Hadronic Calorimeter Shower Size: Challenges and Opportunities for Jet Substructure in the Superboosted Regime

TL;DR

This paper discusses the fundamental limitations of using hadronic calorimeter data for analyzing superboosted jets, highlighting the impact of neutral hadrons and fluctuations on jet substructure measurements and proposing new variables.

Contribution

It identifies intrinsic constraints in jet substructure analysis due to calorimeter physics and introduces zero-cone variables to explore potential information channels.

Findings

15% of jet transverse energy profile remains inaccessible

Fluctuations are not mitigated by global corrections

Neutral fraction correlates with jet flavor

Abstract

Hadrons have finite interaction size with dense material, a basic feature common to known forms of hadronic calorimeters (HCAL). We argue that substructure variables cannot use HCAL information to access the microscopic nature of jets much narrower than the hadronic shower size, which we call superboosted massive jets. It implies that roughly 15% of their transverse energy profile remains inaccessible due to the presence of long-lived neutral hadrons. This part of the jet substructure is also subject to order-one fluctuations. We demonstrate that the effects of the fluctuations are not reduced when a global correction to jet variables is applied. The above leads to fundamental limitations in the ability to extract intrinsic information from jets in the superboosted regime. The neutral fraction of a jet is correlated with its flavor. This leads to an interesting and possibly useful difference between superboosted W/Z/h/t jets and their corresponding backgrounds. The QCD jets that form the background to the signal superboosted jets might also be qualitatively different in their substructure as their mass might lie at or below the Sudakov mass peak. Finally, we introduce a set of zero-cone longitudinal jet substructure variables and show that while they carry information that might be useful in certain situations, they are not in general sensitive to the jet substructure.