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1. Observation of a cross-section enhancement near the $t\overline{t}$ production threshold in $\sqrt{s}=13$  TeV pp collisions with the ATLAS detector

   https://doi.org/10.1088/1361-6633/ae60a0  

   ATLAS (May 2026, Reports on Progress in Physics)

   Abstract A measurement of $t\overline{t}$ production is presented in the invariant-mass region near the pair production threshold, $m_{t\overline{t}}\sim 345$  GeV, in final states with two charged leptons and multiple jets. The measurement is based on $140{\mathrm{fb}}^{-1}$ of proton–proton collision data collected at $\sqrt{s}=13$  TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of $t\overline{t}$ production: a baseline model including only perturbative quantum chromodynamics (pQCD) predictions for the hard process at approximate next-to-next-to leading order accuracy in the strong coupling, and an extended model that, in addition, incorporates non-relativistic QCD simulations that also include the formation of colour-singlet quasi-bound-states near the $t\overline{t}$ threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed $m_{t\overline{t}}$ distributions, in bins of two angular observables sensitive to spin-correlations in the $t\overline{t}$ system. An excess of events is observed over the baseline pQCD prediction, with an observed significance over 8 standard deviations. This excess is consistent with the formation of colour-singlet and spin-singletS-wave quasi-bound $t\overline{t}$ states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of ${9.3}_{-1.3}^{+1.4}$  pb. 

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2. Computation of the expectation value of the spin operator ${\hat{S}}^2$ for the spin-flip Bethe–Salpeter equation

   https://doi.org/10.1088/2516-1075/ad48ed  

   Barker, B A; Seshappan, A; Strubbe, D A (May 2024, Electronic Structure)

   Abstract Spin-flip (SF) methods applied to excited-state approaches like the Bethe–Salpeter equation allow access to the excitation energies of open-shell systems, such as molecules and defects in solids. The eigenstates of these solutions, however, are generally not eigenstates of the spin operator ${\hat{S}}^2$ . Even for simple cases where the excitation vector is expected to be, for example, a triplet state, the value of $⟨{\hat{S}}^2⟩$ may be found to differ from 2.00; this difference is called ‘spin contamination’. The expectation values $⟨{\hat{S}}^2⟩$ must be computed for each excitation vector, to assist with the characterization of the particular excitation and to determine the amount of spin contamination of the state. Our aim is to provide for the first time in the SF methods literature a comprehensive resource on the derivation of the formulas for $⟨{\hat{S}}^2⟩$ as well as its computational implementation. After a brief discussion of the theory of the SF Bethe–Salpeter equation (BSE) and some examples further illustrating the need for calculating $⟨{\hat{S}}^2⟩$ , we present the derivation for the general equation for computing $⟨{\hat{S}}^2⟩$ with the eigenvectors from an SF-BSE calculation, how it is implemented in a Python script, and timing information on how this calculation scales with the size of the SF-BSE Hamiltonian. 

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3. Measurement of off-shell Higgs boson production in the $H^{\ast }\to ZZ\to 4\ell$ decay channel using a neural simulation-based inference technique in 13 TeV pp collisions with the ATLAS detector

   https://doi.org/10.1088/1361-6633/adcd9a  

   The_ATLAS_Collaboration (May 2025, Reports on Progress in Physics)

   Abstract A measurement of off-shell Higgs boson production in the $H^{\ast }\to ZZ\to 4\ell$ decay channel is presented. The measurement uses 140 fb⁻¹of proton–proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider and supersedes the previous result in this decay channel using the same dataset. The data analysis is performed using a neural simulation-based inference method, which builds per-event likelihood ratios using neural networks. The observed (expected) off-shell Higgs boson production signal strength in the $ZZ\to 4\ell$ decay channel at 68% CL is ${0.87}_{-0.54}^{+0.75}$ ( ${1.00}_{-0.95}^{+1.04}$ ). The evidence for off-shell Higgs boson production using the $ZZ\to 4\ell$ decay channel has an observed (expected) significance of 2.5σ(1.3σ). The expected result represents a significant improvement relative to that of the previous analysis of the same dataset, which obtained an expected significance of 0.5σ. When combined with the most recent ATLAS measurement in the $ZZ\to 2\ell 2\nu$ decay channel, the evidence for off-shell Higgs boson production has an observed (expected) significance of 3.7σ(2.4σ). The off-shell measurements are combined with the measurement of on-shell Higgs boson production to obtain constraints on the Higgs boson total width. The observed (expected) value of the Higgs boson width at 68% CL is ${4.3}_{-1.9}^{+2.7}$ ( ${4.1}_{-3.4}^{+3.5}$ ) MeV. 

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4. Test of lepton flavor universality in ${\text{B}}^{\pm }\to {\text{K}}^{\pm }{\mu }^{+}{\mu }^{-}$ and ${\text{B}}^{\pm }\to {\text{K}}^{\pm }{\text{e}}^{+}{\text{e}}^{-}$ decays in proton-proton collisions at $\sqrt{\text{s}}=\text{13}\text{TeV}$

   https://doi.org/10.1088/1361-6633/ad4e65  

   CMS_Collaboration, The (July 2024, Reports on Progress in Physics)

   Abstract A test of lepton flavor universality in ${\text{B}}^{\pm }\to {\text{K}}^{\pm }{\mu }^{+}{\mu }^{-}$ and ${\text{B}}^{\pm }\to {\text{K}}^{\pm }{\text{e}}^{+}{\text{e}}^{-}$ decays, as well as a measurement of differential and integrated branching fractions of a nonresonant ${\text{B}}^{\pm }\to {\text{K}}^{\pm }{\mu }^{+}{\mu }^{-}$ decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at $\sqrt{s}=13\text{TeV}$ recorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions $B({\text{B}}^{\pm }\to {\text{K}}^{\pm }{\mu }^{+}{\mu }^{-})$ to $B({\text{B}}^{\pm }\to {\text{K}}^{\pm }{\text{e}}^{+}{\text{e}}^{-})$ is determined from the measured double ratio $R\left(\text{K}\right)$ of these decays to the respective branching fractions of the ${\text{B}}^{\pm }\to \text{J}/\text{ψ}{\text{K}}^{\pm }$ with $\text{J}/\text{ψ}\to {\mu }^{+}{\mu }^{-}$ and ${\text{e}}^{+}{\text{e}}^{-}$ decays, which allow for significant cancellation of systematic uncertainties. The ratio $R\left(\text{K}\right)$ is measured in the range $1.1<q^2<6.0{\text{GeV}}^2$ , whereqis the invariant mass of the lepton pair, and is found to be $R\left(\text{K}\right)={0.78}_{-0.23}^{+0.47}$ , in agreement with the standard model expectation $R\left(\text{K}\right)\approx 1$ . This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the sameq²range, $B({\text{B}}^{\pm }\to {\text{K}}^{\pm }{\mu }^{+}{\mu }^{-})=(12.42\pm 0.68)\times {10}^{-8}$ , is consistent with the present world-average value and has a comparable precision. 

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5. Tripling of the superconducting critical current density in BaFe ${}_2$ (As ${}_{1-x}$ P ${}_x$ ) ${}_2$ retained after pressure release

   https://doi.org/10.1088/1361-6668/ae702f  

   Liu, Jiangteng; Lopez, Alex; Slone, Daniel; Ren, Guodong; Liu, Zhaoyu; Idrobo, Juan-Carlos; Chu, Jiun-Haw; Eley, Serena (June 2026, Superconductor Science and Technology)

   Abstract Superconducting performance is tunable not only via chemical modification or defect engineering, but also through external parameters such as pressure, though this method remains less readily accessible. In this work, we study how compression influences vortex dynamics and critical currents in an iron-based superconductor. Specifically, we perform magnetization measurements using an off-the-shelf pressure cell to investigate the effects of hydrostatic pressures up to 1.08 GPa on the magnetic properties of BaFe ${}_2$ (As ${}_{0.62}$ P ${}_{0.38}$ ) ${}_2$ crystals across a range of temperatures $T$ and magnetic fields $H$ . Although these pressures minimally affect the superconducting critical temperature, they produce a clear increase in the critical current density $J_{\mathrm{c}}(T,H)$ , a pronounced reduction in the rate of thermally activated vortex motion $S(T,H)$ , and can change the dominant vortex pinning mechanism. Furthermore, the effects of pressure are irreversible: after pressurization and subsequent release at room temperature, high-density microcracks are observed and the crystals retain their enhanced critical current densities. The second magnetization peak vanishes above 18 K after the pressure cycle, which we attribute to a transition from predominantly $\delta \kappa$ pinning to a mixed mechanism of $\delta T_{\mathrm{c}}$ and surface pinning. Lastly, a threefold increase in $J_{\mathrm{c}}$ , a more than 40% reduction in $S$ at 8 K and 0.5 T, and an expanded elastic-creep region were achieved after $1-2$ pressure cycles. These findings demonstrate the potential utility of pressure cycling for improving $J_{\mathrm{c}}$ , which may offer a simpler alternative compared to approaches such as chemical doping or the introduction of artificial pinning centers. 

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