ATLAS

Abstract:

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

Abstract:

This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks — a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: e+light jet, μ+light jet, e+b-jet, and μ+b-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additional same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of s=13 TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb−1 at s=13.6 TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark S~1 production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded S~1 mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings yde = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for ysμ, ybe, and ybμ couplings set to 3.5, respectively.

Abstract:

Processes with τ$$\tau $$-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes τ$$\tau $$-leptons produced in proton–proton collisions only through their decay products. Data analyses involving hadronically decaying τ$$\tau $$-leptons face challenges due to backgrounds from jets misidentified as τ$$\tau $$-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as τ$$\tau $$-leptons (light-quark, gluon, b-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using W(μν)$$W(\mu \nu )$$ enriched event sample, ranges from 15 to 35% depending on the τ$$\tau $$-lepton’s transverse momentum and charged-particle decay multiplicity.

Abstract:

Measurements of tt¯ℓ+ℓ-$$t\bar{t}\ell ^{+}\ell ^{-}$$ production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on s=13$$\sqrt{s} = 13$$ TeV$$\text {T}\text {e}\hspace{-1.00006pt}\text {V}$$ proton–proton collision data with an integrated luminosity of 140fb-1$$140\,\textrm{fb}^{-1}$$, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the tt¯ℓ+ℓ-$$t\bar{t}\,\ell ^+\ell ^-$$ signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including to test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on the relevant operators.

Abstract:

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying τ$$\tau $$-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a τ$$\tau $$-slepton or a τ$$\tau $$-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying τ$$\tau $$-lepton and no other leptons, exactly one hadronically decaying τ$$\tau $$-lepton and at least one other lepton, or two or more hadronically decaying τ$$\tau $$-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 fb-1$$\textrm{fb}^{-1}$$ and 51.8 fb-1$$\textrm{fb}^{-1}$$ of pp collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015–2018 at s=13$$\sqrt{s} = 13$$ TeV and 2022–2023 at s=13.6$$\sqrt{s} = 13.6$$ TeV, respectively. Gluino masses below 2.25 TeV and squark masses up to 1.7 TeV are excluded