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ShowerAlphaQCD.cc
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ShowerAlphaQCD.cc
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// -*- C++ -*-
//
// This is the implementation of the non-inlined, non-templated member
// functions of the ShowerAlphaQCD class.
//
#include "ShowerAlphaQCD.h"
#include "Pythia7/Interface/ClassDocumentation.h"
using namespace Herwig;
ShowerAlphaQCD::~ShowerAlphaQCD() {}
ClassDescription<ShowerAlphaQCD> ShowerAlphaQCD::initShowerAlphaQCD;
// Definition of the static class description member.
void ShowerAlphaQCD::Init() {
static ClassDocumentation<ShowerAlphaQCD> documentation
("This (concrete) class describes the QCD alpha running.");
}
double ShowerAlphaQCD::value(const Energy2 scale) {
double val = alpha_s(scale, 1.0*GeV2, 1);
// ***ACHTUNG*** just a call to the simpler dummy function used
// previously in SG's fragmentation programs
// ***LOOKHERE*** HERE THE FUNCTION alphaQCD( scale ) SHOULD BE DEFINED
// OR CALL THE ONE DEFINED IN PYTHIA7
// (see class Pythia7::O1AlphaS)
return scaleFactor() * val;
}
//////////////////////////////////////////////////////////////////////////
// private stuff:
// ***ACHTUNG*** put somthing more serious here, later
// a parametrization of alpha_s for test purposes only
double ShowerAlphaQCD::alpha_s(double q2, double q2min, int type) {
// hier mit 4 flavours
double lambda = 1.0;
double val = 0.0;
double lambda4 = 0.3;
int cflavour;
// assign different lambda_QCD values for different numbers of
// active flavours in a smooth way
if(q2 < 1.) {
lambda = 0.3280;
cflavour = 27;
} else {
if(q2 < sqr(5.)) {
lambda = lambda4;
cflavour = 25;
} else {
if(q2 < sqr(100.)) {
lambda = 0.2349;
cflavour = 23;
} else {
lambda = 0.1320;
cflavour = 21;
}
}
};
// different choices for q2 in the non-perturbative region, q2 < q2min
if (q2 < q2min) {
if (q2 < 0) {
cerr << "alpha_s: negative q2" << endl;
val = -1.;
} else {
switch (type) {
case 1:
// flat, zero; the default type with no NP effects.
val = 0.;
break;
case 2:
// flat, non-zero alpha_s = alpha_s(q2min).
val = 12.*M_PI/(((double) cflavour)*log(q2min/sqr(lambda)));
break;
case 3:
// linear
val = 12.*M_PI/(((double) cflavour)*log(q2min/sqr(lambda)))
*q2/q2min;
break;
case 4:
// quadratic
val = 12.*M_PI/(((double) cflavour)*log(q2min/sqr(lambda)))
*sqr(q2/q2min);
break;
};
}
} else {
// the common running coupling
val = 12.*M_PI/(((double) cflavour)*log(q2/sqr(lambda)));
};
return (val);
}

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