arXiv subjects

R. Watanabe

Publications and source records attributed to R. Watanabe.

11 recordsLinked to original sources

arXiv2023

Discretization of Camassa-Holm peakon equation using orthogonal polynomials and matrix $LR$ transformations

Discrete integrable systems are closely related to orthogonal polynomials and isospectral matrix transformations. In this paper, we use these relationships to propose a nonautonomous time-discretization of the Camassa-Holm (CH) peakon equation, which describes the motion of peakon waves, which are soliton waves with sharp peaks. We then validate our time-discretization, and clarify its asymptotic behavior as the discrete-time goes to infinity. We present numerical examples to demonstrate that the proposed discrete equation captures peakon wave motions.

nlin.SI↗

arXiv2023

Matrix similarity transformations derived from extended $q$-analogues of the Toda equation and Lotka-Volterra system

The $q$-Toda equation is derived from replacing ordinary derivatives with $q$-derivatives in the famous Toda equation. In this paper, we associate an extension of the $q$-Toda equation with matrix eigenvalue problems, and then show applications of its time-discretization to computing matrix eigenvalues. With respect to the Lotka-Volterra system, we also have the similar discussion on the case of the Toda equation.

nlin.SI↗

arXiv2022

Application of the extended $q$-discrete Toda equation to computing eigenvalues of Hessenberg totally nonnegative matrices

The Toda equation is one of the most famous integrable systems, and its time-discretization is simply the recursion formula of the quotient-difference (qd) algorithm for computing eigenvalues of tridiagonal matrices. An extension of the Toda equation is the q-Toda equation, which is derived by replacing standard derivatives with the so-called q-derivatives involving a parameter q such that 0 < q < 1. In our previous paper, we showed that a discretization of the q-Toda equation is shown to be also applicable to computing tridiagonal eigenvalues. In this paper, we consider another extension of the q-discrete Toda equation and find an application to computing eigenvalues of Hessenberg totally nonnegative (TN) matrices, which are matrices where all minors are nonnegative. There are two key components to our approach. First, we consider the extended q-discrete equation from the perspective of shifted LR transformations, similarly to the discrete Toda and its q-analogue cases. Second, we clarify asymptotic convergence as discrete-time goes to infinity in the qdiscrete Toda equation by focusing on TN properties. We also present two examples to numerically verify convergence to Hessenberg TN eigenvalues numerically.

nlin.SI↗

arXiv2020

First measurement using a nuclear emulsion detector of the $\nu_{\mu}$ charged-current cross section on iron around the 1$\,$GeV energy region

We have carried out $\nu_{\mu}$ charged-current interaction measurement on iron using an emulsion detector exposed to the T2K neutrino beam in the J-PARC neutrino facility. The data samples correspond to 4.0$\times$10$^{19}$ protons on target, and the neutrino mean energy is 1.49$\,$GeV. The emulsion detector is suitable for precision measurements of charged particles produced in neutrino-iron interactions with a low momentum threshold thanks to thin-layered structure and sub-$\mu$m spatial resolution. The charged particles are successfully detected, and their multiplicities are measured using the emulsion detector. The cross section was measured to be $\sigma^{\mathrm{Fe}}_{\mathrm{CC}} = (1.28 \pm 0.11({\mathrm{stat.}})^{+0.12}_{-0.11}({\mathrm{syst.}})) \times 10^{-38} \, {\mathrm{cm}}^{2}/{\mathrm{nucleon}}$. The cross section in a limited kinematic phase space of induced muons, $\theta_{\mu} < 45^{\circ}$ and $p_{\mu} > 400 \, {\rm MeV}/c$, on iron was $\sigma^{\mathrm{Fe}}_{\mathrm{CC \hspace{1mm} phase \hspace{0.5mm} space}} = (0.84 \pm 0.07({\mathrm{stat.}})^{+0.07}_{-0.06}({\mathrm{syst.}})) \times 10^{-38} \, {\mathrm{cm}}^{2}/{\mathrm{nucleon}}$. The cross-section results are consistent with previous values obtained via different techniques using the same beamline, and they are well reproduced by current neutrino interaction models. These results demonstrate the capability of the detector towards the detailed measurements of the neutrino-nucleus interactions around the 1$\,$GeV energy region.

hep-ex↗

arXiv2019

The Belle II Physics Book

We present the physics program of the Belle II experiment, located on the intensity frontier SuperKEKB $e^+e^-$ collider. Belle II collected its first collisions in 2018, and is expected to operate for the next decade. It is anticipated to collect 50/ab of collision data over its lifetime. This book is the outcome of a joint effort of Belle II collaborators and theorists through the Belle II theory interface platform (B2TiP), an effort that commenced in 2014. The aim of B2TiP was to elucidate the potential impacts of the Belle II program, which includes a wide scope of physics topics: B physics, charm, tau, quarkonium, electroweak precision measurements and dark sector searches. It is composed of nine working groups (WGs), which are coordinated by teams of theorist and experimentalists conveners: Semileptonic and leptonic B decays, Radiative and Electroweak penguins, phi_1 and phi_2 (time-dependent CP violation) measurements, phi_3 measurements, Charmless hadronic B decay, Charm, Quarkonium(like), tau and low-multiplicity processes, new physics and global fit analyses. This book highlights "golden- and silver-channels", i.e. those that would have the highest potential impact in the field. Theorists scrutinised the role of those measurements and estimated the respective theoretical uncertainties, achievable now as well as prospects for the future. Experimentalists investigated the expected improvements with the large dataset expected from Belle II, taking into account improved performance from the upgraded detector.

hep-ex↗

arXiv2013

Uniform spin susceptibility and spin-gap phenomenon in the BCS-BEC crossover regime of an ultracold Fermi gas

We investigate the uniform spin susceptibility $χ_{\rm s}$ in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Including pairing fluctuations within the framework of an extended $T$-matrix approximation, we show that $χ_{\rm s}$ exhibits non-monotonic temperature dependence in the normal state. In particular, $χ_{\rm s}$ is suppressed near the superfluid phase transition temperature $T_{\rm c}$ due to strong pairing fluctuations. To characterize this anomalous behavior, we introduce the spin-gap temperature $T_{\rm s}$ as the temperature at which $χ_{\rm s}$ takes a maximum value. Determining $T_{\rm s}$ in the whole BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas in terms of the temperature and the strength of a pairing interaction. We also clarify how the spin-gap phenomenon is related to the pseudogap phenomenon appearing in the single-particle density of states. Our results indicate that an ultracold Fermi gas in the BCS-BEC crossover region is a very useful system to examine the pseudogap phenomenon and the spin-gap phenomenon in a unified manner.

cond-mat.quant-gas↗

arXiv2012

Pseudogap phenomenon in an ultracold Fermi gas with a p-wave pairing interaction

We investigate single-particle properties of a one-component Fermi gas with a tunable p-wave interaction. Including pairing fluctuations associated with this anisotropic interaction within a $T$-matrix theory, we calculate the single-particle density of states, as well as the spectral weight, above the superfluid transition temperature $T_{\rm c}$. Starting from the weak-coupling regime, we show that the so-called pseudogap first develops in these quantities with increasing the interaction strength. However, when the interaction becomes strong to some extent, the pseudogap becomes obscure to eventually disappear in the strong-coupling regime. This non-monotonic interaction dependence is quite different from the case of an s-wave interaction, where the pseudogap simply develops with increasing the interaction strength. The difference between the two cases is shown to originate from the momentum dependence of the p-wave interaction, which vanishes in the low momentum limit. We also identify the pseudogap regime in the phase diagram with respect to the temperature and the p-wave interaction strength. Since the pseudogap is a precursor phenomenon of the superfluid phase transition, our results would be useful for the research toward the realization of p-wave superfluid Fermi gases.

cond-mat.quant-gas↗

arXiv2010

Magnetic and Electronic Raman Scattering at the Nodal Spin-Density-Wave Transition in BaFe2As2

Two magnon excitations and the nodal spin density wave (SDW) gap were observed in BaFe2As2 by Raman scattering. Below the SDW transition temperature (TSDW) nodal SDW gap opens together with new excitations in reconstructed electronic states. The two-magnon peak remains above TSDW and moreover the energy increases a little. The change from the long-range ordered state to the short-range correlated state is compared to the cuprate superconductors.

cond-mat.supr-con↗

arXiv2009

Epitaxial Growth of NdFeAsO Thin Films by Molecular Beam Epitaxy

Epitaxial films of NdFeAsO were grown on GaAs substrates by molecular beam epitaxy (MBE). All elements including oxygen were supplied from solid sources using Knudsen cells. The x-ray diffraction pattern of the film prepared with the optimum growth condition showed no indication of impurity phases. Only (00l) peaks were observed, indicating that NdFeAsO was grown with the c-axis perpendicular to the substrate. The window of optimum growth condition was very narrow, but the NdFeAsO phase was grown with a very good reproducibility. Despite the absence of any appreciable secondary phase, the resistivity showed an increase with decreasing temperature.

cond-mat.supr-con↗

arXiv2009

Pseudogap in fermionic density of states in the BCS-BEC crossover of atomic Fermi gases

We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature $T_{\rm c}$ including pairing fluctuations within a $T$-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature $T^*$ at which the pseudogap structure in DOS disappears. We also introduce another temperature $T^{**}$ at which the BCS-like double-peak structure disappears in the spectral weight. While one finds $T^*>T^{**}$ in the BCS regime, $T^{**}$ becomes higher than $T^*$ in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction.

cond-mat.quant-gas↗

arXiv2006

Letter of Intent for KASKA: High Accuracy Neutrino Oscillation Measurements with anti-nu_es from Kashiwazaki-Kariwa Nuclear Power Station

One of the current most-demanded experiments in neutrino physics is to measure the last mixing angle theta_13. KASKA is an experiment to detect new type of reactor neutrino oscillation and to measure sin^2 2theta_13 accurately using the world's most powerful nuclear reactor complex; Kashiwazaki-Kariwa nuclear power station. KASKA utilizes near and far detectors of identical structure at nearly optimized baselines and underground depths to cancel most of the systematics and reduce backgrounds. The expected sensitivity is sin^2 2theta_13~0.015, which is 10 times better sensitivity than the current upper limit measured by CHOOZ reactor experiment. Extension of KASKA project has potential to accurately measure other anti-nu_e oscillation parameters. Intense and precisely known neutrino flux measured by the KASKA-theta_13 phase can be used to pin down sin^2 2theta_12 at a baseline ~50km and to measure Dm^2_13 for the first time at a baseline ~5km. This Letter of Intent describes physics motivation, detector system and expected performance of the KASKA experiment.

hep-ex↗