# Performance of the ATLAS Track Reconstruction Algorithms in Dense Environments in LHC Run 2

## TL;DR

This paper evaluates the performance of ATLAS track reconstruction algorithms in dense jet cores at 13 TeV, using data-driven methods to quantify efficiency and compare with simulations, addressing challenges posed by high particle multiplicities.

## Contribution

It introduces a novel, data-driven method using dE/dx to measure track reconstruction efficiency in dense environments at the LHC.

## Key findings

- Reconstruction efficiency decreases in dense jet cores at high transverse momentum.
- The measured fraction of unreconstructed charged particles is approximately 6-9%.
- The study provides insights into track reconstruction performance in challenging dense environments.

## Abstract

With the increase in energy of the Large Hadron Collider to a centre-of-mass energy of 13 TeV for Run 2, events with dense environments, such as in the cores of high-energy jets, became a focus for new physics searches as well as measurements of the Standard Model. These environments are characterized by charged-particle separations of the order of the tracking detectors sensor granularity. Basic track quantities are compared between 3.2 fb$^{-1}$ of data collected by the ATLAS experiment and simulation of proton-proton collisions producing high-transverse-momentum jets at a centre-of-mass energy of 13 TeV. The impact of charged-particle separations and multiplicities on the track reconstruction performance is discussed. The efficiency in the cores of jets with transverse momenta between 200 GeV and 1600 GeV is quantified using a novel, data-driven, method. The method uses the energy loss, dE/dx, to identify pixel clusters originating from two charged particles. Of the charged particles creating these clusters, the measured fraction that fail to be reconstructed is $0.061 \pm 0.006 \textrm{(stat.)} \pm 0.014 \textrm{(syst.)}$ and $0.093 \pm 0.017 \textrm{(stat.)}\pm 0.021 \textrm{(syst.)}$ for jet transverse momenta of 200-400 GeV and 1400-1600 GeV, respectively.