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

Abstract

With the increase in energy of the Large Hadron Collider to a centre-of-mass energy of 13 text {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 text {TeV}. 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 text {GeV} is quantified using a novel, data-driven, method. The method uses the energy loss, {text { d}}{} textit{E}/dtextit{x}, 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 {text {(stat.)}} pm 0.014 {text {(syst.)}} and 0.093 pm 0.017 {text {(stat.)}}pm 0.021 {text {(syst.)}} for jet transverse momenta of 200–400 text {GeV} and 1400–1600 text {GeV}, respectively.