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SigmaRatio.h
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SigmaRatio.h
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#ifndef CepGen_StructureFunctions_SigmaRatio_h
#define CepGen_StructureFunctions_SigmaRatio_h
#include <array>
#include <cmath>
namespace CepGen
{
namespace SF
{
class SigmaRatio
{
public:
SigmaRatio() {}
virtual double operator()( double q2, double xbj ) const { return 0.; }
};
class E143Ratio : public SigmaRatio
{
public:
struct Parameterisation
{
double q2_b;
std::array<double,6> a, b, c;
static Parameterisation standard() {
Parameterisation out;
out.q2_b = 0.34;
out.a = { { 0.0485, 0.5470, 2.0621, -0.3804, 0.5090, -0.0285 } };
out.b = { { 0.0481, 0.6114, -0.3509, -0.4611, 0.7172, -0.0317 } };
out.c = { { 0.0577, 0.4644, 1.8288, 12.3708, -43.1043, 41.7415 } };
return out;
}
};
E143Ratio( const Parameterisation& param = Parameterisation::standard() ) : params_( param ) {}
double operator()( double q2, double xbj ) const override
{
const double u = q2/params_.q2_b;
const double xl = log( 25.*q2 );
const double pa = ( 1.+params_.a[3]*xbj+params_.a[4]*xbj*xbj )*pow( xbj, params_.a[5] );
const double pb = ( 1.+params_.b[3]*xbj+params_.b[4]*xbj*xbj )*pow( xbj, params_.b[5] );
const double tt = theta( q2, xbj );
const double q2_thr = params_.c[3]*xbj + params_.c[4]*xbj*xbj+params_.c[5]*xbj*xbj*xbj;
const double ra = params_.a[0]/xl*tt + params_.a[1]/pow( pow( q2, 4 )+pow( params_.a[2], 4 ), 0.25 )*pa,
rb = params_.b[0]/xl*tt + ( params_.b[1]/q2+params_.b[2]/( q2*q2+0.3*0.3 ) )*pb,
rc = params_.c[0]/xl*tt + params_.c[1]*pow( pow( q2-q2_thr, 2 )+pow( params_.c[2], 2 ), -0.5 );
const double r = ( ra+rb+rc ) / 3.;
if ( q2 > params_.q2_b ) return r;
return r * 0.5 * ( 3.*u-u*u*u );
}
private:
double theta( double q2, double xbj ) const {
return 1.+12.*( q2/( q2+1. ) )*( 0.125*0.125/( 0.125*0.125+xbj*xbj ) );
}
Parameterisation params_;
};
}
}
#endif

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