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MEQCD2to2Fast.h
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MEQCD2to2Fast.h
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// -*- C++ -*-
//
// MEQCD2to2Fast.h is a part of Herwig++ - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2007 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_MEQCD2to2Fast_H
#define HERWIG_MEQCD2to2Fast_H
//
// This is the declaration of the MEQCD2to2Fast class.
//
#include "Herwig++/MatrixElement/HwMEBase.h"
#include "ThePEG/Repository/UseRandom.h"
namespace Herwig {
using namespace ThePEG;
using namespace ThePEG::Helicity;
/**
* The MEQCD2to2Fast class implements the matrix elements for
* QCD \f$2\to2\f$ scattering processes using hard coded formulae and
* as such can not include spin correlations. It is designed to be a faster
* replacement for MEQCD2to2 for use in the underlying event.
*
* @see \ref MEQCD2to2FastInterfaces "The interfaces"
* defined for MEQCD2to2Fast.
*/
class MEQCD2to2Fast: public HwMEBase {
public:
/**
* The default constructor.
*/
MEQCD2to2Fast() :_maxflavour(5),_process(0) {
massOption(vector<unsigned int>(2,0));
}
/** @name Virtual functions required by the MEBase class. */
//@{
/**
* Return the order in \f$\alpha_S\f$ in which this matrix
* element is given.
*/
virtual unsigned int orderInAlphaS() const;
/**
* Return the order in \f$\alpha_{EW}\f$ in which this matrix
* element is given.
*/
virtual unsigned int orderInAlphaEW() const;
/**
* The matrix element for the kinematical configuration
* previously provided by the last call to setKinematics(), suitably
* scaled by sHat() to give a dimension-less number.
* @return the matrix element scaled with sHat() to give a
* dimensionless number.
*/
virtual double me2() const;
/**
* Return the scale associated with the last set phase space point.
*/
virtual Energy2 scale() const;
/**
* Add all possible diagrams with the add() function.
*/
virtual void getDiagrams() const;
/**
* Get diagram selector. With the information previously supplied with the
* setKinematics method, a derived class may optionally
* override this method to weight the given diagrams with their
* (although certainly not physical) relative probabilities.
* @param dv the diagrams to be weighted.
* @return a Selector relating the given diagrams to their weights.
*/
virtual Selector<DiagramIndex> diagrams(const DiagramVector & dv) const;
/**
* Return a Selector with possible colour geometries for the selected
* diagram weighted by their relative probabilities.
* @param diag the diagram chosen.
* @return the possible colour geometries weighted by their
* relative probabilities.
*/
virtual Selector<const ColourLines *>
colourGeometries(tcDiagPtr diag) const;
//@}
public:
/** @name Functions used by the persistent I/O system. */
//@{
/**
* Function used to write out object persistently.
* @param os the persistent output stream written to.
*/
void persistentOutput(PersistentOStream & os) const;
/**
* Function used to read in object persistently.
* @param is the persistent input stream read from.
* @param version the version number of the object when written.
*/
void persistentInput(PersistentIStream & is, int version);
//@}
/**
* The standard Init function used to initialize the interfaces.
* Called exactly once for each class by the class description system
* before the main function starts or
* when this class is dynamically loaded.
*/
static void Init();
protected:
/**
* Members to calculate the matrix elements
*/
//@{
/**
* Matrix element for \f$gg\to gg\f$.
*/
double gg2ggME() const {
Energy2 u(uHat()),t(tHat()),s(sHat());
double output = 9./4.*(3.-t*u/s/s-s*u/t/t-s*t/u/u);
double flow[3]={(1.-u*t/s/s-s*t/u/u+t*t/s/u),
(1.-t*u/s/s-s*u/t/t+u*u/s/t),
(1.-t*s/u/u-u*s/t/t+s*s/u/t)};
_flow = 1+UseRandom::rnd3(flow[0],flow[1],flow[2]);
double diag[3]={(sqr(u)+sqr(t))/sqr(s),
(sqr(s)+sqr(u))/sqr(t),
(sqr(s)+sqr(t))/sqr(u)};
if(_flow==1) diag[1]=0;
else if(_flow==2) diag[2]=0;
else if(_flow==3) diag[0]=0;
_diagram=1+UseRandom::rnd3(diag[0],diag[1],diag[2]);
return output;
}
/**
* Matrix element for \f$gg\to q\bar{q}\f$
*/
double gg2qqbarME() const {
Energy2 u(uHat()),t(tHat()),s(sHat());
Energy4 u2(sqr(u)),t2(sqr(t)),s2(sqr(s));
double output =(1./6./u/t-3./8./s2)*(t2+u2);
double flow[2]={u2/(u2+t2),t2/(u2+t2)};
_flow = 1+UseRandom::rnd2(flow[0],flow[1]);
_diagram=3+_flow;
return output;
}
/**
* Matrix element for \f$q\bar{q}\to gg\f$
*/
double qqbar2ggME() const {
Energy2 u(uHat()),t(tHat()),s(sHat());
Energy4 s2(sqr(s)),u2(sqr(u)),t2(sqr(t));
double output = 0.5*(32./27./u/t-8./3./s2)*(t2+u2);
double flow[2] = {u2/(u2+t2),t2/(t2+u2)};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
_diagram=6+_flow;
return output;
}
/**
* Matrix element for \f$qg\to qg\f$
*/
double qg2qgME() const {
Energy2 u(uHat()),t(tHat()),s(sHat());
Energy4 s2(sqr(s)),u2(sqr(u)),t2(sqr(t));
double output = (-4./9./s/u+1./t2)*(s2+u2);
double flow[2]={u2/(s2+u2),s2/(s2+u2)};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
_diagram=9+_flow;
return output;
}
/**
* Matrix elements for \f$\bar{q}g\to \bar{q}g\f$.
*/
double qbarg2qbargME() const {
// scale
Energy2 u(uHat()),t(tHat()),s(sHat());
Energy4 u2(sqr(u)),s2(sqr(s)); // t2(sqr(t))
double flow[2]={u2/(s2+u2),s2/(s2+u2)};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
_diagram=12+_flow;
return (-4./9./s/u+1./t/t)*(s*s+u*u);
}
/**
* Matrix element for \f$qq\to qq\f$
*/
double qq2qqME() const {
Energy2 u(uHat()),t(tHat());
Energy4 s2(sqr(sHat())),u2(sqr(u)),t2(sqr(t));
double output;
if(mePartonData()[0]->id()==mePartonData()[1]->id()) {
output = 0.5*(4./9.*((s2+u2)/t2+(s2+t2)/u2)
-8./27.*s2/u/t);
double flow[2]={(s2+u2)/t2,(s2+t2)/u2};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
}
else {
output = 4./9.*(s2+u2)/t2;
_flow=2;
}
_diagram = 15+_flow;
return output;
}
/**
* Matrix element for \f$\bar{q}\bar{q}\to \bar{q}\bar{q}\f$
*/
double qbarqbar2qbarqbarME() const {
Energy2 u(uHat()),t(tHat());
Energy4 u2(sqr(u)),t2(sqr(t)),s2(sqr(sHat()));
double output;
if(mePartonData()[0]->id()==mePartonData()[1]->id()) {
output = 0.5*(4./9.*((s2+u2)/t2+(s2+t2)/u2)
-8./27.*s2/u/t);
double flow[2]={(s2+u2)/t2,(s2+t2)/u2};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
}
else {
output = 4./9.*(s2+u2)/t2;
_flow = 2;
}
_diagram = 17+_flow;
// final part of colour and spin factors
return output;
}
/**
* Matrix element for \f$q\bar{q}\to q\bar{q}\f$
*/
double qqbar2qqbarME() const {
// type of process
bool diagon[2]={mePartonData()[0]->id()== -mePartonData()[1]->id(),
mePartonData()[0]->id()== mePartonData()[2]->id()};
// scale
Energy2 u(uHat()),t(tHat()),s(sHat());
Energy4 s2(sqr(s)),t2(sqr(t)),u2(sqr(u));
double output;
if(diagon[0]&&diagon[1]) {
output= (4./9.*((s2+u2)/t2+(u2+t2)/s2)
-8./27.*u2/s/t);
double flow[2]={(t2+u2)/s2,(s2+u2)/t2};
_flow=1+UseRandom::rnd2(flow[0],flow[1]);
}
else if(diagon[0]) {
output = (4./9.*(t2+u2)/s2);
_flow=1;
}
else {
output = (4./9.*(s2+u2)/t2);
_flow=2;
}
_diagram=19+_flow;
return output;
}
//@}
protected:
/** @name Clone Methods. */
//@{
/**
* Make a simple clone of this object.
* @return a pointer to the new object.
*/
virtual IBPtr clone() const;
/** Make a clone of this object, possibly modifying the cloned object
* to make it sane.
* @return a pointer to the new object.
*/
virtual IBPtr fullclone() const;
//@}
private:
/**
* The static object used to initialize the description of this class.
* Indicates that this is a concrete class with persistent data.
*/
static ClassDescription<MEQCD2to2Fast> initMEQCD2to2Fast;
/**
* The assignment operator is private and must never be called.
* In fact, it should not even be implemented.
*/
MEQCD2to2Fast & operator=(const MEQCD2to2Fast &);
private:
/**
* Maximum numbere of quark flavours to include
*/
unsigned int _maxflavour;
/**
* Processes to include
*/
unsigned int _process;
/**
* Colour flow
*/
mutable unsigned int _flow;
/**
* Diagram
*/
mutable unsigned int _diagram;
};
}
#include "ThePEG/Utilities/ClassTraits.h"
namespace ThePEG {
/** @cond TRAITSPECIALIZATIONS */
/** This template specialization informs ThePEG about the
* base classes of MEQCD2to2Fast. */
template <>
struct BaseClassTrait<Herwig::MEQCD2to2Fast,1> {
/** Typedef of the first base class of MEQCD2to2Fast. */
typedef Herwig::HwMEBase NthBase;
};
/** This template specialization informs ThePEG about the name of
* the MEQCD2to2Fast class and the shared object where it is defined. */
template <>
struct ClassTraits<Herwig::MEQCD2to2Fast>
: public ClassTraitsBase<Herwig::MEQCD2to2Fast> {
/** Return a platform-independent class name */
static string className() { return "Herwig::MEQCD2to2Fast"; }
/**
* The name of a file containing the dynamic library where the class
* MEQCD2to2Fast is implemented. It may also include several, space-separated,
* libraries if the class MEQCD2to2Fast depends on other classes (base classes
* excepted). In this case the listed libraries will be dynamically
* linked in the order they are specified.
*/
static string library() { return "HwMEHadronFast.so"; }
};
/** @endcond */
}
#endif /* HERWIG_MEQCD2to2Fast_H */

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