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Abstract:

A measurement of off-shell Higgs boson production is performed in the H * → W W channel. The measurement uses a proton–proton collision dataset with an integrated luminosity of 140 fb − 1 collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both W bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95% symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95 % confidence level on the Higgs boson total width of 13.1 (17.3) MeV.

Abstract:

A search for exotic decays of the Higgs boson $H$ into new scalar or pseudoscalar particles that subsequently decay into $b$ -quarks is presented. The search considers $ZH$ production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, $H\to 2a\to 4b$ , second to a pair of scalars with different masses ( $m_{a1}<m_{a2}$ ), either directly, $H\to a_1{a}_2\to 4b$ , or via a longer decay chain, $H\to a_1{a}_2\to 3a_1\to 6b$ . The analysis uses proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross section to the SM prediction times the branching ratio of Higgs bosons decaying into $4b$ or $6b$ , between 4% and 25% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to 2a\to 4b)$ , between 24% and 38% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to a_1{a}_2\to 4b)$ , and between 10% and 20% for $\sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to a_1{a}_2\to 3a_1\to 6b)$ , depending on the masses of the scalar particles.

Abstract:

The MALTA monolithic active pixel detector has been developed to meet the stringent demands of future high-energy physics experiments. To assess its capabilities, we performed fast-timing studies to define a figure of merit for this family of detectors. Conventional laser techniques are hindered by reflections from the sensor's metal layers, which restrict material penetration. We developed a triggered micro-X-ray system designed for precise timing measurements, that employs a micro-X-ray source that generates X-rays from a Cu-Cr target, synchronized with an external trigger signal. After validating the system with an LGAD, we used it to evaluate the timing performance of MALTA and MALTA2 pixel detector prototypes, providing insights into their operational characteristics.

Abstract:

An anomaly detection search for narrow-width resonances beyond the Standard Model that decay into a pair of jets is presented. The search is based on $139{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ recorded during 2015–2018 with the ATLAS detector at the Large Hadron Collider. The analysis is optimized without a particular signal model and aims to be sensitive to a broad range of new physics. It uses two different machine learning strategies to estimate the background in different signal regions. In each region, a weakly supervised classifier is trained to distinguish this background model from data. The analysis focuses on events with high transverse momentum jets reconstructed as large-radius jets. The mass and substructure of these jets are used as inputs to the classifiers. After a classifier-based selection, the distribution of the invariant mass of the two jets is used to search for potential local excesses. The model-independent results of both the anomaly detection methods show no signs of significant local excesses. In addition to model-independent results, a representative set of signal models is injected into the data, and the sensitivity of the methods to these scenarios is reported.

Abstract:

This contribution outlines the HALHF concept, which combines the high gradients achievable in plasma-wakefield acceleration with conventional radio-frequency acceleration. In HALHF, beam-driven plasma-wakefield cells are used to accelerate electrons to high energy. Because plasma-based acceleration of positrons is problematic, conventional RF acceleration is used but to much lower energy. The HALHF concept utilises not only asymmetric energies but also asymmetric bunch charges and asymmetric transverse emittances, leading to comparable luminosity to conventional facilities but much lower capital cost. Possible upgrades to the HALHF facility are discussed, in particular to the tt− threshold and to 550 GeV, where the Higgs self-coupling and tt−H coupling can be measured. Other upgrades include the provision of two interaction points, to implement a γ.γ collider of two possible types and finally a symmetric high-energy collider if the problem of plasma-based positron acceleration can be solved.