Abstract: A search for emerging jets is presented using 51.8 fb-1 of proton-proton collision data at s=13.6 TeV, collected by the ATLAS experiment during 2022 and 2023. The search explores a hypothetical dark sector featuring 'dark quarks' that are charged under a confining gauge group and couple to the standard model (SM) via a new mediator particle. These dark quarks undergo showering and hadronisation within the dark sector, forming long-lived dark mesons that decay back into SM particles. This results in jets that contain multiple displaced vertices known as emerging jets. The analysis targets events with pairs of emerging jets, produced either through a vector mediator, Z ', in the s-channel, or a scalar mediator, Phi, in the t-channel. No significant excess over the SM background is observed. Assuming a dark pion proper decay length between 5 mm and 50 mm, Z ' mediator masses between 600 GeV and 2550 GeV are excluded for quark and dark quark coupling values of 0.01 and 0.1, respectively. For a quark dark-quark coupling of 0.1, Phi mediator masses between 600 GeV and 1375 GeV are excluded. These results represent the first direct search targeting emerging jet pair production via a Z ' mediator, as well as the first study of emerging jet production mediated by a scalar particle exchanged in the t-channel.

Abstract: A scan of the beam energy across the top quark pair tt production threshold is part of the program of future Higgs, top, and electroweak factory projects. In this paper, we provide projections for the achievable precision in the top quark mass (mt), width (Gamma t), and Yukawa coupling (yt) at the electron-positron (e+e-) stage of the Future Circular Collider (FCC-ee). The study includes a detailed assessment of parametric and systematic uncertainties, as well as a rigorous estimate of the effect of point-to-point correlations. We project that mt and Gamma t can be determined with an experimental precision of 6.8 and 11.5 MeV, respectively, when mt is defined in the potential-subtracted (PS) scheme. The impact of theoretical uncertainties due to missing higher orders is found to be of about 35 (25) MeV on mt (Gamma t) at N3LO in non-relativistic QCD. Therefore, improvements in the theoretical accuracy, which is an active area of development, are key to match the achievable experimental precision at a future e+e- collider. We also explore the prospects for a measurement of yt at FCC-ee via a dedicated run above the tt production threshold.

Abstract: A search for decays of the Higgs boson into a Z boson and a light resonance, with a mass of 0.5-3.5 GeV, is performed using the full 140 fb(-1) dataset of 13 TeV proton-proton collisions recorded by the ATLAS detector during LHC Run 2. Leptonic decays of the Z boson and hadronic decays of the light resonance are considered. The resonance can be interpreted as a J/psi or eta(c) meson, an axion-like particle, or a light pseudoscalar predicted in two-Higgs-doublet models. Due to its low mass, this resonance is produced with a high Lorentz boost in the laboratory frame and therefore reconstructed as a single small-radius jet of hadrons. A neural network is used to correct the Monte Carlo simulation of the total expected background using data from sideband regions. Two additional neural networks are used to distinguish signal from background, enhancing the purity of the signal region. A binned profile-likelihood fit is performed on the final-state invariant mass distribution. No significant excess of events relative to the expected background is observed, and upper limits at 95% confidence level are set on the Higgs boson's branching fraction to a Z boson and a light resonance. The exclusion limit is similar to 10% for the lower masses, and increases for higher masses. Upper limits on the effective coupling C-ZH(eff)/A of an axion-like particle to a Higgs boson and.. boson are also set at 95% confidence level, and range from 0.9 to 2 TeV-1.