Custom scales

HEJ 2 comes with a small selection of built-in renormalisation and factorisation scales, as described in the scales setting. In addition to this, user-defined scales can be imported from custom libraries.

Writing the library

Custom scales are defined through C++ functions that take an event and compute the corresponding scale. As an example, let’s consider a function returning the transverse momentum of the softest jet in an event. To make it accessible from HEJ 2, we have to prevent C++ name mangling with extern "C":

#include "HEJ/Event.hh"

extern "C"
double softest_jet_pt(HEJ::Event const & ev){
  const auto softest_jet = sorted_by_pt(ev.jets()).back();
  return softest_jet.perp();
}

After saving this code to some file myscales.cc, we can compile it to a shared library. With the g++ compiler this can be done with the command

g++ $(HEJ-config --cxxflags) -fPIC -shared -Wl,-soname,libmyscales.so -o libmyscales.so myscales.cc

If g++ is used and the library also contains other definitions, it is recommended to add -fvisibility=hidden to the compiler flags and __attribute__((visibility("default"))) after extern "C" for each exported function in order to avoid possible name clashes.

Importing the scale into HEJ 2

Our custom scale can now be imported into HEJ 2 by adding the following lines to the YAML configuration file

import scales:
   /path/to/libmyscales.so: softest_jet_pt

It is also possible to import several scales from one or more libraries:

import scales:
   /path/to/libmyscales1.so: [first_scale, second_scale]
   /path/to/libmyscales2.so: [another_scale, yet_another_scale]

The custom scales can then be used as usual in the scales setting, for example

scales: [H_T, softest_jet_pt, 2*softest_jet_pt]