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LundPlane FastJet contrib
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The LundPlane FastJet contrib provides tools necessary to construct
the Lund Plane of a jet.

It is based on arXiv:1807.04758 by Frederic Dreyer, Gavin Salam, and
Gregory Soyez.

Azimuthal-angle definitions are taken from arXiv:2103.16526, by
Alexander Karlberg, Gavin Salam, Ludovic Scyboz and Rob Verheyen, and
arXiv:2111.01161 by the authors above and Keith Hamilton.

It provides the following C++ classes:

- LundDeclustering
  Contains the declustering variables associated with a node on
  the Lund plane.

- LundGenerator
  A tool to generate a vector of LundDeclustering for a given jet
  corresponding to its primary Lund plane.

- RecursiveLundEEGenerator
  A recursive tool to extract declusterings for a given jet in
  e+e- collisions, i.e. its full Lund structure down to a user-given
  depth

- LundWithSecondary
  A tool to generate primary, and secondary, lund planes of a jet
  according to the definition given as input.

- SecondaryLund, SecondaryLund_{mMDT|dotmMDT|Mass}
  Provides a definition for the leading emission.

There are four C++ examples which can be compiled with

> make example example_secondary example_dpsi_collinear example_dpsi_slice

The example program can be run using 
> ./example < ../data/single-event.dat

and will output the primary lund plane declusterings as well as
generating a jets.json file, which can be read by a python script:

> python3 ./example.py

Some python classes to assist with reading and processing the json file
are provided in the read_lund_json.py module.

The example_secondary program outputs both the primary and secondary
Lund plane declusterings to screen, and can be run with:

> ./example_secondary < ../data/single-event.dat

The example_dpsi_collinear and example_dpsi_slice programs compute the
contribution to the collinear azimuthal delta_psi observable from
2103.16526, and to the slice observable delta_psi_slice from 2111.01161,
respectively. They can be run with

> ./example_dpsi_collinear < ../data/single-ee-event.dat
> ./example_dpsi_slice < ../data/single-ee-event.dat
