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Twobody.cxx

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// -*- C++ -*-  Twobody.cc,v 1.1.1.1 1994/04/18 18:12:23 burnett Exp
//
// This file is part of HepGismo
//
// file: twobody.cpp

// Interface dependencies -------------------------------------------------


#include "gismo/Twobody.h"
// End Interface dependencies ---------------------------------------------

// Implementation dependencies --------------------------------------------
#include "Random.h"
#include <cmath>
// End Implementation dependencies-----------------------------------------


// member function
#include "gismo/GParticle.h"
#include "facilities/error.h"
static inline double sqr(double x){return x*x;}

int TwoBody::decay(GParticle* parent)const
{

   if (parent->numChildren() != 2)
   {  WARNING( "Twobody::decay-- must be two secondaries to decay!");
      return 0;
   }
   GParticle& b = *parent->child(0);
   GParticle& c = *parent->child(1);
   float m1 = b.mass();
   float m2 = c.mass();

   float M = parent->mass();

   float pstar = (sqr(M)-sqr(m1+m2))*(sqr(M)-sqr(m1-m2));
   if (pstar<0.)
   {  WARNING( "*** decay kinematically forbidden");
      return 0;
   }
   pstar = sqrt(pstar)/(2.0*M);
   float costh = Random::flat(-1.0, 1.0);
   float phi   = Random::flat(2*M_PI);
        Vector pcm(0,0,pstar);
        pcm.rotateY(acos(costh));  // rotate about y axis by acos(costh)
        pcm.rotateZ(phi);         // rotate about z axis by phi

   b.setMomentum(pcm);
   c.setMomentum(-pcm);


   return 1;
}

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