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1. Ionization energies and cationic bond dissociation energies of RuB, RhB, OsB, IrB, and PtB

   https://doi.org/10.1063/5.0107086  

   Merriles, Dakota M.; Morse, Michael D. (August 2022, The Journal of Chemical Physics)

   Two-photon ionization thresholds of RuB, RhB, OsB, IrB, and PtB have been measured using resonant two-photon ionization spectroscopy in a jet-cooled molecular beam and have been used to derive the adiabatic ionization energies of these molecules. From the measured two-photon ionization thresholds, IE(RuB) = 7.879(9) eV, IE(RhB) = 8.234(10) eV, IE(OsB) = 7.955(9) eV, IE(IrB) = 8.301(15) eV, and IE(PtB) = 8.524(10) eV have been assigned. By employing a thermochemical cycle, cationic bond dissociation energies of these molecules have also been derived, giving D0(Ru+–B) = 4.297(9) eV, D0(Rh+–B) = 4.477(10) eV, D0(Os–B+) = 4.721(9) eV, D0(Ir–B+) = 4.925(18) eV, and D0(Pt–B+) = 5.009(10) eV. The electronic structures of the resulting cationic transition metal monoborides (MB+) have been elucidated using quantum chemical calculations. Periodic trends of the MB+ molecules and comparisons to their neutral counterparts are discussed. The possibility of quadruple chemical bonds in all of these cationic transition metal monoborides is also discussed. 

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2. Conservation of longitudinal spin polarization of positrons emitted from a thin Ni(100) foil

   https://doi.org/10.1103/PhysRevA.107.062809  

   Cecchini, G. G.; Greaves, R. G.; Mills, A. P. (June 2023, Physical Review A)

   We have discovered that 5 keV bursts of 5 × 107 positrons with an initial longitudinal spin polarization of (28.8 ± 0.7)%, when implanted into a thin Ni(100) crystal, are emitted with 20% efficiency at thermal energies from its surface with (30.9 ± 0.5)% polarization. We conclude that the positron spin polarization is preserved while interacting with the Ni, despite the 0.61 T average transverse magnetization of the Ni at room temperature. The resulting polarized beam has been focused to a 0.025-mm mean-diameter spot when accelerated to 5 keV and will be uniquely suited for experiments on a neutral spin aligned e+-e − plasma, spin- and angle-resolved positronium emission spectroscopy, and critical for producing a triplet positronium Bose-Einstein condensate. 

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3. Positron Beam Loading and Acceleration in the Blowout Regime of a Plasma Wakefield Accelerator

   https://doi.org/10.34133/research.0878  

   Zhou, Shiyu; Ding, Siqin; An, Weiming; Su, Qianqian; Hua, Jianfei; Li, Fei; Mori, Warren B; Joshi, Chan; Lu, Wei (January 2025, Research)

   Plasma wakefield acceleration in the nonlinear blowout regime has achieved marked milestones in electron beam acceleration, demonstrating high acceleration gradients and energy efficiency while preserving excellent beam quality. However, this regime is deemed unsuitable for achieving positron acceleration of comparable results, which is vital for future compact electron–positron colliders. In this article, we find that an intense positron beam loaded at the back of beam-driven blowout cavity can self-consistently induce the focusing field and flatten the longitudinal wakefield, leading to stable, high-efficiency, and high-quality positron acceleration. This is achieved through the formation of an on-axis electron filament induced by positron beam load, which shapes the plasma wakefield in a distinct way compared to electron beam load in the blowout regime. Via a nonlinear analytic model and numerical simulations, we explain the novel beam loading effects of the interaction between the on-axis filament and the blowout cavity. High-fidelity simulations show that a high-charge positron beam can be accelerated with >20% energy transfer efficiency, ~1% energy spread, and ~1 mm·mrad normalized emittance, while considerably depleting the energy of the drive beam. The concept can also be extended to simultaneous acceleration of electron and positron beams and high transformer ratio positron acceleration as well. This development offers a new route for the application of plasma wakefield acceleration into particle physics. 

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4. Action principles and conservation laws for Chew–Goldberger–Low anisotropic plasmas

   https://doi.org/10.1017/S0022377822000642  

   Webb, G.M.; Anco, S.C.; Meleshko, S.V.; Zank, G.P. (August 2022, Journal of Plasma Physics)

   The ideal Chew–Goldberger–Low (CGL) plasma equations, including the double adiabatic conservation laws for the parallel ( $$p_\parallel$$ ) and perpendicular pressure ( $$p_\perp$$ ), are investigated using a Lagrangian variational principle. An Euler–Poincaré variational principle is developed and the non-canonical Poisson bracket is obtained, in which the non-canonical variables consist of the mass flux $${\boldsymbol {M}}$$ , the density $$\rho$$ , the entropy variable $$\sigma =\rho S$$ and the magnetic induction $${\boldsymbol {B}}$$ . Conservation laws of the CGL plasma equations are derived via Noether's theorem. The Galilean group leads to conservation of energy, momentum, centre of mass and angular momentum. Cross-helicity conservation arises from a fluid relabelling symmetry, and is local or non-local depending on whether the gradient of $$S$$ is perpendicular to $${\boldsymbol {B}}$$ or otherwise. The point Lie symmetries of the CGL system are shown to comprise the Galilean transformations and scalings. 

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5. Disclosing the catalog pulsars dominating the Galactic positron flux

   https://doi.org/10.1088/1475-7516/2025/02/029  

   Orusa, Luca; Manconi, Silvia; Donato, Fiorenza; Mauro, Mattia Di (February 2025, Journal of Cosmology and Astroparticle Physics)

   Abstract The cosmic-ray flux of positrons is measured with high precision by the space-borne particle spectrometer AMS-02. The hypothesis that pulsars and their nebulae can significantly contribute to the excess of the AMS-02 positron flux has been consolidated after the observation of aγ-ray emission at GeV and TeV energies of a few degree size around a few sources, that provide indirect evidence that electron and positron pairs are accelerated to very high energies from these sources.By modeling the emission from pulsars in the ATNF catalog, we find that combinations of positron emission from cataloged pulsars and secondary production can fit the observed AMS-02 data. Our results show that a small number of nearby, middle-aged pulsars, particularly B1055-52, Geminga (J0633+1746), and Monogem (B0656+14), dominate the positron emission, contributing up to 80% of the flux at energies above 100 GeV. From the fit to the data, we obtain a list of the most important sources for which we recommend multi-wavelength follow-up observations, particularly in theγ-ray and X-ray bands, to further constrain the injection and diffusion properties of positrons. 

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