subroutine scaleset_HTttbar4(p,mu0)
c--- subroutine to calculate dynamic scale equal to
c--- partonic HT (i.e. scalar sum of top qaurk and anti-top quark, whether or not they pass cuts)
      implicit none
      include 'constants.f'
      include 'npart.f'
      integer j
      double precision p(mxpart,4),mu0,pt
      double precision ptop(4),ptbr(4),MTtop,MTtbr

      do j=1,4
      ptop(j)=p(3,j)+p(4,j)+p(5,j)
      ptbr(j)=p(5,j)+p(6,j)+p(7,j)
      enddo

      MTtop=dsqrt(max(ptop(4)**2-ptop(3)**2,0.01))
      MTtbr=dsqrt(max(ptbr(4)**2-ptbr(3)**2,0.01))

      mu0=(MTtop+MTtbr)/4.0d0

      return
      end