Add pizero decay routine

pizero_decay.f

@@ -0,0 +1,97 @@
+	subroutine pizero_decay(vertex,success,gamma1,gamma2)
+
+C+_____________________________________________________________________________
+!  pizero_decay:  This routine takes the electroproduced pi0 and
+!   produces the 2-photon pair that we actually detect (in a calorimeter
+!   most likely).
+!	
+!   This routine should be called before the single arm mc.
+!
+!   The outgoing pion will be thrown flat in phi and cos(th) in
+!   pi0 rest frame.	
+C-_____________________________________________________________________________
+	USE structureModule
+	implicit none
+	include 'simulate.inc'
+
+	type(event):: vertex
+C Local declarations.
+	real*8 ph,eh
+	real*8 rph,rth1,rth
+	real*8 beta,gamma
+	real*8 pxr1,pyr1,pzr1,pxr2,pyr2,pzr2
+	real*8 gamma1(4), gamma2(4)
+	real*8 bx,by,bz,er,pr
+	real*8 pf1,pf2
+
+	real*8 Mgamma
+
+	real*8 grnd
+
+	logical success
+
+C ============================= Executable Code ================================
+
+	parameter(Mgamma=0.0)
+C calculate beta and gamma for pi0 rest frame
+	ph = vertex%p%P
+	eh = sqrt(ph**2+Mh**2)
+	beta = ph/eh
+	gamma = 1./sqrt(1.-beta*beta)
+
+C Generate center/mass decay angles and momenta.
+	rph = grnd()*2.*pi
+ 100	rth1 = grnd()*2.-1.
+	rth = acos(rth1)
+
+
+	er = Mh/2.0
+	if(er.lt.Mgamma) then
+	   success=.false.
+	   return
+	endif
+	pr = sqrt(er**2-Mgamma**2)
+	pxr1 = pr*sin(rth)*cos(rph)
+	pyr1 = pr*sin(rth)*sin(rph)
+	pzr1 = pr*cos(rth)
+
+	pxr2 = -pxr1
+	pyr2 = -pyr1
+	pzr2 = -pzr1
+C Boost to Lab frame, calculate new angles and momentum, finish drift
+
+	bx = -beta * vertex%up%x
+	by = -beta * vertex%up%y
+	bz = -beta * vertex%up%z
+	call loren(gamma,bx,by,bz,er,pxr1,pyr1,pzr1,
+     >      gamma1(1),gamma1(2),gamma1(3),gamma1(4),pf1)
+
+	call loren(gamma,bx,by,bz,er,pxr2,pyr2,pzr2,
+     >      gamma2(1),gamma2(2),gamma2(3),gamma2(4),pf2)
+
+
+c   do some checks for for energy and momentum conservation in the lab
+
+	if(abs(eh-gamma1(1)-gamma2(1)).gt.0.001) then
+	   write(6,*) 'pi0 decay: bad energy sum'
+	endif
+
+	if(abs(ph*vertex%up%x-gamma1(2)-gamma2(2)).gt.0.001) then
+	   write(6,*) 'pi0 decay: bad px sum'
+	endif
+
+	if(abs(ph*vertex%up%y-gamma1(3)-gamma2(3)).gt.0.001) then
+	   write(6,*) 'pi0 decay: bad py sum'
+	endif
+
+	if(abs(ph*vertex%up%z-gamma1(4)-gamma2(4)).gt.0.001) then
+	   write(6,*) 'pi0 decay: bad pz sum'
+	endif
+	success = .true.
+
+
+	return
+	end
+
+
+