* $Id$
    
C*********************************************************************  
    
      SUBROUTINE LU2ENT_HIJING(IP,KF1,KF2,PECM)    
    
C...Purpose: to store two partons/particles in their CM frame,  
C...with the first along the +z axis.   
#include "lujets_hijing.inc"
#include "ludat1_hijing.inc"
#include "ludat2_hijing.inc"
    
C...Standard checks.    
      MSTU(28)=0    
      IF(MSTU(12).GE.1) CALL LULIST_HIJING(0)  
      IPA=MAX(1,IABS(IP))   
      IF(IPA.GT.MSTU(4)-1) CALL LUERRM_HIJING(21,  
     &'(LU2ENT_HIJING:) writing outside LUJETS_HIJING memory')    
      KC1=LUCOMP_HIJING(KF1)   
      KC2=LUCOMP_HIJING(KF2)   
      IF(KC1.EQ.0.OR.KC2.EQ.0) CALL LUERRM_HIJING(12,  
     &'(LU2ENT_HIJING:) unknown flavour code') 
    
C...Find masses. Reset K, P and V vectors.  
      PM1=0.    
      IF(MSTU(10).EQ.1) PM1=P(IPA,5)    
      IF(MSTU(10).GE.2) PM1=ULMASS_HIJING(KF1) 
      PM2=0.    
      IF(MSTU(10).EQ.1) PM2=P(IPA+1,5)  
      IF(MSTU(10).GE.2) PM2=ULMASS_HIJING(KF2) 
      DO 100 I=IPA,IPA+1    
      DO 100 J=1,5  
      K(I,J)=0  
      P(I,J)=0. 
  100 V(I,J)=0. 
    
C...Check flavours. 
      KQ1=KCHG(KC1,2)*ISIGN(1,KF1)  
      KQ2=KCHG(KC2,2)*ISIGN(1,KF2)  
      IF(KQ1+KQ2.NE.0.AND.KQ1+KQ2.NE.4) CALL LUERRM_HIJING(2,  
     &'(LU2ENT_HIJING:) unphysical flavour combination')   
      K(IPA,2)=KF1  
      K(IPA+1,2)=KF2    
    
C...Store partons/particles in K vectors for normal case.   
      IF(IP.GE.0) THEN  
        K(IPA,1)=1  
        IF(KQ1.NE.0.AND.KQ2.NE.0) K(IPA,1)=2    
        K(IPA+1,1)=1    
    
C...Store partons in K vectors for parton shower evolution. 
      ELSE  
         IF(KQ1.EQ.0.OR.KQ2.EQ.0) CALL LUERRM_HIJING(2, 
     &        '(LU2ENT_HIJING:) requested flavours can not develop'//
     $        'parton shower')   
        K(IPA,1)=3  
        K(IPA+1,1)=3    
        K(IPA,4)=MSTU(5)*(IPA+1)    
        K(IPA,5)=K(IPA,4)   
        K(IPA+1,4)=MSTU(5)*IPA  
        K(IPA+1,5)=K(IPA+1,4)   
      ENDIF 
    
C...Check kinematics and store partons/particles in P vectors.  
      IF(PECM.LE.PM1+PM2) CALL LUERRM_HIJING(13,   
     &'(LU2ENT_HIJING:) energy smaller than sum of masses')    
      PA=SQRT(MAX(0.,(PECM**2-PM1**2-PM2**2)**2-(2.*PM1*PM2)**2))/  
     &(2.*PECM) 
      P(IPA,3)=PA   
      P(IPA,4)=SQRT(PM1**2+PA**2)   
      P(IPA,5)=PM1  
      P(IPA+1,3)=-PA    
      P(IPA+1,4)=SQRT(PM2**2+PA**2) 
      P(IPA+1,5)=PM2    
    
C...Set N. Optionally fragment/decay.   
      N=IPA+1   
      IF(IP.EQ.0) CALL LUEXEC_HIJING   
    
      RETURN    
      END