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Abstract:

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark (U1$$U_1$$) as predicted by an ultraviolet-complete extension of the SM, referred to as the ‘4321’ model. Given the assumed decay of U1$$U_1$$ into third-generation SM fermions, the final state can contain multiple τ$$\tau $$-leptons and b-quarks. This search is based on a dataset of pp collisions at s=13$$\sqrt{s}=13$$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to 140fb-1$$140~\textrm{fb}^{-1} $$. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an R-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).

Abstract:

A calibration of the ATLAS flavour-tagging algorithms using a new calibration procedure based on optimal transportation maps is presented. Simultaneous, continuous corrections to the b-jet, c-jet, and light-flavour jet classification probabilities from jet-tagging algorithms in simulation are derived for b-jets using tt¯→eμννbb$$t\bar{t} \rightarrow e\mu \nu \nu bb$$ data. After application of the derived calibration maps, closure between simulation and observation is achieved for jet flavour observables used in ATLAS analyses of Large Hadron Collider (LHC) Run 2 proton-proton collision data. This continuous calibration opens up new possibilities for the future use of jet flavour information in LHC analyses and also serves as a guide for deriving high-dimensional corrections to simulation via transportation maps, an important development for a broad range of inference tasks.

Abstract:

Measurements of integrated and differential cross-sections for W±Z production in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of s=13 TeV are used, corresponding to an integrated luminosity of 140 fb−1. The W±Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons or muons. The integrated cross-section per lepton flavour for the production of W±Z is measured in the detector fiducial region with a relative precision of 4%. The measured value is compared with the Standard Model prediction at a precision of up to next-to-next-to-leading-order in QCD and next-to-leading-order in electroweak. Cross-sections for W+Z and W−Z production and their ratio are presented. The W±Z production is also measured differentially as functions of various kinematic variables, including new observables sensitive to CP-violation effects. All measurements are compared with state-of-the-art Standard Model predictions from fixed-order calculations or Monte Carlo generators based on next-to-leading-order matrix elements interfaced with parton showers. An effective field theory interpretation of the measurements is performed, considering both CP-conserving and CP-violating dimension-6 operators modifying the W±Z production. In the absence of observed deviations from the Standard Model, limits on CP-conserving Wilson coefficients are extracted using the transverse mass of the W±Z system. For CP-violating coefficients a machine learning approach is used to construct an observable with enhanced sensitivity to CP-violation effects.

Abstract:

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and is produced in the modified Tower 180 nm CMOS imaging technology. The pixel pitch is 36.4 µm and the sensing layer consists of either high resistivity epitaxial or Czochralski silicon. The implementation of a small collection electrode features a small pixel capacitance and offers low noise. Typically a detection threshold of around 200 e⁻ is used. It is calibrated making use of a dedicated charge injection circuit on chip and an Fe-55 source. In this contribution, MALTA2 sensors are characterized in terms of hit detection efficiency inside the pixel at fine threshold steps. A comparison is made for samples before and after irradiation at different substrate voltages and with different doping concentration of the internal n-layer. Data was taken at CERN SPS test beam campaigns in 2023 and 2024, using the MALTA beam telescope consisting of six MALTA tracking planes with 4 µm spatial and 2 ns timing resolution. A reconstruction of the most probable signal amplitude from binary hit data is performed. A two-dimensional map of the collected charge is obtained with sub-pixel resolution. The presented method provides an in-beam alternative to grazing angle studies or Edge-TCT for determining a charge collection profile.