8.2. Custom scale setterΒΆ

You can write a custom calculator to set the factorisation, renormalisation and resummation scales. It has to be implemented as a C++ class which derives from the Scale_Setter_Base base class and implements only the constructor and the Calculate method.

Here is a snippet for a very simple one, which sets all three scales to the invariant mass of the two incoming partons.

#include "PHASIC++/Scales/Scale_Setter_Base.H"
#include "ATOOLS/Org/Message.H"

using namespace PHASIC;
using namespace ATOOLS;

namespace PHASIC {

  class Custom_Scale_Setter: public Scale_Setter_Base {


    Custom_Scale_Setter(const Scale_Setter_Arguments &args) :
      m_scale.resize(3); // by default three scales: fac, ren, res
                         // but you can add more if you need for COUPLINGS
      SetCouplings(); // the default value of COUPLINGS is "Alpha_QCD 1", i.e.
                      // m_scale[1] is used for running alpha_s
                      // (counting starts at zero!)

    double Calculate(const std::vector<ATOOLS::Vec4D> &p,
                  const size_t &mode)
      double muF=(p[0]+p[1]).Abs2();
      double muR=(p[0]+p[1]).Abs2();
      double muQ=(p[0]+p[1]).Abs2();

      m_scale[stp::fac] = muF;
      m_scale[stp::ren] = muR;
      m_scale[stp::res] = muQ;

      // Switch on debugging output for this class with:
      // Sherpa "OUTPUT=2[Custom_Scale_Setter|15]"
      DEBUG_FUNC("Calculated scales:");

      return m_scale[stp::fac];



// Some plugin magic to make it available for SCALES=CUSTOM

Scale_Setter_Base *ATOOLS::Getter
operator()(const Scale_Setter_Arguments &args) const
  return new Custom_Scale_Setter(args);

void ATOOLS::Getter<Scale_Setter_Base,Scale_Setter_Arguments,
PrintInfo(std::ostream &str,const size_t width) const
  str<<"Custom scale scheme";

If the code is compiled into a library called libCustomScale.so, then this library is loaded dynamically at runtime with the switch SHERPA_LDADD: CustomScale either on the command line or in the run section, cf. Customization. This then allows to use the custom scale like a built-in scale setter by specifying SCALES: CUSTOM (cf. SCALES).