  We present a measurement of the charged particle reconstruction inefficiency
inside of jet cores, using data collected by the ATLAS experiment in 2015 of
$pp$ collisions produced at the LHC, at a center-of-mass energy of 13 TeV. The
determination of this inefficiency is important for jet energy scale and mass
calibration, as well as multiple other performance studies and analyses. A data
driven method is used, where the fraction of lost particle tracks is determined
from energy deposition $dE/dx$ in the pixel detector. The fraction of lost
tracks is found to be less than 5%, which is an improvement since the previous
study, and agrees well within systematic uncertainties with a Monte Carlo
simulation.
