Performance of the ATLAS track reconstruction algorithms in dense environments in LHC Run 2

With the increase in energy of the Large Hadron Collider to a centre-of-mass energy of 13 Formula: see text 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 fbFormula: see text 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 Formula: see text. The impact of charged-particle separations and multiplicities on the track reconstruction performance is discussed. The track reconstruction efficiency in the cores of jets with transverse momenta between 200 and 1600 Formula: see text is quantified using a novel, data-driven, method. The method uses the energy loss, Formula: see text, 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 Formula: see text and Formula: see text for jet transverse momenta of 200-400 Formula: see text and 1400-1600 Formula: see text, respectively.