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1. Narrow bandwidth, low-emittance positron beams from a laser-wakefield accelerator

   https://doi.org/10.1038/s41598-024-56281-1  

   Streeter, M_J V; Colgan, C; Carderelli, J; Ma, Y; Cavanagh, N; Los, E E; Ahmed, H; Antoine, A F; Audet, T; Balcazar, M D; et al (December 2024, Scientific Reports)

   Abstract The rapid progress that plasma wakefield accelerators are experiencing is now posing the question as to whether they could be included in the design of the next generation of high-energy electron-positron colliders. However, the typical structure of the accelerating wakefields presents challenging complications for positron acceleration. Despite seminal proof-of-principle experiments and theoretical proposals, experimental research in plasma-based acceleration of positrons is currently limited by the scarcity of positron beams suitable to seed a plasma accelerator. Here, we report on the first experimental demonstration of a laser-driven source of ultra-relativistic positrons with sufficient spectral and spatial quality to be injected in a plasma accelerator. Our results indicate, in agreement with numerical simulations, selection and transport of positron beamlets containing$$N_{e+}\ge 10^5$$ $N_{e+}\ge {10}^5$ positrons in a 5% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. Particle-in-cell simulations show that positron beams of this kind can be guided and accelerated in a laser-driven plasma accelerator, with favourable scalings to further increase overall charge and energy using PW-scale lasers. The results presented here demonstrate the possibility of performing experimental studies of positron acceleration in a laser-driven wakefield accelerator. 

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2. Random transverse and longitudinal field Ising chains

   https://doi.org/10.1088/1742-5468/adfe5a  

   Pető, Tamás; Iglói, Ferenc; Kovács, István A (September 2025, Journal of Statistical Mechanics: Theory and Experiment)

   Abstract Motivated by experimental results on compounds like ${\text{LiHo}}_x{\text{Y}}_{1-x}{\text{F}}_4$ , we consider an Ising chain with random bonds in the simultaneous presence of random transverse and longitudinal fields. We study the low-energy properties of the model at zero temperature by the strong disorder renormalisation group method. In the absence of random longitudinal fields, the model showcases a trivial quantum-ordered and quantum-disordered fixed-point and a non-trivial infinite disorder critical point. In the absence of random transverse fields, the behaviour is dictated by the classical random-field Ising fixed-point. In the simultaneous presence of both a longitudinal and transverse random field, the RG trajectories are attracted to a set of disordered fixed-points, in which the disorder is either due to random quantum fluctuations, or due to classical random-field effects. Between the two regimes there is a smooth cross-over, which becomes sharp at the infinite disorder fixed-point. This local separatrix defines the relevant scaling direction, where the correlation-length is shown to diverge with an exponent ${\nu }_h\approx 1$ . 

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3. Low-energy magnetic states of Tb adatom on graphene

   https://doi.org/10.1088/1361-648X/ad8fe9  

   Shaikh, Monirul; Klein, Alison; Wysocki, Aleksander L (November 2024, Journal of Physics: Condensed Matter)

   Abstract Electronic structure and magnetic interactions of a Tb adatom on graphene are investigated from first principles using combination of density functional theory and multiconfigurational quantum chemistry techniques including spin–orbit coupling (SOC) . We determine that the six-fold symmetry hollow site is the preferred adsorption site and investigate electronic spectrum for different adatom oxidation states including Tb³⁺, Tb²⁺, Tb¹⁺, and Tb⁰. For all charge states, the Tb $4f^8$ configuration is retained with other adatom valence electrons being distributed over $5d_{xy}$ , $5d_{x2+y2}$ , and $6s/5d_0$ single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the $5d6s$ spins are parallel to the4fspin. For Tb³⁺, the energy levels can be described by theJ = 6 multiplet split by the graphene crystal field (CF). For other oxidation states, the interaction of4felectrons with spin and orbital degrees of freedom of $6s5d$ electrons in the presence of SOC results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene CF. Stable magnetic moment is predicted for Tb³⁺and Tb²⁺adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm⁻¹controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb¹⁺and Tb⁰adatoms. Our results indicate that the occupation of the $6s5d$ orbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms. 

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4. SZ–X-Ray Surface Brightness Fluctuations in the SPT-XMM Clusters

   https://doi.org/10.3847/1538-4357/adcd74  

   Romero, Charles E; Gaspari, Massimo; Schellenberger, Gerrit; Benson, Bradford A; Bleem, Lindsey E; Bulbul, Esra; Forman, William; Kraft, Ralph; Nulsen, Paul; Reichardt, Christian L; et al (May 2025, The Astrophysical Journal)

   Abstract The hot plasma in galaxy clusters, the intracluster medium, is expected to be shaped by subsonic turbulent motions, which are key for heating, cooling, and transport mechanisms. The turbulent motions contribute to the nonthermal pressure, which, if not accounted for, consequently imparts a hydrostatic mass bias. Accessing information about turbulent motions is thus of major astrophysical and cosmological interest. Characteristics of turbulent motions can be indirectly accessed through surface brightness fluctuations. This study expands on our pilot investigations of surface brightness fluctuations in the Sunyaev–Zel’dovich and in X-ray data by examining, for the first time, a large sample of 60 clusters using both SPT-SZ and XMM-Newton data and spans the redshift range 0.2 < z < 1.5, thus constraining the respective pressure and density fluctuations within 0.6R₅₀₀. We deem density fluctuations to be of sufficient quality for 32 clusters, finding mild correlations between the peak of the amplitude spectra of density fluctuations and various dynamical parameters. We infer turbulent velocities from density fluctuations with an average Mach number ${\mathcal{M}}_{\text{3D}}=0.52\pm 0.14$ , in agreement with numerical simulations. For clusters with inferred turbulent Mach numbers from fluctuations in both pressure, ${\mathcal{M}}_{\text{P}}$ , and density, ${\mathcal{M}}_{\rho }$ , we find broad agreement between ${\mathcal{M}}_{\text{P}}$ and ${\mathcal{M}}_{\rho }$ . Our results suggest either a bimodal or a skewed unimodal Mach number distribution, with the majority of clusters being turbulence-dominated (subsonic) while the remainder are shock-dominated (supersonic). 

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5. Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions

   https://doi.org/10.1088/1361-6633/ae0fc3  

   Collaboration, The STAR (October 2025, Reports on Progress in Physics)

   Abstract Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformationβ₂and triaxialityγ, and sometimes even a pear-like octupole deformationβ₃. The STAR experiment introduced a new ‘imaging-by-smashing’ technique ((STAR Collaboration) 2024Nature63567; Jia 2025Rep. Prog. Phys.88092301) to image the nuclear global shape by colliding nuclei at ultra-relativistic speeds and analyzing outgoing debris. Features of nuclear shape manifest in collective observables like anisotropic flowv_nand radial flow via mean transverse momentum $\left[p_{\mathrm{T}}\right]$ . We present new measurements of the variances ofv_n(n = 2, 3, and 4) and $\left[p_{\mathrm{T}}\right]$ , and the covariance of $v_n^2$ with $\left[p_{\mathrm{T}}\right]$ , in collisions of highly deformed²³⁸U and nearly spherical¹⁹⁷Au. Ratios of these observables between the two systems effectively suppress common final-state effects, isolating the strong impact of uranium’s deformation. By comparing results with state-of-the-art hydrodynamic model calculations, we extract ${\beta }_{2\mathrm{U}}$ and ${\gamma }_{\mathrm{U}}$ values consistent with those deduced from low-energy nuclear structure measurements. Measurements ofv₃and its correlation with $\left[p_{\mathrm{T}}\right]$ also provide the first experimental suggestion of a possible octupole deformation for²³⁸U. These findings provide significant support for using high-energy collisions to explore nuclear shapes on femtosecond timescales, with implications for both nuclear structure and quark-gluon plasma studies. 

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