racking in dense environments and its inefficiency measurement using pixel dE/dx

We present a measurement of the charged particle reconstruction ine ciency 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 ine ciency 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.