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1. 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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2. Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton–proton collisions at $\sqrt{s}=13\text{TeV}$

   https://doi.org/10.1088/1361-6633/ae2207  

   Collaboration, The CMS (December 2025, Reports on Progress in Physics)

   Abstract A search for pseudoscalar or scalar bosons decaying to a top quark pair ( $\text{t}\overline{\text{t}}$ ) in final states with one or two charged leptons is presented. The analyzed proton–proton collision data was recorded at $\sqrt{s}=13\text{TeV}$ by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138  ${\text{fb}}^{-1}$ . The invariant mass $m_{\mathrm{t}\overline{\mathrm{t}}}$ of the reconstructed $\text{t}\overline{\text{t}}$ system and variables sensitive to its spin and parity are used to discriminate against the standard model $\text{t}\overline{\text{t}}$ background. Interference between pseudoscalar or scalar boson production and the standard model $\text{t}\overline{\text{t}}$ continuum is included, leading to peak-dip structures in the $m_{\mathrm{t}\overline{\mathrm{t}}}$ distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic $\text{t}\overline{\text{t}}$ production threshold, while good agreement is found for high $m_{\mathrm{t}\overline{\mathrm{t}}}$ . The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet ${}^1{\text{S}}_0^{\left[1\right]}$ $\text{t}\overline{\text{t}}$ quasi-bound state ${\eta }_{\mathrm{t}}$ , inspired by nonrelativistic QCD, is added. Upper limits at 95% confidence level are set on the coupling between the pseudoscalar or scalar bosons and the top quark for boson masses in the range 365–1000 GeV, relative widths between 0.5% and 25%, and two background scenarios with or without ${\eta }_{\mathrm{t}}$ contribution. 

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3. Observation of a pseudoscalar excess at the top quark pair production threshold

   https://doi.org/10.1088/1361-6633/adf7d3  

   Collaboration, The CMS (August 2025, Reports on Progress in Physics)

   Abstract A search for resonances in top quark pair ( $\mathrm{t}\bar{\mathrm{t}}$ ) production in final states with two charged leptons and multiple jets is presented, based on proton–proton collision data collected by the CMS experiment at the CERN LHC at $\sqrt{s}=13\text{TeV}$ , corresponding to 138 fb⁻¹. The analysis explores the invariant mass of the $\mathrm{t}\bar{\mathrm{t}}$ system and two angular observables that provide direct access to the correlation of top quark and antiquark spins. A significant excess of events is observed near the kinematic $\mathrm{t}\bar{\mathrm{t}}$ threshold compared to the non-resonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar ( ${}^1{\text{S}}_0^{\left[1\right]}$ ) quasi-bound toponium state, as predicted by non-relativistic quantum chromodynamics. Using a simplified model for ${}^1{\text{S}}_0^{\left[1\right]}$ toponium, the cross section of the excess above the pQCD prediction is measured to be $8.8{}_{-1.4}^{+1.2}\text{pb}$ . 

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4. Sequence modeling of higher-order wave modes of quasi-circular, spinning, non-precessing binary black hole mergers

   https://doi.org/10.1088/1361-6382/ae2bd6  

   Tiki, Victoria; Pham, Kiet; Huerta, E A (December 2025, Classical and Quantum Gravity)

   Abstract Higher-order gravitational wave modes from quasi-circular, spinning, non-precessing binary-black-hole (BBH) mergers encode rich information about the nonlinear dynamics of strong-field gravity. We present a transformer-based sequence-completion surrogate that, given an early-inspiral segment, forecasts the subsequent late inspiral, merger, and ringdown. The intended applications are (i) patching or completing expensive or interrupted numerical-relativity (NR) simulations and (ii) providing late-time cross-checks and rapid hybridization studies. The training set is built from theNRHybSur3dq8surrogate, which provides spherical-harmonic modes up to $\ell \text{⩽}4$ (excluding (4, 0) and $(4,\pm 1)$ , and including (5, 5)) for mass ratios $q\text{⩽}8$ , dimensionless spin components $s_{1,2}^z\in [-0.8,0.8]$ , and inclination angles $\theta \in [0,\pi ]$ . Waveforms are supplied on the interval $t\in [-5000\mathrm{M},-100\mathrm{M})$ and the model autoregressively generates the plus and cross polarizations $(h_{+},h_{\times })$ on $t\in [-100\mathrm{M},130\mathrm{M}]$ . Training on the Delta supercomputer with 16 NVIDIA A100 GPUs required $\sim 15$  h on more than 14 million hybrid waveforms. Evaluation on a held-out test set of $\mathrm{840,000}$ samples yields mean and median overlaps of 0.996 and 0.997, respectively, with respect to the surrogate ground truth. The model generalises reasonably well to out-of-distribution NR simulations from the SXS catalogue: across 521 NR waveforms with mass ratios up toq = 15 and spin magnitudes $|s|\text{⩽}0.998$ , we obtain a median overlap of 0.969, rising to 0.998 for face-on/off orientations. Within the surrogate’s training domain, the transformer provides accurate, millisecond-scale sequence completion of BBH waveforms, including higher-order modes, from early inspiral to ringdown. The NR tests indicate promising performance on higher-fidelity, out-of-distribution data, particularly for detector-favourable orientations. 

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5. 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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