High Energy Accelerator Research Organization

COMET experiment is planned to be conducted at the Hadron facility of J-PARC. This experiment searches for the neutrino-less transition of a muon to an electron in a nuclear field known as “<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>–e conversion,” which is one of the Charged Lepton Flavor Violation processes. A detector solenoid (DS), which is located at the downstream end of a muon beam line for COMET, is the world's largest cryogen-free superconducting magnet, which is directly cooled by cryocoolers. The DS was fabricated, and the commissioning tests were performed in 2024. The DS has successfully been cooled down to 4.2K and excited to the rated current without training quenches. In addition, the preliminary magnetic field measurement using a 3-axis Hall probe is carried out, and these results have proven the integrity of the magnet.

Abstract In this work, we present the proof of concept of a characterization of the temperature&amp;#xD;dependence of the quality factor Q(T ) for a massive silicon substrate suspended in a&amp;#xD;cryogenic environment using the Gentle Nodal Suspension (GeNS) method. Measurements&amp;#xD;were conducted in a low-vibration cryostat, analyzing three distinct resonant vibrational&amp;#xD;modes of the sample. Quality factor values and associated uncertainties were extracted&amp;#xD;using multiple statistical approaches. The results highlight the suitability of the&amp;#xD;experimental method and confirm crystalline silicon as a promising material for last-stage&amp;#xD;test mass suspensions in cryogenic conditions, as required in next-generation gravitational&amp;#xD;wave detectors. A comparison between experimental data and FEM simulations confirms&amp;#xD;the accuracy of the setup. This study, part of the broader development of monolithic&amp;#xD;silicon suspensions, demonstrates the effectiveness of the GeNS technique for assessing&amp;#xD;mechanical dissipation in large-scale substrates.

Abstract. The volume increase that accompanies many hydration reactions can stress and fracture the surrounding rock, a process commonly called reaction-induced fracturing. Reaction-induced fracturing accelerates the rate of hydration by creating new pathways for fluids to migrate into reactive rock and by generating new reactive surface areas. The evolution of reaction-induced fractures also depends on the background stress state, which varies among different tectonic environments. We investigate the impact of tectonic stresses on reaction-induced fracturing using 2-D hydraulic-chemical-mechanical distinct element models. The results indicate that the general pattern of reaction-induced fractures depends on the orientation of background tectonic stresses relative to fluid-supplying channels. A spalling fracture pattern characterized by short cracks parallel to and along fluid-supplying channels occurs when the maximum principal tectonic stress is parallel to the channels whereas a branching fracture pattern characterized by long tensile cracks that propagate in a hierarchical manner into unreacted parts of the rock is expected when the tectonic stress is hydrostatic or when the maximum principal tectonic stress is normal to fluid-supplying channels. Spalling localizes hydration and fluid flow along the channels whereas branching promotes spatially extensive hydration and fluid flow away from the fluid supply. The results indicate tectonic stresses may guide the hydration distribution in the oceanic lithosphere at mid-ocean ridges and outer rises and in the cold mantle wedge corner in subduction zones.

In light of the latest Planck and Atacama Cosmology Telescope (P-ACT) joint results on the primordial scalar power spectrum, we show that the R 2 inflation model extended with a non-minimally coupled scalar field χ — namely the χ-extended R 2 inflation model — can naturally accommodate a larger spectral index ns and a small positive running αs at cosmic microwave background (CMB) scales, both of which are consistent with the latest P-ACT constraints. This is because the χ field contributes a blue-tilted component to the primordial power spectrum, which both modifies the large-scale power and, as a result, significantly enhances power on small scales. The deviation of the ns and αs from the single field R 2 inflation is related to the non-minimal coupling constant ξ. The consequent enhancement in the primordial power spectrum can be large enough to lead to the formation of primordial black holes (PBHs) of mass ≲1020 g as dark matter candidates. Furthermore, future observations of the small-scale power spectrum, CMB spectral distortions, and stochastic gravitational waves will provide decisive tests of this model and its predictions for PBHs. We stress its strong connection to the seesaw mechanism for the generation of the observed small masses.

Precise control over multiple coordination sites is critical for directing molecular assembly on surfaces, a challenge well suited to N-heterocyclic carbenes (NHCs) owing to their strong σ-donation and tunable steric properties. In previously reported bisNHC systems, on-surface carbene generation from salt precursors requires elevated temperatures, driving adsorption into thermodynamically favored flat-lying configurations that obscure intrinsic binding behavior. Here, we employ isolable bisNHC derivatives bearing isopropyl and tert-butyl substituents, enabling their direct deposition as unprotected molecular precursors. Scanning tunneling microscopy and near-edge X-ray absorption fine structure measurements reveal that both derivatives adopt flat-lying geometries; however, their lateral assembly behavior differs markedly. The isopropyl substituted derivative forms mutually repulsive chains, whereas the more sterically hindered tert-butyl derivative (bisNHCtBu) unexpectedly assembles into a close-packed, dipole-driven molecular lattice. The bisNHCtBu lattice is stable at room temperature but thermally fragile, underscoring the difficulty of accessing this architecture via protected-precursor approaches. The bisNHCtBu lattice exhibits a wide spectroscopic bandgap and contains aligned metal-binding cavities that enable observation of spin-flip excitations of deposited dysprosium atoms. This molecular lattice provides a versatile platform for probing emergent electronic and magnetic properties and the single molecule reactivity of surface-confined, low-coordinate metal complexes.

The spin supersolid-a magnetic analogue of the supersolid that simultaneously exhibits solid and superfluid orders-has emerged as a promising sub-Kelvin refrigerant with strong low-energy fluctuations and associated entropic effects1. However, the stringent prerequisites have so far confined its presence to certain magnetic insulators. Here we report the discovery of a metallic spin supersolid in a rare-earth compound EuCo2Al9 (ECA), which is a good metal with excellent electrical and thermal conductivity. The high-spin Eu2+ ions form a three-dimensional lattice with stacked triangular layers, in which the spin-supersolid state is stabilized through a mechanism involving both Ruderman-Kittel-Kasuya-Yosida (RKKY) and dipolar couplings. Neutron diffraction shows microscopic evidence of spin supersolidity, demonstrating the coexistence of out-of-plane and in-plane spin orders in this alloy. Our RKKY-dipolar model successfully captures the metallic spin-supersolid Y and V phases in ECA, along with the 1/3 magnetization plateau. The observed nonclassical magnetization behaviours within these phases point to significant quantum fluctuations, probably enhanced by the conduction electrons. The resistivity measurements provide a transport probe for the spin-supersolid transitions, because of scattering of conduction electrons from local moments. Through the adiabatic demagnetization process, ECA achieves ultralow cooling to 106 mK, exhibiting a giant magnetocaloric effect that manifests sharp anomalies in the magneticGrüneisen ratio. ECA emerges as one of the first metallic spin supersolids, combining low cooling temperature, large magnetic entropy and ultrahigh thermal conductivity for high-performance sub-Kelvin refrigeration.