Volume,166B, number 1 PHYSICS LETTERS 2 January 1986 PION PRODUCTION MECHANISM IN HIGH ENERGY HEAVY ION COLLISIONS Y KITAZOE a, M SANO b, H TOKI c and S NAGAMIYAd u Department of Physics, Kocht Medical School, Nankoku, Kocht 781 51, Japan h Institute for Nuclear Stud)', University of Tokyo, Tanashl, Tokyo 188, Japan ' Department of Physics, Tok)o Metropohtan Untversttp, Setaga)~a, Tokyo 158 Japan a Department of Physws, Umverslty of Tok)o, Bunkyo-ku Hongo, Tokyo 158, Japan Received 15 January 1985 revised manuscript recewed 29 October 1985 We study the mechanism of plon production m high energy heavy ion colhslons with the nuclear cascade model developed m a previous paper We describe time-dependently the reaction rates of the processes N +N-~ N + ~, N + A---,N+N A ---, N + ~r, and N + ~r ---,A, to discuss the achievement of the chemical equlhbrlum The importance of final ~-N and er-N interactions is pointed out The present cascade model reproduces well the observed plon yields in Ar + KC1 high multiplicity events, without introducing the nuclear compression effect The reason of the agreement is discussed by decomposing a variety of conditions employed in this model, and by reproducing the situations of Cugnon's cascade model and others Stock et al [1] made the first systematic calcula- tions of pmn productmn m Ingh energy heavy ion col- llsmns by using the cascade model of Cugnon et al [2] and reported that the pmn yxelds for high multx- phclty events [3] are largely overestimated m terms of the cascade model They took this discrepancy serious, since the cascade model descnbes precisely the microscopic colhslon processes between particles As a result, the discrepancy was interpreted as due to the nuclear compressmn effect, winch takes out some kinetic energy available to produce pmns Recently, Sano et al [4] corrected the procedure of extracting the nuclear equation of state as proposed by Stock et al, and argued that the nuclear compres- stun effect would have to be extremely stiff to repro- duce the observed plon yields Ca_hay et al [5] dis- cussed other effects for reductmn of the pmn yields m the cascade model [2], namely the off-mass-shell effect and renormalrzatlon of the pmn sources On the other hand, Kruse et al [6] have pointed out by us- mg the cascade model [7] with a self~zonslstent mean field that the mare drop m the pmn yields as due to the transformatmn of kmetac energy into potential energy during the Ingh-denstty phase of the reaction In thas way, the problem of pmn productmn m rela- tivistic nuclear colhsmns remains unresolved as an open question 0370-2693/86/$ 03 50 © Elsevier Science Pubhshers B V (North-Holland Physics Pubhstung Dlvlsmn) Katazoe et al [8] have also developed a new cas- cade model program, winch improves upon the unde- sxred aspects of Cugnon's cascade model The calcu- lated results [8,9] reproduced systematxcally the ex- perimental proton data, winch otherwise have been interpreted as evidence for collectwe motion For the plon yaelds, tins program overestmaated the h~gh-mul- txphclty data (E L = 0.4-1 6 GeV/nucleon) m Ar + KCI central colllsmns by 50% [10] Tins overestlma- tmn was not surprising, since the program assumed spontaneous emlssmn and no absorptxon of plons Recently, however, we have found that an xmprove- ment of the sorting procedure * ~ m NN colhslons leads to a further overestmaatlon of the pmn yield, though it does not affect the proton spectra In a previous paper [ 10], the colhslon partner of a nucleon to be scattered at a time step was determined by selecting a nucleon which exists m the nearest neighbor and by com- panng their relatwe distance with the interaction radms This method was found to be unsatisfactory when the part- ner has another nucleon and their relative distance IS shorter than the distance first mentioned In the present verslon, firstly we search all collaslon partners to be scattered Sec- ondly, we sort these pairs w~th mcreasangorder of their rela- tive distances. In thrs way, we can get a nucleon pair with the shortest relative distance m the system, that with the second shortest relatwe distance, and so on Collaslonsare undergone according to this ordering 35