Calculating Track-Based Observables for the LHC

TL;DR

This paper develops a QCD-based formalism using track functions to accurately calculate charged particle observables at the LHC, accounting for hadronization effects and improving measurement precision.

Contribution

It introduces a novel formalism with track functions for perturbative calculations of track-based observables, including their renormalization group evolution and applications to LHC processes.

Findings

Track functions absorb infrared divergences in charged particle observables.

The formalism matches well with Pythia parton shower simulations.

Application to Higgs plus jet events demonstrates reduced smearing in measurements.

Abstract

By using observables that only depend on charged particles (tracks), one can efficiently suppress pile-up contamination at the LHC. Such measurements are not infrared safe in perturbation theory, so any calculation of track-based observables must account for hadronization effects. We develop a formalism to perform these calculations in QCD, by matching partonic cross sections onto new non-perturbative objects called track functions which absorb infrared divergences. The track function T_i(x) describes the energy fraction x of a hard parton i which is converted into charged hadrons. We give a field-theoretic definition of the track function and derive its renormalization group evolution, which is in excellent agreement with the Pythia parton shower. We then perform a next-to-leading order calculation of the total energy fraction of charged particles in e+ e- -> hadrons. To demonstrate the implications of our framework for the LHC, we match the Pythia parton shower onto a set of track functions to describe the track mass distribution in Higgs plus one jet events. We also show how to reduce smearing due to hadronization fluctuations by measuring dimensionless track-based ratios.