Abstract: This paper presents two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb(-1) of root s = 13TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events are required to feature an energetic jet, large missing transverse momentum, and at least one low-momentum charged particle that serves as a candidate higgsino decay product. In the first search, targeting higgsino mass splittings in the range of 0.3-1 GeV, the higgsinos are expected to predominantly decay into pions that are identified as low-momentum charged particles with large transverse impact parameters due to the long higgsino lifetime (c tau approximate to O(0.1-10mm)), and neural networks are used to discriminate between signal and background processes. The second search targets larger mass splittings in the range of 1-3 GeV, where the higgsinos are expected to decay promptly into low-momentum leptons, one of which is identified by dedicated low-momentum electron or muon taggers based on neural networks utilising tracking and calorimeter information. No significant excess above the Standard Model prediction is observed in either search and the results are interpreted within simplified models, to set lower limits on the masses of the higgsino-like charginos and neutralinos. Together, these searches exclude chargino masses below 126 GeV at 95% confidence level for mass splittings between the chargino and lightest neutralino in the range of 0.3-2 GeV. This represents the first ATLAS constraints in a portion of this parameter space and surpasses the limits previously set by other experiments.

Abstract: Low-lying states of Zr-94 were investigated via low-energy multi-step Coulomb excitation. From the measured gamma-ray yields, 16 reduced E2 transition probabilities between low-spin states were determined, together with the spectroscopic quadrupole moments of the 2(1,2)+ states. Based on this information, for the first time in the Zr isotopic chain, the shapes of the 0(1,2)(+) states including their deformation softness were inferred in a model-independent way using the quadrupole sum rules approach. The ground state of Zr-94 possesses a rather diffuse shape associated with a spherical configuration, while the 0(2)(+) state is triaxial tending towards oblate and more strongly deformed. The observed features of shape coexistence in Zr-94 are consistent with both Monte-Carlo shell-model predictions and IBM-CM calculations, and provide model-independent constraints on the shape character assigned in the IBM-CM to the intruder configuration in Zr92-96.

Abstract: A search is presented for massive long-lived particles in events featuring at least one displaced vertex and at least one displaced muon, using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider from 2022 to 2024 at a centre-of-mass energy of 13.6 TeV. The data sample corresponds to an integrated luminosity of 164 fb(-1). The analysis targets scenarios in which long-lived particles decay inside the ATLAS inner detector, resulting in a topology of at least one massive, displaced vertex (DV) with multiple associated tracks, and at least one muon with a large transverse impact parameter relative to the primary interaction point. The muon is not required to be associated with the DV. Two signal regions are defined by the transverse distance of the reconstructed DV from the interaction point. Background contributions are estimated by using fully data-driven techniques. No significant excess above the expected background is observed. Upper limits at 95% confidence level are set on the visible cross-section and on the production cross-sections of several benchmark models of R-parity-violating supersymmetry.