K+/ pi + enhancement in heavy-ion collisions.

Kaon production from heavy-ion collisions is a topic of great interest. For heavy-ion collisions at =1 GeV/ nucleon, it has been shown that the kaon production probability is sensitive to the nuclear equation of state at high densities. ' At ultrarelativistic energy collisions available at the future heavy-ion collider, it has been argued that a kaon might carry the signature for the quark-gluon plasma. . Recently, there are preliminary data from the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory on kaon production from the reaction of Si on Au at an incident energy of 14.5 GeV/nucleon. The expected ratio of K+/n. + is about 5% from the proton-nucleus collisions at the same energy. The measured ratio is about 20%. The factor of 4 enhancement has stimulated the speculation that a quark-g)uon plasma might have been formed in the collision. However, measurements of nuclear stopping power at this energy indicates that the projectile is stopped by the target with the resulting energy density less than that for the formation of the quark-gluon plasma. From the analysis of the transverse energy distribution in such reaction, it has been shown that the data are consistent with the assumption that a fast-moving hadronic fireball has been formed from the projectile and part of the target. It is thus unlikely that the enhanced production of kaons is due to the formation of the quark-gluon plasma. To find other explanations to this large value of K+ jn+ ratio is therefore of great interest. The first possibility one can think of is the associated production mN~AK from the interaction between the secondary pions and the nucleons. Indeed, in a recent study of A production from p annihilation in nuclei, we have found that the associated production from the secondary pions contributes almost equally to A production as that from the direct annihilation. To take into account the contributioh from the associated production to kaon production, we need to consider the interaction of pion with both the spectator and the participant nucleons in the target. As pions are produced most likely outside the target in high-energy collisions due to the time dilation of the production process in a fast moving frame, the contribution from the interaction of pions with the spectator nucleons is expected to be unimportant. The interaction of the pion with the participant nucleons will not lead to an appreciable contribution to kaon production either as the threshold energy far this process is much higher than the average energy of the pion in the frame of the participants. However, the number of created pions is very large, and the contribution from the pion-pion interaction might be important. En this paper, we shall demonstrate this important contribution to the K+/m. + ratio in high-energy heavy-ion collisions in an expanding fireball model. Similar conclusion have been reported previously by Kapusta and Mekjian for a hadronic matter that is composed of only pions. This paper is organized as follows. In Sec. II the hydrochemical model is introduced to describe the fiieba11 expansion. The rates for the relevant reactions are calculated in Sec. III. Section IV gives the results of the calculations. Discussions and conclusions are given in Sec. V. In the Appendix the details of solving the hydrochemical model are described.