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Kinematics.h
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// -*- C++ -*-
//
// Kinematics.h is a part of Herwig++ - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2011 The Herwig Collaboration
//
// Herwig++ is licenced under version 2 of the GPL, see COPYING for details.
// Please respect the MCnet academic guidelines, see GUIDELINES for details.
//
//
#ifndef HERWIG_Kinematics_H
#define HERWIG_Kinematics_H
// This is the declaration of the Kinematics class.
#include <ThePEG/Config/ThePEG.h>
#include "ThePEG/Vectors/ThreeVector.h"
#include "ThePEG/Vectors/LorentzRotation.h"
#include "ThePEG/Repository/UseRandom.h"
#include <ThePEG/Vectors/Lorentz5Vector.h>
namespace Herwig {
using namespace ThePEG;
/** \ingroup Utilities
* This is a pure static class which provides some useful methods
* for kinematics computation, as the two body decays.
*
* NB) For other useful kinematical methods (and probably even those
* implemented in Kinematics class!):
* @see UtilityBase
*/
class Kinematics {
public:
/**
* Calculate the momenta for a two body decay
* The return value indicates success or failure.
* @param p The momentum of the decaying particle
* @param m1 The mass of the first decay product
* @param m2 The mass of the second decay product
* @param unitDir1 Direction for the products in the rest frame of
* the decaying particle
* @param p1 The momentum of the first decay product
* @param p2 The momentum of the second decay product
*/
static bool twoBodyDecay(const Lorentz5Momentum & p,
const Energy m1, const Energy m2,
const Axis & unitDir1,
Lorentz5Momentum & p1, Lorentz5Momentum & p2) {
Energy min=p.mass();
if ( min >= m1 + m2 && m1 >= ZERO && m2 >= ZERO ) {
Momentum3 pstarVector = unitDir1 * pstarTwoBodyDecay(min,m1,m2);
p1 = Lorentz5Momentum(m1, pstarVector);
p2 = Lorentz5Momentum(m2,-pstarVector);
// boost from CM to LAB
Boost bv=p.boostVector();
p1.boost( bv );
p2.boost( bv );
return true;
}
return false;
}
/**
* Calculate the momenta for a two body decay
* The return value indicates success or failure.
* @param p The momentum of the decaying particle
* @param m1 The mass of the first decay product
* @param m2 The mass of the second decay product
* @param cosThetaStar1 Polar angle in rest frame
* @param phiStar1 Azimuthal angle in rest frame
* @param p1 The momentum of the first decay product
* @param p2 The momentum of the second decay product
*/
static bool twoBodyDecay(const Lorentz5Momentum & p,
const Energy m1, const Energy m2,
const double cosThetaStar1,
const double phiStar1,
Lorentz5Momentum & p1, Lorentz5Momentum & p2) {
return twoBodyDecay(p,m1,m2,unitDirection(cosThetaStar1,phiStar1),p1,p2);
}
/**
* As the name implies, this takes the momentum p0 and does a flat three
* body decay into p1..p3. The argument fcn is used to add additional
* weights. If it is not used, the default is just flat in phasespace.
* The return value indicates success or failure.
*/
static bool threeBodyDecay(Lorentz5Momentum p0, Lorentz5Momentum &p1,
Lorentz5Momentum &p2, Lorentz5Momentum &p3,
double (*fcn)(Energy2,Energy2,Energy2,InvEnergy4) = NULL);
/**
* For the two body decay M -> m1 + m2 it gives the module of the
* 3-momentum of the decay product in the rest frame of M.
*/
static Energy pstarTwoBodyDecay(const Energy M,
const Energy m1, const Energy m2) {
return ( M > ZERO && m1 >=ZERO && m2 >= ZERO && M > m1+m2 ?
Energy(sqrt(( sqr(M) - sqr(m1+m2) )*( sqr(M) - sqr(m1-m2) ))
/ (2.0*M) ) : ZERO);
}
/**
* It returns the unit 3-vector with the given cosTheta and phi.
*/
static Axis unitDirection(const double cosTheta, const double phi) {
return ( fabs( cosTheta ) <= 1.0 ?
Axis( cos(phi)*sqrt(1.0-cosTheta*cosTheta) ,
sin(phi)*sqrt(1.0-cosTheta*cosTheta) , cosTheta) : Axis() );
}
/**
* This just generates angles. First flat -1..1, second flat 0..2Pi
*/
static void generateAngles(double & ct, double & az) {
ct = UseRandom::rnd()*2.0 - 1.0; // Flat from -1..1
az = UseRandom::rnd()*Constants::twopi;
}
private:
/**
* Pure static class so default constructor private
*/
Kinematics();
/**
* Pure static class so copy constructor private
*/
Kinematics(const Kinematics & x);
/**
* The assignment operator is private and must never be called.
* In fact, it should not even be implemented.
*/
Kinematics & operator=(const Kinematics & x);
/**
* Maximum number of attempts to generate a 3, 4 or 5 body decay.
*/
static const unsigned int _ntry=100;
};
}
#endif /* HERWIG_Kinematics_H */

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