On the use of charged-track information to subtract neutral pileup

TL;DR

This paper evaluates the effectiveness of using charged-track information to subtract neutral pileup in jets, finding moderate correlations and that existing methods outperform NpC-based approaches, with zeroing playing a significant role.

Contribution

The study critically assesses NpC pileup mitigation, clarifies the role of zeroing in jet cleansing, and compares linear combinations of mitigation techniques.

Findings

NpC correlation with neutral pileup is moderate in simulations.

Zeroing improves pileup mitigation but has limitations.

Linear combinations of NpC and area-based methods offer limited gains.

Abstract

The use of charged pileup tracks in a jet to predict the neutral pileup component in that same jet could potentially lead to improved pileup removal techniques, provided there is a strong local correlation between charged and neutral pileup. In Monte Carlo simulation we find that the correlation is however moderate, a feature that we attribute to characteristics of the underlying non-perturbative dynamics. Consequently, `neutral-proportional-to-charge' (NpC) pileup mitigation approaches do not outperform existing, area-based, pileup removal methods. This finding contrasts with the arguments made in favour of a new method, "jet cleansing", in part based on the NpC approach. We identify the critical differences between the performances of linear cleansing and trimmed NpC as being due to the former's rejection of subjets that have no charged tracks from the leading vertex, a procedure that we name "zeroing". Zeroing, an extreme version of the "charged-track trimming" proposed by ATLAS, can be combined with a range of pileup-mitigation methods, and appears to have both benefits and drawbacks. We show how the latter can be straightforwardly alleviated. We also discuss the limited potential for improvement that can be obtained by linear combinations of the NpC and area-subtraction methods.