Top tagging : an analytical perspective

TL;DR

This paper analyzes top tagging techniques from QCD principles, proposing IRC safe variants of CMS top tagger, developing new substructure identification methods, and performing resummed calculations to compare with parton showers.

Contribution

It introduces IRC safe top tagger variants, new three-pronged substructure techniques, and applies triple-collinear splitting functions for analytical calculations.

Findings

IRC safe top tagger variants perform comparably to CMS tagger

Resummed calculations align with parton shower results

Identifies key features affecting tagger performance at high p_t

Abstract

In this paper we study aspects of top tagging from first principles of QCD. We find that the method known as the CMS top tagger becomes collinear unsafe at high $p_t$ and propose variants thereof which are IRC safe, and hence suitable for analytical studies, while giving a comparable performance to the CMS tagger. We also develop new techniques to identify three-pronged jet substructure, based on adaptations of the Y-splitter method and its combination with grooming. A novel feature of our studies, relative to previous calculations of two-pronged substructure, is our use of triple-collinear splitting functions, which owes to the presence of two mass scales of the same order, $m_t$ and $m_W$, in the signal jet. We carry out leading logarithmic resummed calculations for the various top-taggers, for both background and signal jets, and compare the results to those from parton showers. We also identify and comment on the main features driving tagger performance at high $p_t$ and discuss the role of non-perturbative effects.