arXiv subjects

Kota Sasaki

Publications and source records attributed to Kota Sasaki.

3 recordsLinked to original sources

arXiv2026

Complete one-loop QED corrections to $D_s^+$ leptonic decays and impact on the CKM unitarity test

Recently, a violation of the CKM unitarity condition has been reported in the latest charm-meson data and the latest lattice results, once the universal electroweak correction is taken into account. In this article, we analytically derive for the first time the complete one-loop electroweak (EW) and QED corrections to the $D_{s}^+ \to \ell^+ ν_\ell$ decays for $\ell = μ, τ$. Our analysis incorporates both short-distance EW-QED corrections, which are beyond the leading-logarithmic approximation (the so-called Sirlin factor), and long-distance soft-photon corrections depending on the maximum total energy of undetected photons with their resummation. Although the inclusive photon QED corrections to the meson leptonic decays are well known, they do not match the actual measurement circumstances in $D_s^+ \to μ^+ ν_μ$. We find $ |V_{cs}|_{D_s} = 0.991 \pm 0.007 $ from the latest data on $D_s^+$ leptonic decays. We show that properly including these radiative corrections is essential to bring the second-column CKM unitarity tests into agreement with the Standard Model expectation. The study emphasizes that the current limiting factor in confirming CKM unitarity is the precision of QED corrections, and it points out that improving lattice simulations, taking the QED corrections into account, would be desirable for a more robust confirmation.

hep-ph↗

arXiv2026

Baryon-Meson Sum Rule for $b \to s ν\barν$

We derive a robust sum rule among the branching fractions of $Λ_b \to Λν\barν$ and $B \to K^{(\ast)} ν\barν$, assuming that right-handed neutrinos are decoupled. Despite the presence of 18 independent Wilson coefficients in the effective Hamiltonian, this relation remains exact. Remarkably, it is found that the coefficients of this baryon-meson sum rule are numerically identical to those of the $b\to c$ semileptonic sum rule among the branching fractions of $Λ_b \to Λ_c τ\barν$ and $B \to D^{(\ast)}τ\barν$. Once the decay rate of $B \to K^{\ast} ν\barν$ is measured, the decay rate of $Λ_b \to Λν\barν$ can be determined in a model-independent manner for new-physics scenarios involving only left-handed neutrino interactions. This clearly demonstrates that observables in baryonic and mesonic $b \to s ν\barν$ transitions will serve as a powerful probe for discriminating among new-physics scenarios.

hep-ph↗

arXiv2020

Magnetic excitations in magnetization plateaux of a frustrated spin ladder

Magnetization plateaux emerging in quantum spin systems due to spontaneously breaking of translational symmetry have been reported both theoretically and experimentally. The broken symmetry can induce reconstruction of elementary excitations such as Goldstone and Higgs modes, whereas its microscopic mechanism and reconstructed quasi-particle in magnetization-plateau phases have remained unclear so far. Here we theoretically study magnetic excitations in the magnetization-plateau phases of a frustrated spin ladder by using dynamical density-matrix renormalization-group method. Additionally, analytical approaches with perturbation theory are performed to obtain intuitive view of magnetic excitations. Comparison between numerical and analytical results indicates the presence of a reconstructed quasi-particle originating from spontaneously broken translational symmetry, which is realized as a collective mode of spin trimer called trimeron.

cond-mat.str-el↗