348 Search Results

In this paper we review the physics opportunities at linear $e^+e^-$ colliders with a special focus on high centre-of-mass energies and beam polarisation, take a fresh look at the various accelerator technologies available or under development and, for the first time, discuss how a facility first equipped with a technology mature today could be upgraded with technologies of tomorrow to reach much higher energies and/or luminosities. In addition, we will discuss detectors and alternative collider modes, as well as opportunities for beyond-collider experiments and R&D facilities as part of a linear collider facility (LCF). The material of this paper willmore » support all plans for $e^+e^-$ linear colliders and additional opportunities they offer, independently of technology choice or proposed site, as well as R&D for advanced accelerator technologies. This joint perspective on the physics goals, early technologies and upgrade strategies has been developed by the LCVision team based on an initial discussion at LCWS2024 in Tokyo and a follow-up at the LCVision Community Event at CERN in January 2025. It heavily builds on decades of achievements of the global linear collider community, in particular in the context of CLIC and ILC.« less

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (𝑝_T) dependence of radial flow fluctuations (𝑣₀⁡(𝑝_T)) over 0.5 < 𝑝_T < 10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at $$\sqrt{𝑠_{NN}}$$ = 5.02 TeV. The data revealmore » three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in 𝑝_T, and centrality-independent shape in 𝑝_T. The comparison with a hydrodynamic model demonstrates the sensitivity of 𝑣₀⁡(𝑝_T) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.« less

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair ($$t\overline{t}$$) invariant mass in proton-proton collisions using 140 fb⁻¹ of data at $$\sqrt{s}=13$$ TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the $$t\overline{t}$$ invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The $$t\overline{t}$$ system is reconstructed in the single-lepton final state, requiring exactly onemore » isolated electron or muon and at least four jets with at least two identified as originating from b-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of Y_t < 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.« less

Jet flavour tagging enables the identification of jets originating from heavy-flavour quarks in proton–proton collisions at the Large Hadron Collider, playing a critical role in its physics programmes. This paper presents GN2, a transformer-based flavour tagging algorithm deployed by the ATLAS Collaboration that represents a different methodology compared to previous approaches. Designed to classify jets based on the flavour of their constituent particles, GN2 processes low-level tracking information in an end-to-end architecture and incorporates physics-informed auxiliary training objectives to enhance both interpretability and performance. Its performance is validated in both simulation and collision data. The measured c-jet (light-jet) rejection inmore » data is improved by a factor of 3.5 (1.8) for a 70% b-jet tagging efficiency, compared to the previous algorithm. GN2 provides substantial benefits for physics analyses involving heavy-flavour jets, such as measurements of Higgs boson pair production and the couplings of bottom and charm quarks to the Higgs boson, and demonstrates the impact of advanced machine learning methods in experimental particle physics.« less

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 ofmore » 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.« less

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 additionalmore » 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 $$\sqrt{s}=13$$ TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb⁻¹ at $$\sqrt{s}=13.6$$ TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark ($$\tilde{S}$$₁) production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded $$\tilde{S}$$₁ $$\tilde{S}$$₁ mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings y_de = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for y_sμ, y_be, and y_bμ couplings set to 3.5, respectively.« less

Higgs boson production cross-sections via gluon–gluon fusion and vector-boson fusion in proton–proton collisions are measured in the $$H\rightarrow WW^*\rightarrow \ell \nu \ell \nu$$ decay channel. The Large Hadron Collider delivered proton–proton collisions at a centre-of-mass energy of 13 TeV between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $$140\,\text {fb}^{-1}.$$ The total cross-sections for Higgs boson production by gluon–gluon fusion and vector-boson fusion times the $$H\rightarrow WW^*$$ branching ratio are measured to be $$12.4^{+1.3}_{-1.2}\,\text {pb}$$ and $$0.79^{+0.18}_{-0.16}\,\text {pb},$$ respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterisedmore » through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory.« less

ATLAS, a general-purpose experiment at the Large Hadron Collider (LHC), makes use of a large internationally-distributed computing infrastructure, including over 10⁶ TB of managed data on disk and tape and almost one million simultaneously running CPU cores. Upgrades for the High-Luminosity LHC (HL-LHC) will increase the required computing resources by a factor of 3–4 by the beginning of the 2030s, and by an order of magnitude before the conclusion of data taking at the beginning of the 2040s. These resources are spread over around 100 computing sites worldwide. Efforts are underway within the experiment to evaluate and mitigate various aspectsmore » of the environmental impact of the sites, with the additional long-term goal of making recommendations to the sites that will significantly reduce the total expected environmental impact in the HL-LHC era. These efforts take several forms: building awareness in the experiment community, adjusting aspects of the computing policy, and modifications of data center configurations, either in ways that take advantage of particular features of ATLAS workloads or in generic ways that reduce the environmental impact of the computing resources. This paper describes the ongoing investigations and approaches that have already provided useful and actionable outcomes.« less

A search for the dimuon decay of the Higgs boson is presented based on 𝑝⁢𝑝 collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 165 fb⁻¹ at $$\sqrt{s}$$ = 13.6 TeV. To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of 3.4⁢𝜎 (2.5⁢𝜎 expected). The best-fit signal strength is 𝜇 = 1.4 ± 0.4. This result provides evidence for the 𝐻 → 𝜇⁢𝜇 decay with ATLAS data and offers a direct probe of themore » Higgs-boson Yukawa coupling to second-generation fermions.« less

A measurement of the top-quark pole mass $$m$$$^{pole}_{t}$$ is presented in $$t\bar{t}$$ events with an additional jet, $$t\bar{t}$$+ 1-jet, produced in pp collisions at $$\sqrt{s} = 13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb⁻¹. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $$t\bar{t}$ + 1-jet differential cross-section as a function of the inverse of the invariant mass of the $$t\bar{t}$$ + 1-jet system. Iterative Bayesian Unfolding is used to correct the datamore » to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp → $$t\bar{t}$$j(2 → 3), where top quarks are taken as stable particles, and the process pp → $$b\bar{b}$$l⁺ vl^–$$\overline{ν}$$j (2 → 7), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ² fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check.« less