Abstract: This paper presents a study of the radiation hardness of the hadronic Tile Calorimeter of the ATLAS experiment in the LHC Run 2. Both the plastic scintillators constituting the detector active media and the wavelength-shifting optical fibres collecting the scintillation light into the photodetector readout are elements susceptible to radiation damage. The dedicated calibration and monitoring systems of the detector (caesium radioactive sources, laser and minimum bias integrator) allow to assess the response of these optical components. Data collected with these systems between 2015 and 2018 are analysed to measure the degradation of the optical instrumentation across Run 2. Moreover, a simulation of the total ionising dose in the calorimeter is employed to study and model the degradation profile as a function of the exposure conditions, both integrated dose and dose rate. The measurement of the relative light output loss in Run 2 is presented and extrapolations to future scenarios are drawn based on current data. The impact of radiation damage on the cell response uniformity is also analysed.

Abstract: For the High-Luminosity upgrade of the Large Hadron Collider, the ATLAS experiment will replace its current Inner Detector with an all-silicon Inner Tracker (ITk), consisting of pixel and strip detectors. The strip detector will consist of a central region or “barrel” assembled with staves and forward regions or “end-caps” assembled with petals. The ITk will nominally operate with liquid CO2 cooling at-35 degrees C; however, in the event of cooling system failures, it is possible that sensors will experience temperatures below-35 degrees C. At these low temperatures, it has been observed that the silicon sensors within modules – the fundamental readout units of the detector – can physically crack, rendering the modules inoperable. Understanding and resolving the issue of sensor cracking was one of the most important and urgent issues for the ITk project. This paper presents part of the mitigation strategies developed for petals. These mitigation strategies are based on modifications to the choice of adhesive and its deposition pattern for module assembly and petal loading. The most promising mitigation strategy presented here prevents cracking to temperatures as low as-45 degrees C, which can be expected in case of cooling system problems, with a small percentage of cracks observed after being cycled to-55 degrees C, which can be expected in case of catastrophic cooling system failures.

Abstract: We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved – cosmology, astrophysics, nuclear, and particle physics – in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

Abstract: The inner tracking system of the ATLAS experiment will be upgraded to a full silicon detector in 2030 for HL-LHC. The new tracking system is called ITk, the Inner Tracker. The ITk requirements include operational efficiency higher than 99% and noise hit occupancy smaller than 0.1%. During the pre-production phase of the ITk project, many short-strip modules were observed to exhibit so-called “Cold Noise (CN)”, wherein clusters of strips displayed very high noise when the modules were operated at temperatures below-35 degrees C. To investigate the CN impact and ensure the quality of module production, huge amount of effort have been put in by the collaboration. This paper focuses on the impact of CN on the tracking performance by examining two short strip modules that exhibit CN: one is non-irradiated, while the other one has been irradiated to the maximum expected end-of-lifetime fluence. For each module, the global and single strip tracking performance are evaluated. The global performance study shows that the non-irradiated module can be operated within specifications with a threshold around 1fC, but it is not possible to operate the irradiated module as required. In the single strip analysis, it was found that while CN does not affect the charge collection, it reduces the operating window and leaves less margin for detector operation. In the non-irradiated module, less than 3% of strips fail the detector requirements in the CN regions. For the irradiated module, about 20% of strips fail the requirements in the low CN region and around 52% fail in the high CN region. The fraction of strips that cannot operate due to CN throughout their lifetime can be predicted according to the measured noise at the required noise occupancy level and its expected median collected charge. In cases when the noise hit occupancy caused by CN is kept below 1% with a threshold smaller than 0.45 fC, at least 60% of strips could meet the operating requirements by the end of detector's lifetime. Thus, the module is likely to satisfy the operating requirements in terms of global efficiency and global noise occupancy.

Abstract: This article reports the laser calibration of the hadronic Tile Calorimeter of the ATLAS experiment in the LHC Run 2 data campaign. The upgraded Laser II calibration system is described. The system was commissioned during the first LHC Long Shutdown, exhibiting a stability better than 0.8% for the laser light monitoring. The methods employed to derive the detector calibration factors with data from the laser calibration runs are also detailed. These allowed to correct for the response fluctuations of the 9852 photomultiplier tubes of the Tile Calorimeter with a total uncertainty of 0.5% plus a luminosity-dependent sub-dominant term. Finally, we report the regular monitoring and performance studies using laser events in both standalone runs and during proton collisions. These studies include channel timing and quality inspection, and photomultiplier linearity and response dependence on anode current.