Jet Reconstruction

This package implements sequential Jet Reconstruction (clustering) algorithms, which are used in high-energy physics as part of event reconstruction for $pp$ and $e^+e^-$ colliders.

Algorithms

Algorithms used are based on the C++ FastJet package (https://fastjet.fr, hep-ph/0512210, arXiv:1111.6097), reimplemented natively in Julia.

The algorithms include anti-${k}_\text{T}$, Cambridge/Aachen, inclusive $k_\text{T}$, generalised $k_\text{T}$ for $pp$ events; and the Durham algorithm and generalised $k_\text{T}$ for $e^+e^-$.

Reconstruction Interface

The main interface for reconstruction is jet_reconstruct, called as, e.g.,

jet_reconstruct(particles; algorithm = JetAlgorithm.AntiKt, R = 1.0)

or with some of the optional arguments,

jet_reconstruct(particles; algorithm = JetAlgorithm.GenKt, R = 0.4, 
                p = 0.5, recombine = +, strategy = RecoStrategy.Best)

Where particles is a collection of 4-vector objects (see Input Particle Types) to reconstruct and the algorithm is either given explicitly or implied by the power value.

For the case of generalised $k_T$ (for $pp$ and $e^+e^-$) both the algorithm (GenKt, EEKt) and p are needed.

The R value determines the cone size; in the case of the Durham algorithm the R value is ignored.

The object returned is a ClusterSequence, which internally tracks all merge steps and is used for Inclusive and Exclusive Selections.

Algorithm Types

Each known algorithm is referenced using a JetAlgorithm scoped enum value.

$pp$ Algorithms

For the three $pp$ algorithms with fixed p values, the p value can be given instead of the algorithm name. However, this should be considered deprecated and will be removed in a future release.

Strategy

Generally one does not need to manually specify a strategy, but Algorithm Strategy describes how to do this, if desired.

Inclusive and Exclusive Selections

To obtain final jets both inclusive ($p_T$ cut) and exclusive ($n_{jets}$ or $d_{ij}$ cut) selections are supported:

• inclusive_jets(clusterseq::ClusterSequence, ptmin = 0.0)
• exclusive_jets(clusterseq::ClusterSequence; dcut = nothing, njets = nothing)

(For exclusive_jets either dcut or njets is needed, but not both.)

Sorting

Sorting vectors is trivial in Julia, no special sorting methods are provided. As an example, to sort exclusive jets of $>5.0$ (usually GeV, depending on your EDM) from highest energy to lowest:

sorted_jets = sort!(inclusive_jets(cs::ClusterSequence; ptmin=5.0), 
  by=JetReconstruction.energy, rev=true)

Jet Constituents and Jet Parents

There are two ways to retrieve jet constituents. The first way is just to retrieve the indexes of the constituent jets. These indexes refer to the original collection of particles passed in to the reconstruction.

• constituent_indexes

The alternative it to retrieve the actual jets from the reconstruction sequence. In this case the returned array contains references to the jet objects (of type T) used internally in the reconstruction.

• constituents

Note that in both these cases the cluster sequence object from the reconstruction is required (to avoid circular dependencies and improve memory management reconstructed jets do not contain a link back to their cluster sequence).

To retrieve a jet's parents:

• parent_jets

This will return a tuple of the target jet's parents, or nothing when one or both parents are missing (the only case when a jet has one parent is when it undergoes a beam merge step).

References

Although it has been developed further since the CHEP2023 conference, the CHEP conference proceedings, 10.1051/epjconf/202429505017, should be cited if you use this package:

@article{refId0,
    author = {{Stewart, Graeme Andrew} and {Gras, Philippe} and {Hegner, Benedikt} and {Krasnopolski, Atell}},
    doi = {10.1051/epjconf/202429505017},
    journal = {EPJ Web of Conf.},
    pages = {05017},
    title = {Polyglot Jet Finding},
    url = {https://doi.org/10.1051/epjconf/202429505017},
    volume = 295,
    year = 2024,
    eprint={2309.17309},
    archivePrefix={arXiv},
    primaryClass={hep-ex}
}

The original paper on arXiv is:

@misc{stewart2023polyglot,
      title={Polyglot Jet Finding}, 
      author={Graeme Andrew Stewart and Philippe Gras and Benedikt Hegner and Atell Krasnopolski},
      year={2023},
      eprint={2309.17309},
      archivePrefix={arXiv},
      primaryClass={hep-ex}
}

Authors and Copyright

Code in this package is authored by:

• Atell Krasnopolski <delta_atell@protonmail.com>
• Graeme A Stewart <graeme.andrew.stewart@cern.ch>
• Philippe Gras <philippe.gras@cern.ch>

and is Copyright 2022-2024 The Authors, CERN.

The code is under the MIT License.