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// -*- C++ -*-
//
// ShowerAlpha.h is a part of Herwig - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2017 The Herwig Collaboration
//
// Herwig is licenced under version 3 of the GPL, see COPYING for details.
// Please respect the MCnet academic guidelines, see GUIDELINES for details.
//
#ifndef HERWIG_ShowerAlpha_H
#define HERWIG_ShowerAlpha_H
//
// This is the declaration of the ShowerAlpha class.
//
#include "ThePEG/Interface/Interfaced.h"
#include "ShowerAlpha.fh"
namespace Herwig {
using namespace ThePEG;
/** \ingroup Shower
*
* This class is the abstract class from which all types of running couplings
* used in the Showering derive from.
* The main purpose of this class, and the ones that derive from it, is
* to allow systematic uncertainties for the initial-state radiation and,
* independently, the final-state radiation effects, to be evaluated.
*
* This is achieved by allowing a multiplicative factor,
* which is 1.0 for the "central value",
* for the scale argument, \f$\mu^2\f$, of the running coupling. This
* factor, \f$f\f$ is given by the scaleFactor() member and the coupling
* returned by the value() member is such that
* \f[\alpha(\mu^2)\to \alpha(f\times\mu^2).\f]
* This scale factor is a parameter which is settable by the user, via the
* interface.
* Although, of course, it is not clear my how much we should scale
* in order to get a \f$1\sigma\f$ systematic error (but factors:
* 1/2 and 2 are quite common), this method provides a double side error,
* and it appears more sensible than the rough and one-sided evaluation
* obtained
* via turning off the I.S.R. and/or F.S.R. (possibilities which are,
* anyway, provided by Herwig).
*
* @see \ref ShowerAlphaInterfaces "The interfaces"
* defined for ShowerAlpha.
*/
class ShowerAlpha: public Interfaced {
public:
/** @name Standard constructors and destructors. */
//@{
/**
* The default constructor.
*/
ShowerAlpha() : _scaleFactor( 1.0 ) {}
//@}
public:
/**
* Methods to return the coupling and the scaleFactor
*/
//@{
/**
* Pure virtual method that is supposed to return the
* running alpha value evaluated at the input scale.
* @param scale The scale
* @return The coupling
*/
virtual double value(const Energy2 scale) const = 0;
/**
* Virtual method, which
* should be overridden in a derived class to provide an
* overestimate approximation of the alpha value.
*/
virtual double overestimateValue() const = 0;
/**
* Virtual method which returns the ratio of the running alpha
* value at the input scale to the overestimated value.
* @param scale The scale
* @return The ratio
*/
virtual double ratio(const Energy2 scale,double factor=1.) const = 0;
/**
* It returns the factor that multiplies the
* scale argument, \f$\mu^2\f$, of the running coupling.
* This is supposed to be <I>1.0</I> in normal conditions (central values)
* whereas different values can be useful for systematics evaluation
* for Initial State radiation or Final State radiation effects.
*/
double scaleFactor() const {return _scaleFactor;}
/**
* Initialize this coupling.
*/
virtual void initialize () {}
//@}
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();
private:
/**
* The assignment operator is private and must never be called.
* In fact, it should not even be implemented.
*/
ShowerAlpha & operator=(const ShowerAlpha &);
private:
/**
* The scale factor
*/
double _scaleFactor;
};
}
#endif /* HERWIG_ShowerAlpha_H */

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