factor of about 2 when using an alpha projectile instead of of a deuteron whereas the K+ yield increases by a factor of about 4. According to transport calculations, the K+ enhancement depends both on the number of hadron-hadron collisions: their center-of-mass energy increases with increasing number of projectile nucleons.