[Paper Review] Looking inside jets: an introduction to jet substructure and boosted-object phenomenology

Simone Marzani, Grégory Soyez+1 more|arXiv (Cornell University)|Jan 29, 2019

High-Energy Particle Collisions Research58 citations

TL;DR

A pedagogical set of lecture notes introducing jet substructure, grooming, and boosted-object phenomenology in QCD, with emphasis on theory, observables, and applications at the LHC.

ABSTRACT

The study of the internal structure of hadronic jets has become in recent years a very active area of research in particle physics. Jet substructure techniques are increasingly used in experimental analyses by the LHC collaborations, both in the context of searching for new physics and for Standard Model measurements. On the theory side, the quest for a deeper understanding of jet substructure algorithms has contributed to a renewed interest in all-order calculations in QCD. This has resulted in new ideas about how to design better observables and how to provide a solid theoretical description for them. In the last years, jet substructure has seen its scope extended, for example, with an increasing impact in the study of heavy-ion collisions, or with the exploration of deep-learning techniques. Furthermore, jet physics is an area in which experimental and theoretical approaches meet together, where cross-pollination and collaboration between the two communities often bear the fruits of innovative techniques. Despite the wealth of literature on this topic, we feel that a comprehensive and, at the same time, pedagogical introduction to jet substructure is still missing. This makes the endeavour of approaching the field particularly hard, as newcomers have to digest an increasing number of substructure algorithms and techniques, too often characterised by opaque terminology and jargon. Furthermore, while first-principle calculations in QCD have successfully been applied in order to understand and characterise the substructure of jets, they often make use of calculational techniques, such as resummation, which are not the usual textbook material. Thus, the idea of combining our experience in different aspects of jet substructure phenomenology to put together this set of lecture notes, which we hope could help and guide someone who moves their first steps in the physics of jet substructure.

Motivation & Objective

• Motivate the study of jet substructure as a tool for distinguishing boosted electroweak-scale resonances from QCD jets.
• Provide a self-contained, pedagogical introduction to QCD at colliders relevant to jet physics.
• Survey jet algorithms, grooming techniques, and observables used to analyze jet internal structure.
• Present analytic and Monte Carlo comparisons to understand jet mass distributions and grooming impacts.
• Highlight applications to searches, measurements, and the interface between theory and experiment.

Proposed method

• Review the theory of QCD and factorisation in soft/collinear limits relevant to jets.
• Explain perturbative calculations and the role of IRC safety in jet observables.
• Discuss jet algorithms (sequential recombination and cones) and experimental considerations.
• Analyze jet mass distributions with and without grooming to illustrate all-order resummation concepts.
• Introduce substructure tools (groomers, prong-finders, jet-shape observables) and their analytic understanding.
• Compare analytic predictions with Monte Carlo simulations to assess non-perturbative effects.

Experimental results

Research questions

• RQ1 How can jet substructure observables be analytically understood in the soft and collinear limits of QCD?
• RQ2 What is the impact of grooming techniques on jet mass distributions and related observables?
• RQ3 How do grooming and substructure tools enable discrimination between signal jets from boosted resonances and background QCD jets?
• RQ4 What is the role of resummation and factorisation approaches (e.g., coherent branching, SCET) in describing jet observables?
• RQ5 How can jet substructure methods be robustly applied to searches and measurements at the LHC?

Key findings

• Jet substructure techniques have become central to LHC analyses for both searches and SM measurements.
• Grooming reduces sensitivity to soft, non-perturbative effects, facilitating comparisons with first-principle calculations.
• Analytic understanding of jet mass under grooming demonstrates the interplay of all-order resummation and non-perturbative corrections.
• Quark/gluon discrimination and multi-prong tagging can be studied within analytic frameworks using angularities, ECFs, and related observables.
• Substructure methods extend beyond boosted bosons to heavy-ion physics and novel ML-based approaches.