subroutine scalar_dm(i1,i2,za,zb,amp) 
      implicit none 
      include 'constants.f' 
      include 'dm_params.f' 
      include 'zprods_decl.f' 
      integer i1,i2
      double complex amp(2,2) 
      double precision Bp,Beta,sab

      amp(1,2)=czip 
      amp(2,1)=czip
      
      sab=Dble(za(i1,i2)*zb(i2,i1))
!      write(6,*) 'in scalar dm ',sab 
!      pause
      Beta=one-4d0*xmass**2/sab 
      Beta=dsqrt(beta) 
      Bp=one+Beta
      
      amp(1,1)=-za(i1,i2)*Bp+xmass**2/zb(i2,i1)/Bp     
      amp(2,2)=zb(i2,i1)*Bp-xmass**2/za(i1,i2)/Bp

      return 
      end