Performance in beam tests of irradiated Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector

The High Granularity Timing Detector (HGTD) will be installed in the ATLAS experiment tomitigate pile-up effects during the High Luminosity (HL) phase of the Large Hadron Collider (LHC)at CERN. Low Gain Avalanche Detectors (LGADs) will provide high-precision measurements of the timeof arrival of particles at the HGTD, improving the particle-vertex assignment. To cope with thehigh-radiation environment, LGADs have been optimized by adding carbon in the gain layer, thusreducing the acceptor removal rate after irradiation. Performances of several carbon-enrichedLGAD sensors from different vendors, and irradiated with high fluences of1.5 and 2.5 × 10$^{15}$ n$_{eq}$/cm$^{2}$, have been measured in beam test campaigns during theyears 2021 and 2022 at CERN SPS and DESY. This paper presents the results obtained with datarecorded by an oscilloscope synchronized with a beam telescope which provides particle positioninformation within a resolution of a few μm. Collected charge, time resolution and hitefficiency measurements are presented. In addition, the efficiency uniformity is also studied as afunction of the position of the incident particle inside the sensor pad.