Operational Experience and Performance with the ATLAS Pixel detector at the Large Hadron Collider at CERN

The 4-layer Pixel Detector, a subsystem of the \textsc{ATLAS} experiment on which the tracking performance critically relies, has undergone significant hardware and readout upgrades to meet the challenges imposed by the higher collision energy, pile-up and luminosity delivered by the Large Hadron Collider (LHC) . The LHC has recently surpassed the record breaking instantaneous luminosities of 2 ×1034 cm−2s−1. The key status and performance metrics of the ATLAS Pixel Detector are summarized, and the operational experience and requirements to ensure optimum data quality and data taking efficiency are described, with special emphasis on radiation damage experience.

The tracking performance of the ATLAS detector at the Large Hadron Collider (LHC) relies critically on its 4-layer Pixel Detector, located at the core of the ATLAS tracker.The ATLAS pixel detector consists of four barrel layers and a total of six disk layers, three at each end of the barrel region.It has undergone significant hardware and readout upgrades to meet the challenges imposed by the higher collision energy, pileup and luminosity that are delivered by the LHC.
The key status and performance metrics of the ATLAS Pixel Detector are summarised, and the operational experience and requirements to ensure optimum data quality and data taking efficiency will be described, with special emphasis on radiation damage experience.By the end of the proton-proton collision runs in 2018, the IBL had received an integrated fluence of approximately φ = 9 × 10 14 1 MeV neq/cm 2 .The innermost of the three outer layers (B-layer) has been exposed to about half the fluence of the IBL, and lower fluences for other layers.The ATLAS collaboration is continually evaluating the impact of radiation on the Pixel Detector.In particular, signs of degradation are visible but are not impacting yet the tracking performance, including a trend of decreasing charge collection, dE/dX, occupancy reduction with integrated luminosity, underdepletion effects with IBL, effects of annealing that are significant for the inner-most layers.A quantitative analysis of all these effects will be presented and discussed, as well as the operational issues and mitigation techniques adopted during the LHC run and the ones foreseen during the LHC Long Shutdown 2.

Introduction
ATLAS [1] is one of the four major experiments at the Large Hadron Collider (LHC) at CERN.It is a general-purpose particle physics experiment designed to exploit the full discovery potential that the LHC provides.

Radiation damage effects
Figure 2 shows the simulated 1 MeV equivalent neutron fluence absorbed by the barrel layers of the ATLAS Pixel detector at η = 0 during Run 2 [7].Two different types of radiation damage, described in the following sections, are visible in the Pixel detector: non-ionizing damage to the sensor bulk and ionizing damage to the front-end chip [8].

Radiation damage in sensor bulk
Radiation damage in the sensor bulk affects the electric field profile and the charge collection efficiency (Figure 4) [7]; it affects the depletion voltage and generates charge trapping affecting collected charge and dE/dX.Figure 3 shows the gradual decrease of the measured dE/dx and cluster size of the B-Layer as a function of delivered luminosity in Run 2 [7].On the other hand, changes in the electric field profile affects the cluster charge distribution.Bias voltage values in different layers are regularly increased to partially recover the depletion area and charge collection efficiency [7].The analog and digital thresholds were reduced in 2018 to compensate for the loss of charge collection efficiency due to radiation damage, as marked in Figure 5 [7].

Radiation damage in the front-end chip
Radiation damages on SiO 2 and SiO 2 -Si induce trapped charges which increase low-voltage leakage current in FE-I4.IBL low-voltage leakage current increases due to radiation damage at low total ionizing dose (TID).Moreover, it causes drift on the analog threshold and time over threshold (ToT) of the FE-I4.This requires regular tunings to bring the mean back to the target value and reduce the RMS. Figure 6 shows the ToT and its RMS evolution of IBL during 2018 [9].

Detector performance
The tracking performance has been significantly improved since the insertion of the IBL. Figure 7 indicates that the impact parameter resolution (σ d 0 ) is improved nearly by a factor of two at low p T as a result of adding the IBL point measurement at smaller radius and with high spatial resolution [10].Figure 8 shows the IBL spatial resolution in Run 2 [7], measured from the corrected transverse positions of two reconstructed IBL clusters associated to a charged particle track in the module overlap region.The spatial resolution (∼10 µm in r-φ ) is found to slightly degrade over the period of Run 2 as a result of the decrease of collected charge and change in Lorentz angle1 .

Conclusion
Thanks to the tremendous effort of the operation team, the ATLAS Pixel Detector has been operated successfully and has delivered excellent performance and data quality despite the increasing luminosity of the LHC during Run 2, in which the overall performance has been improved further thanks to the installation of IBL.The radiation damage effects are visible in several observables, but have no significant impact on the quality of reconstructed physics objects.The ATLAS Pixel community is preparing towards Run 3.

Figure 1 :
Figure 1: Layout of the ATLAS Pixel detector [4].Left figure: 3D model of the barrel layers and end-cap disks.Right figure: transverse layout and radial position of the barrel layers.

Figure 3 :
Figure 3: Average cluster size (in φ and z) and dE/dx as a function of delivered luminosity during Run 2 for B-Layer [7].

Figure 4 :
Figure 4: Charge collection efficiency for IBL planar modules as a function of integrated luminosity [7].

Figure 5 :
Figure 5: Efficiency for B-Layer hits associated to a reconstructed track as a function of |η| [7].

Figure 6 :
Figure 6: Evolution of mean and RMS of ToT over all pixels in IBL measured in calibration scans.Each color/symbol series corresponds to a single tuning of the detector [9].

Figure 7 :
Figure 7: Unfolded transverse impact parameter resolution measured from data in 2015 including the IBL, in contrast with 2012 data (no IBL) [10].

Figure 8 :
Figure8: Spatial resolution of IBL hits associated to reconstructed particle tracks in di-jet events as a function of the integrated luminosity[7].