arXiv subjects

Taishi Sano

Publications and source records attributed to Taishi Sano.

3 recordsLinked to original sources

arXiv2026

Schrödinger Symmetry in Spherically-symmetric Static Mini-superspaces with Matter Fields

Schrödinger symmetry has been shown to emerge in a ``fluid limit" from the full superspace to several mini-superspace models. To investigate one aspect of the robustness of this emergent symmetry, we consider two spherically-symmetric static mini-superspace models with matter fields at the classical level: (i) a Maxwell field with a cosmological constant and (ii) $n$ massless scalar fields. By developing a method based on canonical transformations, we demonstrate that for model (i), 3D Schrödinger symmetry emerges, and the solution is the (anti-)de Sitter Reissner-Nordström spacetime, and for model (ii), $(2+n)$D Schrödinger symmetry appears, and the solution is a generalized Janis-Newman-Winicour spacetime and its ``interior", a Kantowski-Sachs type closed universe. Furthermore, for the vacuum model, we find that 2D Schrödinger symmetry holds with different lapse functions and mini-superspace coordinates, suggesting the potential, yet unconfirmed, covariance of the symmetry. Finally, we propose a physical interpretation of the symmetry under the Hamiltonian constraint $H$: symmetry generators commuting with $H$ map a solution to another one, while those non-commuting with $H$ generate a new theory with the Schrödinger symmetry and the transformed configuration is a solution to the new theory. These results reinforce the robustness of the emergent Schrödinger symmetry and open new frontiers for exploring dynamics of matter and gravity.

gr-qc↗

arXiv2026

Vector Horndeski black holes in nonlinear electrodynamics

On a spherically symmetric and static background, we study the existence of linearly stable black hole (BH) solutions in nonlinear electrodynamics (NED) with a Horndeski vector-tensor (HVT) coupling, with and without curvature singularities at the center ($r=0$). Incorporating the electric charge $q_E$ and the magnetic charge $q_M$, we first show that nonsingular BHs can exist only if $q_M = 0$. We then study the stability of purely electric BHs by analyzing the behavior of perturbations in the metric and the vector field. Nonsingular electric BHs are unstable due to a Laplacian instability in the vector perturbation near the regular center. In the absence of the HVT coupling ($β=0$), singular BHs in power-law NED theories can be consistent with all linear stability conditions, while Born-Infeld BHs encounter strong coupling due to a vanishing propagation speed as $r \to 0$. In power-law NED and Born-Infeld theories with $β\neq 0$, the electric fields for singular BHs are regular near $r=0$, while the metric functions behave as $\propto r^{-1}$. Nevertheless, we show that Laplacian instabilities occur for regions inside the outer horizon $r_h$, unless the HVT coupling constant $β$ is significantly smaller than $r_h^2$. For $β\neq 0$, we also reconstruct the NED Lagrangian so that one of the metric functions takes the Reissner-Nordström form. In this case, there exists a branch where all squared propagation speeds are positive, but the ghost and strong coupling problems are present around the BH center. Thus, the dominance of the HVT coupling generally leads to BH instability in the high-curvature regime.

gr-qc↗

arXiv2024

Complex scaling calculation of phase shifts for positron collisions with positive ions

We present phase-shift calculations for positron collisions with positive ions using a complex scaling method (CSM). Based on the findings of this study [R. Suzuki, T. Myo, and K. Katō, Prog. Theor. Phys. 113, 1273 (2005).], we propose a modification of the phase shift in the CSM calculation, in which phase shifts are derived only from the complex eigenenergies of the CSM Hamiltonian. This modification is based on the fact that the contributions of high-lying complex eigenenergies can be approximated as a constant value in the case of a small collision energy, where neither target excitation nor positronium formation occurs. The proposed modification limits the contribution of the complex eigenenergies to the vicinity of the collision energy, which is also intuitively acceptable. We present a geometrical formulation of the modification and demonstrative calculations of positron scattering off positive ions. Our results agree well with those reported in the literature for the targets Ne, Ar, Kr, Xe, H, He, He$^+$, and Li$^{2+}$. The phase shifts of positron scattering off a Li$^+$ ion have also been reported.

physics.atom-ph↗