For the High Luminosity upgrade of the Large Hadron Collider (HL-LHC), the ATLAS11© Copyright 2026 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license. Inner Detector will be replaced by the all-silicon Inner Tracker (ITk), designed to operate in a significantly more challenging radiation environment and at higher collision rates. The ITk Pixel Detector will employ both planar and 3D silicon sensor technologies, with the latter selected for the innermost layer due to its superior radiation hardness. The 3D pixel sensors, produced by FBK (Italy) and SINTEF (Norway), are assembled into triplet modules, where three single bare modules are interconnected via a flex circuit. This configuration represents the final detector layout and thus a substantial step from previous R&D studies performed on single-chip assemblies. This work focuses on the experimental characterisation of triplet modules in beam-test conditions. Modules equipped with both the preproduction readout chip ITkPixV1.1 and the production ITkPixV2 were assembled and tested in laboratory setups, irradiated at the CERN IRRAD facility and at RARiS (Japan), and subsequently exposed to pion beams at the CERN SPS. Results on the performance of 3D triplet modules in their final hardware configuration are presented, with particular emphasis on test-beam measurements of hit efficiency as a function of fluence, and on charge-collection performance enabled by the Time-over-Threshold (ToT) capability of the ITkPixV2 readout chip.

The High-Luminosity Large Hadron Collider (HL-LHC) will deliver instantaneous luminosities up to five times higher than those of the current LHC, reaching an unprecedented $7.5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$. This significant increase in luminosity — resulting in an average pileup of over 200 interactions per bunch crossing — will pose serious challenges to existing detector systems. To maintain tracking performance under these conditions, the ATLAS Inner Detector (ID) will be replaced by an all-silicon Inner Tracker (ITk), providing coverage up to $|\eta| < 4$ and designed to withstand fluences up to $1.9 \times 10^{16}\,\text{n}_{\text{eq}}/\text{cm}^2$. The ITk will consist of five pixel layers and four strip layers in the barrel region. The innermost pixel layer will use triplet modules based on 3D silicon sensors, offering enhanced radiation hardness. The second layer will feature quad modules with $100 \mu \text{m}$ planar sensors, while the outer two layers will use quad modules with $150 \mu \text{m}$ planar sensors. Testbeam activities are essential for characterizing the performance of ITk modules under the extreme conditions expected at the HL-LHC. In this talk, we will present results from the most recent testbeam campaigns conducted in 2024 and 2025 at the North Experimental Area at CERN. These measurements were carried out using a $120 \text{GeV/c}$ pion beam from the H6 beamline and a high-resolution Mimosa telescope for precise track reconstruction. A variety of module geometries were tested, including single-chip cards (SCC), quad, and triplet configurations, with particular focus on the performance of irradiated sensors. These results provide critical input for the final design validation and quality assurance processes ahead of large-scale production. Among the tested modules was a 3D sensor from SINTEF featuring a new passivation method, irradiated to a fluence of $1.7 \times 10^{16}\,\text{n}_{\text{eq}}/\text{cm}^2$, which achieved an efficiency of $96\%$. Two planar sensors from FBK, irradiated to $0.5 \times 10^{16}\,\text{n}_{\text{eq}}/\text{cm}^2$, demonstrated efficiencies of approximately $99\%$. Other notable modules tested include an HPK quad module, a Micron SCC, and an FBK SCC, all equipped with an ITkPixV2, and two linear triplet modules—one produced by FBK and the other by SINTEF.

To be ready for the challenging conditions of the High Luminosity phase of the LHC accelerator at CERN, the ATLAS Inner Detector will be completely replaced with a new all-silicon Inner Tracker, the ITk. Sensors in the innermost layer will be exposed to a fluence up to 1.9$\cdot$10$^{16}$~n$_{eq}$/cm$^2$ (considering a safety factor of 1.5) at the half of the HL-LHC program, after which it is scheduled to be replaced together will the full innermost system. Pixel sensors with 3D technology have been chosen to instrument it due to their radiation hardness. Sensors with 25x100~$\mu$m$^2$ pixel pitch will be used in the central region of the innermost layer (barrel) while sensors with a pitch of 50x50~$\mu$m$^2$ will instrument its two side regions (end-caps). The Fondazione Bruno Kessler (FBK) has been chosen as one of the two vendors for the production of these sensors. This paper will present the performance of 3D pre-production sensors with both pixel pitches produced by FBK measured in test beams with devices irradiated up to and beyond the sensor end-of-life fluence.

The Detector Safety System is the last line of defence to protect the ATLAS detector against abnormal and potentially even unforeseen situations. It is designed to return the detector to a safe state based on predefined actions triggered by alarms which are triggered on their part by specific sets of conditions. Every alarm whether it results in an action taken or not is followed up by the operations team that assesses the criticality, takes countermeasures and identifies the point of failure. From experience abnormal situations can result either from faults or from side effects of planned interventions which were either not properly identified despite the mandatory planning and review or where a mistake during execution occurred. In many cases there are multiple interventions ongoing simultaneously in order to profit from shutdown periods. The rapid analysis of alarms while the incident is ongoing is often complicated due to the complexity of the ATLAS detector and its infrastructure and the large number of responsible groups and experts. A new Alarm Helper tool was designed to assist the operation team, particularly the operator in the control room responsible for infrastructure and safety (SLIMOS – Shift Leader in Matters of Safety), by providing real-time information about ongoing interventions and the possible related causes of failure. The new tool will combine historical events, documentation, and limited knowledge about ongoing interventions. It extends the Expert System which visualizes and simulates infrastructure inter-dependencies and allows to trace faults or alarms to a list of potential points of failure. The new tool also proposes which experts should be contacted in the particular circumstances.

This contribution presents test beam results of SINTEF 3D pixel sensors designed for the Inner Tracker (ITk) of the ATLAS detector at the High Luminosity LHC (HL-LHC). The sensors are required to withstand extreme radiation doses and to maintain efficiency above 96-97% after a lifetime operation at the ITk. We present details on the production and design of these sensors, the setup for the experiment at CERN, and the analysis of the test beam data. Results are promising, showing excellent position resolution and high efficiency after irradiation. The sensors meet the operational efficiency targets for both perpendicular and tilted configurations, validating their design and performance for future HL-LHC operations.

The ATLAS Technical Coordination Expert System is a knowledge-based application which describes and simulates the ATLAS experiment based on its components and their relationships with differing levels of granularity but with an emphasis on general infrastructure. It facilitates the sharing of knowledge and improves the communication among experts with different backgrounds and domains of expertise. The developed software has become essential for the planning of interventions as it gives easily insight into their consequences. Furthermore, it has also proven to be useful for exploring the most effective ways to improve the ATLAS operation and reliability by identifying points of failure with significant impact. The underlying database describes more than 13,000 elements with 89,000 relationships among them. It combines information from diverse domains such as detector control and safety systems, gas and water supplies, cooling, ventilation, cryogenics, and electricity distribution. As the most recent addition, a tool to identify the most probable cause of a failure state has been developed. This paper discusses the graph-based algorithm currently implemented by that tool and shows its behaviour based on the parameters entered by the user. An example in form of a real failure event is given which demonstrates the potential of the Expert System for understanding major failures faster in urgent situations.

The ITk detector, the new ATLAS silicon tracking system for the High Luminosity LHC (HL-LHC), will be equipped with 3D pixel sensor modules in the innermost layer (L0).The pixel cell dimensions will be 25×100 µm 2 in the barrel and 50×50 µm 2 in the end-caps, with one readout electrode at the centre of each pixel and four bias electrodes at the corners.Sensors from pre-production wafers (50×50 µm 2 ) produced by FBK have been bump-bonded to ITkPixV1.1 chips at IZM. Bare modules have been assembled in Genoa on Single Chip Cards (SCCs) and characterized in laboratory measurements and in test beam campaigns.Some of these modules have been irradiated in Bonn and at the CERN IRRAD facility.Preliminary results of their characterization after irradiation are shown, including measurements performed during test beam campaigns at CERN SPS in Summer 2022.

The ATLAS detector requires a huge infrastructure consisting of numerous interconnected systems forming a complex mesh which undergoes constant maintenance and upgrades. The ATLAS Technical Coordination Expert System provides, by the means of a user interface, a quick and deep understanding of the infrastructure, which helps to plan interventions by foreseeing unexpected consequences, and to understand complex events when time is crucial in the ATLAS control room. It is an object-oriented expert system based on the knowledge composed of inference rules and information from diverse domains such as detector control and safety systems, gas, water, cooling, ventilation, cryogenics, and electricity distribution. This paper discusses the latest developments in the inference engine and the implementation of the most probable cause algorithm based on them. One example from the annual maintenance of the 15°C water circuit chillers is discussed.

Cellulose-based paper electronics is an attractive technology to meet the growing demands for naturally abundant, biocompatible, biodegradable, flexible, inexpensive, lightweight and highly miniaturizable sensory materials. The price reduction of industrial carbon nanotube (CNT) grades offers opportunities to manufacture electrically conductive papers whose resistivity is responsive to environmental stimuli, such as the presence of water or organic solvents. Here, a highly sensitive paper nanocomposite is developed by integrating CNTs into a hierarchical network of pulp fibers and nanofibrillated cellulose. The aqueous-phase dynamic web forming process enables the scalable production of sensory paper nanocomposites with minimal nanoparticle loss due to the tailored interfacial bonding between CNT and cellulose components. The resulting materials are applied as multifunctional liquid sensors, such as leak detection and wave monitoring. The sensitivity to liquid water spans an outstanding four orders of magnitude even after 30 cycles and 6-month natural aging, due to the hydroexpansion of the hierarchical cellulose network, which alters the intertube distance between neighboring CNTs. The re-organization of percolated CNTs modifies the electron transport in wet areas of the sheet, which can be predicted by an equivalent circuit of resistors for the rapid detection and quantification of various liquids over large surfaces. (c) 2021 Published by Elsevier Ltd.

This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton–proton collision data at \n$$\\sqrt{s}=13$$\n\n TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb\n$$^{-1}$$\n\n. No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark (\n$$\\tilde{t}_1$$\n\n) or the heavier top squark (\n$$\\tilde{t}_2$$\n\n), excluding at 95% confidence level \n$$\\tilde{t}_1$$\n\n and \n$$\\tilde{t}_2$$\n\n masses up to about 1220 and 875 GeV, respectively.

When planning an intervention on a complex experiment like ATLAS, the detailed knowledge of the system under intervention and of the interconnection with all the other systems is mandatory. In order to improve the understanding of the parties involved in an intervention, a rule-based expert system has been developed. On the one hand this helps to recognise dependencies that are not always evident and on the other hand it facilitates communication between experts with different backgrounds by translating between vocabularies of specific domains. To simulate an event this tool combines information from different areas such as detector control (DCS) and safety (DSS) systems, gas, cooling, ventilation, and electricity distribution. The inference engine provides a list of the systems impacted by an intervention even if they are connected at a very low level and belong to different domains. It also predicts the probability of failure for each of the components affected by an intervention. Risk assessment models considered are fault tree analysis and principal component analysis. The user interface is a web-based application that uses graphics and text to provide different views of the detector system adapted to the different user needs and to interpret the data

For the purpose of withstanding very high radiation doses, silicon pixel sensors with a 3D electrode geometry are being developed. Detectors of this kind are highly interesting for harch radiation environments such as expected in the High Luminosity LHC, but also for space physics and medical applications. In this paper, prototype sensors developed at SINTEF are presented and results from tests in a pion beam at CERN are given. These tests shows that these 3D sensors perform as expected with full efficiency at bias voltages between 5 and 15V.