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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. Observation of quantum entanglement in top quark pair production in proton–proton collisions at $\sqrt{s}=13$  TeV

   https://doi.org/10.1088/1361-6633/ad7e4d  

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

   Abstract Entanglement is an intrinsic property of quantum mechanics and is predicted to be exhibited in the particles produced at the Large Hadron Collider. A measurement of the extent of entanglement in top quark-antiquark ( $\mathrm{t}\overline{\mathrm{t}}$ ) events produced in proton–proton collisions at a center-of-mass energy of 13 TeV is performed with the data recorded by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 36.3 fb⁻¹. The events are selected based on the presence of two leptons with opposite charges and high transverse momentum. An entanglement-sensitive observableDis derived from the top quark spin-dependent parts of the $\mathrm{t}\overline{\mathrm{t}}$ production density matrix and measured in the region of the $\mathrm{t}\overline{\mathrm{t}}$ production threshold. Values of $D<-1/3$ are evidence of entanglement andDis observed (expected) to be $-{0.480}_{-0.029}^{+0.026}$ ( $-{0.467}_{-0.029}^{+0.026}$ ) at the parton level. With an observed significance of 5.1 standard deviations with respect to the non-entangled hypothesis, this provides observation of quantum mechanical entanglement within $\mathrm{t}\overline{\mathrm{t}}$ pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced. 

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3. 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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4. 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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5. The Comprehensive Archive of Substellar and Planetary Accretion Rates

   https://doi.org/10.3847/1538-3881/ad06b8  

   Betti, S K; Follette, K B; Ward-Duong, K; Peck, A E; Aoyama, Y; Bary, J; Dacus, B; Edwards, S; Marleau, G-D; Mohamed, K; et al (November 2023, The Astronomical Journal)

   Abstract Accretion rates ( $\dot{M}$ ) of young stars show a strong correlation with object mass (M); however, extension of the $\dot{M}–M$ relation into the substellar regime is less certain. Here, we present the Comprehensive Archive of Substellar and Planetary Accretion Rates (CASPAR), the largest compilation to date of substellar accretion diagnostics. CASPAR includes: 658 stars, 130 brown dwarfs, and 10 bound planetary mass companions. In this work, we investigate the contribution of methodological systematics to scatter in the $\dot{M}–M$ relation and compare brown dwarfs to stars. In our analysis, we rederive all quantities using self-consistent models, distances, and empirical line flux to accretion luminosity scaling relations to reduce methodological systematics. This treatment decreases the original 1σscatter in the $\log \dot{M}–\log M$ relation by ∼17%, suggesting that it makes only a small contribution to the dispersion. The CASPAR rederived values are best fit by $\dot{M}\propto M^{2.02\pm 0.06}$ from 10M_Jto 2M_⊙, confirming previous results. However, we argue that the brown-dwarf and stellar populations are better described separately and by accounting for both mass and age. Therefore, we derive separate age-dependent $\dot{M}–M$ relations for these regions and find a steepening in the brown-dwarf $\dot{M}–M$ slope with age. Within this mass regime, the scatter decreases from 1.36 dex to 0.94 dex, a change of ∼44%. This result highlights the significant role that evolution plays in the overall spread of accretion rates, and suggests that brown dwarfs evolve faster than stars, potentially as a result of different accretion mechanisms. 

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