Viaux Maira, Nicolas

Fiducial and total 𝑊 ± and 𝑍 boson cross sections, their ratios and the ratio of top-antitop-quark pair and 𝑊 -boson fiducial cross sections are measured in proton–proton collisions at a centre-of-mass energy of √𝑠 = 13.6 TeV, corresponding to an integrated luminosity of 29 fb−1 of data collected in 2022 by the ATLAS experiment at the Large Hadron Collider. The measured fiducial cross-section values for 𝑊 + → 𝓁+𝜈, 𝑊 − → 𝓁−𝜈̄, and 𝑍 → 𝓁+𝓁− (𝓁 = 𝑒 or 𝜇) boson productions are 4250 ± 150 pb, 3310 ± 120 pb, and 744 ± 20 pb, respectively, where the uncertainty is the total uncertainty, including that arising from the luminosity of about 2.2%. The measurements are in agreement with Standard-Model predictions calculated at next-to-next-to-leading-order in 𝛼𝑠 , next-to-nextto-leading logarithmic accuracy and next-to-leading-order electroweak accuracy

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/γ∗) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production.

The observation of the electroweak production of single-top-quarks is made using 255 pb−1 of proton-proton collision data recorded at s√=5.02 TeV with the ATLAS detector at the Large Hadron Collider. An event selection is used to identify single-top-quark candidates arising from t-channel production with the top quark decaying semi-leptonically. Events passing the selection are then used to measure the inclusive cross-section for the combined production of single-top-quarks and antiquarks, σ(tq+t¯q), and the ratio Rt between these two. They are measured to be σ(tq+t¯q)=27.1+4.4−4.1(stat.)+4.4−3.7(syst.) pb and Rt=2.73+1.43−0.82(stat.)+1.01−0.29(syst.). The individual single-top-quark (tq) and single-top-antiquark (t¯q) production cross-sections are measured to be σ(tq)=19.8+3.9−3.1(stat.)+2.9−2.2(syst.) pb and σ(t¯q)=7.3+3.2−2.1(stat.)+2.8−1.5(syst.) pb. All measurements are in good agreement with the Standard Model predictions.