Charged-particle reconstruction at the energy frontier

26 April 2017 | By

Figure 1: Illustration of isolated measurements (left) in the ATLAS pixel detector and merged measurements (right) due to very collimated tracks. Merged measurements are more common in higher energetic jets and are harder to distinguish. The ATLAS event reconstruction software was optimized for Run 2 and is now better able to resolve merged measurements. Different colors represent energy deposits from different charged particles traversing the sensor and the particles trajectories are shown as arrows. (Image: ATLAS Collaboration/CERN)

A new age of exploration dawned at the start of Run 2 of the Large Hadron Collider, as protons began colliding at the unprecedented centre-of-mass energy of 13 TeV. The ATLAS experiment now frequently observes highly collimated bundles of particles (known as jets) with energies of up to multiple TeV, as well as tau-leptons and b-hadrons that pass through the innermost detector layers before decaying. These energetic collisions are prime hunting grounds for signs of new physics, including massive, hypothetical new particles that would decay to much lighter and therefore highly boosted bosons.

In these very energetic jets, the average separation of charged particles is comparable to the size of individual inner detector elements. This easily creates confusion within the algorithms responsible for reconstructing charged particle trajectories (tracks). Therefore, without careful consideration, this can limit the track reconstruction efficiency in these dense environments. This would result in poor identification of long-lived b-hadrons and hadronic tau decays, and difficulties in calibrating the energy and mass of jets.

Figure 2: Efficiency to reconstruct a track of a charged particle from decays of a tau-lepton, rho-meson and B0-hadron as a function of these particles initial transverse momentum. At higher transverse momentum, merged measurements are more abundant and therefore the efficiency drops. This effect is exacerbated by a higher charged-particle multiplicity in the decay, as clearly visible for the tau-lepton’s decay into five charged particles (green circles). (Image: ATLAS Collaboration/CERN)

Similar to increasing the magnification of a microscope, in preparation for Run 2, the ATLAS event reconstruction software was optimized to better resolve these close-by particles. As a result, at angular separations between a jet and a charged particle below 0.02, the reconstruction efficiency for a charged particle track is still around 80% for jets with a transverse momentum of 1400 to 1600 GeV in simulated dijet events. This has maximised the potential for discovery, allowing for more detailed measurements of the newly opened kinematic regime.

Similar to increasing the magnification of a microscope, in preparation for Run 2, the ATLAS event reconstruction software was optimized to better resolve close-by particles.

Recently published results give a general overview of the new track reconstruction algorithm, highlighting the ATLAS detector’s excellent performance in reconstructing charged particles in dense environments. The results also present, for the first time, a novel method for determining in situ (i.e. from data) the efficiency of reconstructing tracks in such an environment. The study uses the ionization energy loss (dE/dx), measured with the ATLAS pixel detector, to deduce the probability of failing to reconstruct a track. The obtained results confirm the excellent performance expected from studies on simulated data.

Figure 3: The ionization energy loss (dE/dx) of charged particles in the ATLAS pixel detector. Three distinct distributions were created to extract the track reconstruction performance in the core of jets: isolated measurements (blue); merged measurements (green); and the data (black circles) which, due to specific selections, should resemble isolated measurements. A possible inefficiency of the track reconstruction is determined by fitting the green and blue distributions to the data (the result is shown as a red line). The fitted contribution of the green distribution to the data corresponds to an inefficiency of the track reconstruction. (Image: ATLAS Collaboration/CERN)