Upgrade of the ATLAS detector at the LHC (2023-26)

Particle physics is a research-driven area concerned with establishing what are the most elementary particles in the Universe and how they interact with one another via fundamental forces. The Large Hadron Collider (LHC) at CERN (the European Laboratory for Particle Physics in Geneva, Switzerland) is an experimental facility at the forefront of particle physics research today. The discovery, in 2012, of the Higgs particle at the LHC was one of the major advances in science in recent decades.

The LHC is used to produce proton-proton collisions at very high energy, which are recorded by very large cutting-edge particle detectors. The ATLAS detector is one such detector system at the LHC. The UK has a long-standing track record of involvement in the ATLAS project, since its inception, covering the design and construction of the original detector, as well as the exploitation of the data collected to progress our fundamental understanding of elementary particles and interactions. The ATLAS experiment exploits a broad physics programme ranging from searches for new particles at the highest mass scales to making precision measurements to characterise, and establish the parameters of, the Standard Model (the theoretical model of particle physics), and potentially discovering deviations from its predictions.

The science case is driven by the aims of elucidating and characterising the nature of the Higgs sector and of electroweak symmetry breaking at the TeV scale, and extending searches for new phenomena - or characterising them if already discovered - to higher mass scales and rarer processes, complemented by improved precision in a range of measurements. To achieve these science goals, a programme of luminosity upgrades is planned which will maintain the LHC at the high energy frontier of particle physics into the 2040s, delivering a dataset more than 10 times larger than that which the LHC is expected to produce until the end of its current phase of operation, in 2025.

The main objective of this proposal is to complete major ongoing upgrades of the ATLAS detector. More specifically, the funding being sought is for the UK's contributions to so-called "Phase-II" upgrades of ATLAS, to be delivered by a consortium of 15 institutions. The detector, trigger and data acquisition upgrades are essential to be able to collect and fully exploit the much-increased data rate that the LHC will deliver to ATLAS during its High-Luminosity phase (HL-LHC), from 2029 onwards.

In addition to the required resources to complete the Construction of the Phase-II upgrades, this proposal also includes a request of resource for the subsequent Integration & Commissioning of the upgraded systems at CERN. The necessary programme of work (for Construction, Integration and Commissioning) extends for six years, starting in April 2023, and is detailed in the present proposal. The request for resources presented here is, however, only for the first three years of the programme, with the funding to completion to be sought at a subsequent time.

The UK responsibilities and deliverables for the ATLAS detector Phase-II Upgrades are in the following systems: the new all-silicon Inner Tracker, the hardware-based calorimeter trigger, the Event Filter software trigger system, the Data Acquisition system and the development of the experiment's Computing and Software.

Results arising from the ATLAS experiment will be published in scientific journals, at national and international conferences and, through other publications and events, communicated to the wider public, including schools. The scientific results will benefit the world-wide community of researchers in both experimental and theoretical particle physics. Other academic beneficiaries include researchers in other disciplines using particle accelerators, detectors, advanced electronics, software and computing.