Calculating the Charge of a Jet

TL;DR

This work develops a quantitative framework for jet charge, addressing its infrared un-safety by defining observables at the hadron level. It cleanly separates perturbative jet-structure from nonperturbative hadronization using fragmenting jet functions and dihadron fragmentation functions, and also presents a full nonperturbative evolution of a jet-charge distribution with a Monte Carlo implementation (JetFrag). The authors validate their approach against Pythia and demonstrate reasonable agreement, while highlighting the importance of FF uncertainties and dihadron correlations for the width. The methodology provides a systematic path to precision jet-charge predictions and can be extended to other track-based observables sensitive to soft radiation.

Abstract

Jet charge has played an important role in experimentally testing the Parton Model and the Standard Model, and has many potential LHC applications. The energy-weighted charge of a jet is not an infrared-safe quantity, so hadronization must be taken into account. Here we develop the formalism to calculate it, cleanly separating the nonperturbative from the perturbative physics, which we compute at one-loop order. We first study the average and width of the jet charge distribution, for which the nonperturbative input is related to (dihadron) fragmentation functions. In an alternative and novel approach, we consider the full nonperturbative jet charge distribution and calculate its evolution and jet algorithm corrections, which has a natural Monte Carlo-style implementation. Our numerical results are compared to Pythia and agree well in almost all cases. This calculation can directly be extended to similar track-based observables, such as the total track momentum generated by an energetic parton.

Figures (11)

• Figure 1: The jet charge distribution in Pythia at the hadronic and partonic level, for a $d$-quark jet with $E = 100$ GeV, using the $e^+ e^-$ anti-$k_T$ algorithm with R=0.5.
• Figure 2: The jet algorithm dependence of the average and width of the jet charge distribution, as function of $\kappa$ and $R$.
• Figure 3: The variation of the average and width of the jet charge distribution, keeping $2E \tan(R/2) = 100$ GeV fixed.
• Figure 4: Jet charge distribution obtained using the JetFrag Monte Carlo described in Sec. \ref{['sec:shower']}. We use the Gaussian (orange solid) or step function (green dashed) toy model described in the text for the nonperturbative input.
• Figure 5: The average charge $\langle Q_1^q \rangle$ at LO and NLO for a $k_T$-like quark jet with R=0.5 and $\kappa=1$. The bands correspond to the perturbative uncertainties as explained in the text.