arorie.f

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C*************************************************************


        SUBROUTINE arorie(S,X1,X3,JL)
C     
        common/hiparnt/hipr1(100),ihpr2(50),hint1(100),ihnt2(50)
        SAVE  /hiparnt/
        common/lujets/n,k(9000,5),p(9000,5),v(9000,5)
        SAVE  /lujets/
        common/ranseed/nseed
        SAVE  /ranseed/
C     
        w=sqrt(s)
        x2=2.-x1-x3
        e1=.5*x1*w
        e3=.5*x3*w
        p1=sqrt(e1**2-p(jl,5)**2)
        p3=sqrt(e3**2-p(jl+1,5)**2)
        cbet=1.
        IF(p1.GT.0..AND.p3.GT.0.) cbet=(p(jl,5)**2
     &           +p(jl+1,5)**2+2.*e1*e3-s*(1.-x2))/(2.*p1*p3)
        IF(abs(cbet).GT.1.0) cbet=max(-1.,min(1.,cbet))
        bet=acos(cbet)
     
C.....MINIMIZE PT1-SQUARED PLUS PT3-SQUARED.....
        IF(p1.GE.p3) THEN
           psi=.5*ulangl(p1**2+p3**2*cos(2.*bet),-p3**2*sin(2.*bet))
           pt1=p1*sin(psi)
           pz1=p1*cos(psi)
           pt3=p3*sin(psi+bet)
           pz3=p3*cos(psi+bet)
        ELSE IF(p3.GT.p1) THEN
           psi=.5*ulangl(p3**2+p1**2*cos(2.*bet),-p1**2*sin(2.*bet))
           pt1=p1*sin(bet+psi)
           pz1=-p1*cos(bet+psi)
           pt3=p3*sin(psi)
           pz3=-p3*cos(psi)
        ENDIF
     
        del=2.0*hipr1(40)*atl_ran(nseed)
        p(jl,4)=e1
        p(jl,1)=pt1*sin(del)
        p(jl,2)=-pt1*cos(del)
        p(jl,3)=pz1
        p(jl+2,4)=e3
        p(jl+2,1)=pt3*sin(del)
        p(jl+2,2)=-pt3*cos(del)
        p(jl+2,3)=pz3
        p(jl+1,4)=w-e1-e3
        p(jl+1,1)=-p(jl,1)-p(jl+2,1)
        p(jl+1,2)=-p(jl,2)-p(jl+2,2)
        p(jl+1,3)=-p(jl,3)-p(jl+2,3)
        RETURN
        END