Measurement of the charged-particle multiplicity inside jets from $\sqrt{s}=8$ TeV $pp$ collisions with the ATLAS detector

TL;DR

This paper measures the average number of charged particles within high-energy jets from proton-proton collisions at 8 TeV, providing data to improve understanding of jet fragmentation and quark-gluon discrimination.

Contribution

It presents the first detailed measurement of charged-particle multiplicity inside jets over a wide momentum range at 8 TeV, including separation of quark and gluon jet contributions.

Findings

Charged-particle multiplicity increases with jet transverse momentum.

Data shows discrepancies with some existing models of jet fragmentation.

Results help refine models of quark and gluon jet behavior.

Abstract

The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with $p_\text{T}>500$ MeV inside high-momentum jets in dijet events using 20.3 fb$^{-1}$ of data recorded with the ATLAS detector in $pp$ collisions at $\sqrt{s}=8$ TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.