Abstract
We present an algorithm for deriving partonic flavour labels to be applied to truth particle jets in Monte Carlo event simulations. The inputs to this approach are final pre-hadronisation partons, to remove dependence on unphysical details such as the order of matrix element calculation and shower generator frame recoil treatment. These are clustered using standard jet algorithms, modified to restrict the allowed pseudojet combinations to those in which tracked flavour labels are consistent with QCD and QED Feynman rules. The resulting algorithm is shown to be portable between the major families of shower generators, and largely insensitive to many possible systematic variations: it hence offers significant advantages over existing ad hoc labelling schemes. However, it is shown that contamination from multi-parton scattering simulations can disrupt the labelling results. Suggestions are made for further extension to incorporate more detailed QCD splitting function kinematics, robustness improvements, and potential uses for truth-level physics object definitions and tagging.
Highlights
The rise of jet substructure methods at the LHC has prompted a resurgence in attempts to distinguish “quark” and “gluon” hadronic jets from each other, primarily for use in searches for BSM phenomena
Even where the parton momenta are physical in their chosen frame, it is often the case that different generations of parton evolution are represented in frames different from the lab frame relevant to final state observables;
This label is discovered by looping over all partons, including those in the hard process final state, through all the intermediate stages of the parton shower and Multi-parton interactions (MPI), down to the final partons described in Sect
Summary
The rise of jet substructure methods at the LHC has prompted a resurgence in attempts to distinguish “quark” and “gluon” hadronic jets from each other, primarily for use in searches for BSM phenomena. The potential unphysical distinction between “matrix element” and “parton shower” partons – problematic for consistency of labelling at different perturbative orders and for “resummation-corrected” matrix elements such as those in the POWHEG method [1] where there is no clear kinematic distinction between ME and PS emissions. All of these limitations and assumptions cause problems in practice, notably in the inability to use the Sherpa event generator [2] whose event record is complexified by the use of matrix-element/parton-shower merging and matching (MEPS) and a dipole shower formalism with 2 → 3 parton branchings [3]. I.e. the 1 → 2 parton splitting formulations used in the Pythia [4,5,6] and Herwig generator families [7,8], are themselves prob-
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