C***********************************************************************
C $Id: arzcms.F,v 1.2 1996/04/10 12:33:45 mclareni Exp $

      REAL FUNCTION ARZCMS(S,SM1,SM2)

C...ARiadne function get Z component in CMS

C...Returns the z component of a particle momentum when placed in the
C...cms system of itself and an other particle given the two particle
C...masses SM1 and SM2 and the total energy squared S

#include "arimpl.f"

      ARZCMS=-1
      AA=(S-(SM1+SM2)**2)
      IF (AA.LT.0.0) RETURN
      ARZCMS=0.5*SQRT(AA*(S-(SM1-SM2)**2)/S)

      RETURN

C**** END OF ARZCMS ****************************************************
      END