CDECK  ID>, HWBAZF.

*CMZ :-        -26/04/91  11.11.54  by  Bryan Webber

*-- Author :    Ian Knowles

C-----------------------------------------------------------------------

      SUBROUTINE HWBAZF(IPAR,JPAR,VEC1,VEC2,VEC3,VEC)

C-----------------------------------------------------------------------

C     Azimuthal correlation functions for Collins' algorithm,

C     see I.G.Knowles, Comp. Phys. Comm. 58 (90) 271 for notation.

C-----------------------------------------------------------------------

      INCLUDE 'HERWIG61.INC'

      DOUBLE PRECISION Z1,Z2,DOT12,DOT23,DOT31,TR,FN(7),VEC1(2),VEC2(2),

     & VEC3(2),VEC(2)

      INTEGER IPAR,JPAR

      LOGICAL GLUI,GLUJ

      IF (.NOT.AZSPIN) RETURN

      Z1=PPAR(4,JPAR)/PPAR(4,IPAR)

      Z2=1.-Z1

      GLUI=IDPAR(IPAR).EQ.13

      GLUJ=IDPAR(JPAR).EQ.13

      IF (GLUI) THEN

         IF (GLUJ) THEN

C           Branching: g--->gg

            FN(2)=Z2/Z1

            FN(3)=1./FN(2)

            FN(4)=Z1*Z2

            FN(1)=FN(2)+FN(3)+FN(4)

            FN(5)=FN(2)+2.*Z1

            FN(6)=FN(3)+2.*Z2

            FN(7)=FN(4)-2.

         ELSE

C           Branching: g--->qqbar

            FN(1)=(Z1*Z1+Z2*Z2)/2.

            FN(2)=0.

            FN(3)=0.

            FN(4)=-Z1*Z2

            FN(5)=-(2.*Z1-1.)/2.

            FN(6)=-FN(5)

            FN(7)=FN(1)

         ENDIF

      ELSE

         IF (GLUJ) THEN

C           Branching: q--->gq

            FN(1)=(1.+Z2*Z2)/(2.*Z1)

            FN(2)=Z2/Z1

            FN(3)=0.

            FN(4)=0.

            FN(5)=FN(1)

            FN(6)=(1.+Z2)/2.

            FN(7)=-FN(6)

         ELSE

C           Branching: q--->qg

            FN(1)=(1.+Z1*Z1)/(2.*Z2)

            FN(2)=0.

            FN(3)=Z1/Z2

            FN(4)=0.

            FN(5)=(1.+Z1)/2.

            FN(6)=FN(1)

            FN(7)=-FN(5)

         ENDIF

      ENDIF

      DOT12=VEC1(1)*VEC2(1)+VEC1(2)*VEC2(2)

      DOT23=VEC2(1)*VEC3(1)+VEC2(2)*VEC3(2)

      DOT31=VEC3(1)*VEC1(1)+VEC3(2)*VEC1(2)

      TR=1./(FN(1)+FN(2)*DOT23+FN(3)*DOT31+FN(4)*DOT12)

      VEC(1)=((FN(2)+FN(5)*DOT23)*VEC1(1)

     &       +(FN(3)+FN(6)*DOT31)*VEC2(1)

     &       +(FN(4)+FN(7)*DOT12)*VEC3(1))*TR

      VEC(2)=((FN(2)+FN(5)*DOT23)*VEC1(2)

     &       +(FN(3)+FN(6)*DOT31)*VEC2(2)

     &       +(FN(4)+FN(7)*DOT12)*VEC3(2))*TR

      END