gen_stop: Keep 1/sqrt(pt) sampling for the virt part, seems to work somewhat better.

src/Phase/gen_stop.f

@@ -78,35 +78,13 @@ c--- generate the pt of jet number ijet
 c--- rapidity limited by E=pT*coshy
         wt=wt/16._dp/pi**3
 
-c       ! some other ways to generate in pT
-c       ! directly in pT
-c       pt = sqrts*r(ijet)/2._dp
-c       xjac = r(ijet)*sqrts**2/4._dp
-c       ! 1/pt
-c       pt = sqrts*r(ijet)/(sqrts+2*r(ijet)-sqrts*r(ijet))
-c       xjac = r(ijet)*sqrts**3 / (2*r(ijet) - sqrts*(r(ijet)-1))**3
-c       ! -log(pT)
-c       pt = exp(log(sqrts/2._dp) - (1-r(ijet))/r(ijet))
-c       xjac = exp(2._dp*log(sqrts/2._dp) - 2._dp*(1-r(ijet))/r(ijet))/r(ijet)**2
-
-        ! for nnlo double real ipsgen=1
-        !if ((kcase == kbq_tpq_jet) .and. (kpart == kreal) .and. usescet .and. (ipsgen == 1)) then
-c           if (ijet == 1) then
-                ! in 1/sqrt(pT)
-                !pt = r(ijet)**2*sqrts/2._dp
-                !xjac = pt*sqrts/2._dp*2._dp*r(ijet)
-c          else
-c              ! 1/pt^2
-c              pt = 1._dp / sqrt(4._dp/sqrts**2 + (1-r(ijet))/r(ijet))
-c              xjac = sqrts**4 / 2._dp / (sqrts**2*(r(ijet)-1._dp) - 4*r(ijet))**2
-c           endif
-        ! for nlo real, ipsgen == 2
-        !elseif ((kcase == kbq_tpq) .and. (kpart == kreal) .and. (.not. usescet) .and. (ipsgen == 2)) then
-            !error stop "todo"
-        !else
-            !error stop "please explicitly setup gen_stop for this case"
+        if (kpart == kvirt) then
+            pt = r(ijet)**2*sqrts/2._dp
+            xjac = pt*sqrts/2._dp*2._dp*r(ijet)
+        else
             call genpt(r(ijet),5._dp,.false.,pt,xjac)
-        !endif
+        endif
+
         wt=wt*xjac
 
         etamax=sqrts/2._dp/pt