Subthreshold K⁺ production in deuteron and alpha induced nuclear reactions

Double differential cross sections d²σ/dpdΩ have been measured for π⁺ and K⁺ emitted around midrapidity in d + A and α + A collisions at a beam kinetic energy of 1.15 GeV/nucleon. The total π⁺ yield increases by a factor of about 2 when using an alpha projectile instead 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 and on the energy available in those collisions: their center-of-mass energy increases with increasing number of projectile nucleons.