ar3jet.f

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        SUBROUTINE ar3jet(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     
        c=1./3.
        IF(k(jl,2).NE.21 .AND. k(jl+1,2).NE.21) c=8./27.
        exp1=3
        exp3=3
        IF(k(jl,2).NE.21) exp1=2
        IF(k(jl+1,2).NE.21) exp3=2
        a=0.24**2/s
        yma=alog(.5/sqrt(a)+sqrt(.25/a-1))
        d=4.*c*yma
        sm1=p(jl,5)**2/s
        sm3=p(jl+1,5)**2/s
        xt2m=(1.-2.*sqrt(sm1)+sm1-sm3)*(1.-2.*sqrt(sm3)-sm1+sm3)
        xt2m=min(.25,xt2m)
        ntry=0
1       IF(ntry.EQ.5000) THEN
                x1=.5*(2.*sqrt(sm1)+1.+sm1-sm3)
                x3=.5*(2.*sqrt(sm3)+1.-sm1+sm3)
                RETURN
        ENDIF
        ntry=ntry+1
     
        xt2=a*(xt2m/a)**(atl_ran(nseed)**(1./d))
     
        ymax=alog(.5/sqrt(xt2)+sqrt(.25/xt2-1.))
        y=(2.*atl_ran(nseed)-1.)*ymax
        x1=1.-sqrt(xt2)*exp(y)
        x3=1.-sqrt(xt2)*exp(-y)
        x2=2.-x1-x3
        neg=0
        IF(k(jl,2).NE.21 .OR. k(jl+1,2).NE.21) THEN
        IF((1.-x1)*(1.-x2)*(1.-x3)-x2*sm1*(1.-x1)-x2*sm3*(1.-x3).
     &  le.0..OR.x1.LE.2.*sqrt(sm1)-sm1+sm3.OR.x3.LE.2.*sqrt(sm3)
     &  -sm3+sm1) neg=1
        x1=x1+sm1-sm3
        x3=x3-sm1+sm3
        ENDIF
        IF(neg.EQ.1) goto 1
     
        fg=2.*ymax*c*(x1**exp1+x3**exp3)/d
        xt2m=xt2
        IF(fg.LT.atl_ran(nseed)) goto 1
     
        RETURN
        END