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subroutine ggggxx(ga,gb,gc,gd,g, vertex)
c
c This subroutine computes the portion of the amplitude of the four-point
c coupling of 4 massless color octet gauge bosons (gluons) corresponding
c to the color structure f^{a,b,e} f{c,d,e}.
c To optain the complete amplitude, this coupling must be called three
c times (once for each color structure) with the following permutations:
c call ggggxx(ga,gb,gc,gd,g,v1)
c call ggggxx(ga,gc,gd,gb,g,v2)
c call ggggxx(ga,gd,gb,gc,g,v3)
c
c f^{a,b,e} f{c,d,e}*v1+
c f^{a,c,e} f{d,b,e}*v2+
c f^{a,d,e} f{b,c,e}*v3
c (See 2.9.1 of the manual for more information).
c
c input:
c complex ga(0:3) : Boson with adjoint color index a
c complex gb(0:3) : Boson with adjoint color index b
c complex gc(0:3) : Boson with adjoint color index c
c complex gd(0:3) : Boson with adjoint color index d
c real g : coupling of w31 with w-/w+
c
implicit none
double complex ga(6),gb(6),gc(6),gd(6),vertex
double complex dv1(0:3),dv2(0:3),dv3(0:3),dv4(0:3),
& dvertx,v12,v13,v14,v23,v24,v34
double precision pga(0:3),pgb(0:3),pgc(0:3),pgd(0:3),g
save dv1,dv2,dv3, dv4
c save dv1,dv2,dv3,dv4,dvertx,v12,v13,v14,v23,v24,v34
#ifdef HELAS_CHECK
double precision pm
double precision epsi
parameter( epsi = 2.0d-5 )
double precision rZero
parameter( rZero = 0.0d0 )
integer stdo
parameter( stdo = 6 )
#endif
c
#ifdef HELAS_CHECK
pga(0) = dble( ga(5))
pga(1) = dble( ga(6))
pga(2) = dimag(ga(6))
pga(3) = dimag(ga(5))
pgb(0) = dble( gb(5))
pgb(1) = dble( gb(6))
pgb(2) = dimag(gb(6))
pgb(3) = dimag(gb(5))
pgc(0) = dble( gc(5))
pgc(1) = dble( gc(6))
pgc(2) = dimag(gc(6))
pgc(3) = dimag(gc(5))
pgd(0) = dble( gd(5))
pgd(1) = dble( gd(6))
pgd(2) = dimag(gd(6))
pgd(3) = dimag(gd(5))
if ( abs(ga(1))+abs(ga(2))
& +abs(ga(3))+abs(ga(4)).eq.rZero ) then
write(stdo,*) ' helas-warn : ga in ggggxx is zero vector'
endif
if ( abs(ga(5))+abs(ga(5)).eq.rZero ) then
write(stdo,*)
& ' helas-error : ga in ggggxx has zero momentum'
endif
if ( abs(gb(1))+abs(gb(2))
& +abs(gb(3))+abs(gb(4)).eq.rZero ) then
write(stdo,*) ' helas-warn : gb in ggggxx is zero vector'
endif
if ( abs(gb(5))+abs(gb(5)).eq.rZero ) then
write(stdo,*)
& ' helas-error : gb in ggggxx has zero momentum'
endif
if ( abs(gc(1))+abs(gc(2))
& +abs(gc(3))+abs(gc(4)).eq.rZero ) then
write(stdo,*) ' helas-warn : gc in ggggxx is zero vector'
endif
if ( abs(gc(5))+abs(gc(5)).eq.rZero ) then
write(stdo,*)
& ' helas-error : gc in ggggxx has zero momentum'
endif
if ( abs(gd(1))+abs(gd(2))
& +abs(gd(3))+abs(gd(4)).eq.rZero ) then
write(stdo,*) ' helas-warn : gd in ggggxx is zero vector'
endif
if ( abs(gd(5))+abs(gd(5)).eq.rZero ) then
write(stdo,*)
& ' helas-error : gd in ggggxx has zero momentum'
endif
pm = max( abs(pga(0)),abs(pgb(0)),abs(pgc(0)),abs(pgd(0)),
& abs(pga(1)),abs(pgb(1)),abs(pgc(1)),abs(pgd(1)),
& abs(pga(2)),abs(pgb(2)),abs(pgc(2)),abs(pgd(2)),
& abs(pga(3)),abs(pgb(3)),abs(pgc(3)),abs(pgd(3)) )
if ( abs(ga(5)+gb(5)+gc(5)+gd(5))
& +abs(ga(6)+gb(6)+gc(6)+gd(6)).ge.pm*epsi) then
write(stdo,*)
& ' helas-error : ga,gb,gc,gd in ggggxx'
write(stdo,*)
& ' : have not balanced momenta'
endif
if ( g.eq.rZero ) then
write(stdo,*) ' helas-error : g in ggggxx is zero coupling'
endif
#endif
dv1(0) = dcmplx(ga(1))
dv1(1) = dcmplx(ga(2))
dv1(2) = dcmplx(ga(3))
dv1(3) = dcmplx(ga(4))
dv2(0) = dcmplx(gb(1))
dv2(1) = dcmplx(gb(2))
dv2(2) = dcmplx(gb(3))
dv2(3) = dcmplx(gb(4))
dv3(0) = dcmplx(gc(1))
dv3(1) = dcmplx(gc(2))
dv3(2) = dcmplx(gc(3))
dv3(3) = dcmplx(gc(4))
dv4(0) = dcmplx(gd(1))
dv4(1) = dcmplx(gd(2))
dv4(2) = dcmplx(gd(3))
dv4(3) = dcmplx(gd(4))
v12 = dv1(0)*dv2(0)-dv1(1)*dv2(1)-dv1(2)*dv2(2)-dv1(3)*dv2(3)
v13 = dv1(0)*dv3(0)-dv1(1)*dv3(1)-dv1(2)*dv3(2)-dv1(3)*dv3(3)
v14 = dv1(0)*dv4(0)-dv1(1)*dv4(1)-dv1(2)*dv4(2)-dv1(3)*dv4(3)
v23 = dv2(0)*dv3(0)-dv2(1)*dv3(1)-dv2(2)*dv3(2)-dv2(3)*dv3(3)
v24 = dv2(0)*dv4(0)-dv2(1)*dv4(1)-dv2(2)*dv4(2)-dv2(3)*dv4(3)
v34 = dv3(0)*dv4(0)-dv3(1)*dv4(1)-dv3(2)*dv4(2)-dv3(3)*dv4(3)
dvertx = v14*v23 -v13*v24
vertex = dcmplx( dvertx ) * (g*g)
c if (abs(dvertx) .gt. 1d40) then
c write(*,*) 'Careful',abs(dvertx)
c write(*,*) v12,v13,v14
c write(*,*) v23,v24,v34
c endif
c
return
end
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