arXiv subjects

Takahiro Sasaki

Publications and source records attributed to Takahiro Sasaki.

At least 19 recordsLinked to original sources

arXiv2026

Intimate relationship between spin configuration in the triplet pair and superconductivity in UTe$_2$

Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$_2$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $T_{\rm c}$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $H_{c2}$ when the magnetic field is applied along the $c$ axis ($H \parallel c$). In addition, we found that $H_{\rm c2}$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $H \parallel b$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $H_{\rm c2}$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$_2$ and superfluid $^3$He, focusing on their spin-triplet pairing states.

cond-mat.supr-con↗

arXiv2025

SILVIA: Ultra-precision formation flying demonstration for space-based interferometry

We propose SILVIA (Space Interferometer Laboratory Voyaging towards Innovative Applications), a mission concept designed to demonstrate ultra-precision formation flying between three spacecraft separated by 100 m. SILVIA aims to achieve sub-micrometer precision in relative distance control by integrating spacecraft sensors, laser interferometry, low-thrust and low-noise micro-propulsion for real-time measurement and control of distances and relative orientations between spacecraft. A 100-meter-scale mission in a near-circular low Earth orbit has been identified as an ideal, cost-effective setting for demonstrating SILVIA, as this configuration maintains a good balance between small relative perturbations and low risk for collision. This mission will fill the current technology gap towards future missions, including gravitational wave observatories such as DECIGO (DECihertz Interferometer Gravitational wave Observatory), designed to detect the primordial gravitational wave background, and high-contrast nulling infrared interferometers like LIFE (Large Interferometer for Exoplanets), designed for direct imaging of thermal emissions from nearby terrestrial planet candidates. The mission concept and its key technologies are outlined, paving the way for the next generation of high-precision space-based observatories.

astro-ph.IM↗

arXiv2020

Where to Map? Iterative Rover-Copter Path Planning for Mars Exploration

In addition to conventional ground rovers, the Mars 2020 mission will send a helicopter to Mars. The copter's high-resolution data helps the rover to identify small hazards such as steps and pointy rocks, as well as providing rich textual information useful to predict perception performance. In this paper, we consider a three-agent system composed of a Mars rover, copter, and orbiter. The objective is to provide good localization to the rover by selecting an optimal path that minimizes the localization uncertainty accumulation during the rover's traverse. To achieve this goal, we quantify the localizability as a goodness measure associated with the map, and conduct a joint-space search over rover's path and copter's perceptual actions given prior information from the orbiter. We jointly address where to map by the copter and where to drive by the rover using the proposed iterative copter-rover path planner. We conducted numerical simulations using the map of Mars 2020 landing site to demonstrate the effectiveness of the proposed planner.

cs.RO↗

arXiv2015

Temperature dependence of meson screening masses; a comparison of effective model with lattice QCD

Temperature dependence of pion and sigma-meson screening masses is evaluated by the Polyakov-loop extended Nambu--Jona-Lasinio model with the entanglement vertex (EPNJL model). We propose a practical way of calculating meson screening masses in the NJL-type effective models. The method based on the Pauli-Villars regularization solves the well-known difficulty that the evaluation of screening masses is not easy in the NJL-type effective models. The method is applied to analyze temperature dependence of pion screening masses calculated with state-of-the-art lattice simulations with success in reproducing the lattice QCD results. We predict the temperature dependence of pole mass by using EPNJL model.

hep-ph↗

arXiv2014

Effective model approach to meson screening masses at finite temperature

Temperature dependence of pion and sigma-meson screening masses is evaluated by the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model with the entanglement vertex. We propose a practical way of calculating meson screening masses in the NJL-type effective models. The method based on the Pauli-Villars regularization solves the well-known difficulty that the evaluaton of screening masses is not easy in the NJL-type effective models.The PNJL model with the entanglement vertex and the Pauli-Villars regularization well reproduces lattice QCD results on temperature dependence of the chiral condensate and the Polyakov loop. The method is applied to analyze temperature dependence of pion screening mass calculated with state-of-the-art lattice simulations with success in reproducing the lattice QCD results.

hep-ph↗

arXiv2014

Heavy quark potential at finite imaginary chemical potential

We investigate chemical-potential ($μ$) dependence of the static-quark free energies in both the real and imaginary $μ$ regions, using the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. Static-quark potentials are evaluated from Polyakov-loop correlators in the deconfinement phase and the imaginary $μ=iμ_{\rm I}$ region and extrapolated to the real $μ$ region with analytic continuation. As the analytic continuation, the potential calculated at imaginary $μ=iμ_{\rm I}$ is expanded into a Taylor-expansion series of $iμ_{\rm I}/T$ up to 4th order and the pure imaginary variable $iμ_{\rm I}/T$ is replaced by the real one $μ_{\rm R}/T$. At real $μ$, the 4th-order term weakens $μ$ dependence of the potential sizably. Also, the color-Debye screening mass is extracted from the color-singlet potential at imaginary $μ$, and the mass is extrapolated to real $μ$ by analytic continuation. The screening mass thus obtained has stronger $μ$ dependence than the prediction of the leading-order thermal perturbation theory at both real and imaginary $μ$.

hep-lat↗

arXiv2013

Phase structure and Hosotani mechanism in QCD-like theory with compact dimensions

We investigated the phase diagram of $SU(3)$ gauge theory in four dimension with one compact dimension by using the perturbative one-loop effective potential. Effects of the adjoint and fundamental fermions are investigated and then the rich phase structure in the quark-mass and compact-size scale is realized. Our results are qualitatively consistent with the recent lattice calculation and clearly show that the lattice calculation can be understood from the Hosotani mechanism. Moreover, we show the result obtained by using the flavor twisted boundary condition for fundamental fermion which does not break the $Z_3$ symmetry, explicitly.

hep-ph↗

arXiv2013

Color screening potential at finite density in two-flavor lattice QCD with Wilson fermions

We investigate chemical-potential (μ) dependence of static-quark free energies in both the real and imaginary μregions, performing lattice QCD simulations at imaginary μand extrapolating the results to the real μregion with analytic continuation. Lattice QCD calculations are done on a 16^{3}\times 4 lattice with the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. Static-quark potential is evaluated from the Polyakov-loop correlation functions in the deconfinement phase. As the analytic continuation, the potential calculated at imaginary μ=iμ_{\rm I} is expanded into a Taylor-expansion series of iμ_{\rm I}/T up to 4th order and the pure imaginary variable iμ_{\rm I}/T is replaced by the real one μ_{\rm R}/T. At real μ, the 4th-order term weakens μdependence of the potential sizably. At long distance, all of the color singlet and non-singlet potentials tend to twice the single-quark free energy, indicating that the interactions between heavy quarks are fully color-screened for finite μ. For both real and imaginary μ, the color-singlet q{\bar q} and the color-antitriplet qq interaction are attractive, whereas the color-octet q{\bar q} and the color-sextet qq interaction are repulsive. The attractive interactions have stronger μ/T dependence than the repulsive interactions. The color-Debye screening mass is extracted from the color-singlet potential at imaginary μ, and the mass is extrapolated to real μby analytic continuation. The screening mass thus obtained has stronger μdependence than the prediction of the leading-order thermal perturbation theory at both real and imaginary μ.

hep-lat↗

arXiv2013

Determination of quark-hadron transition from lattice QCD and neutron-star observation

We determine the quark-hadron transition line in the whole region of temperature (T) and baryon-number chemical potential (μ_B) from lattice QCD results and neutron-star mass measurements, making the quark-hadron hybrid model that is consistent with the two solid constraints. The quark part of the hybrid model is the Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with entanglement vertex that reproduces lattice QCD results at mu_B/T=0, while the hadron part is the hadron resonance gas model with volume-exclusion effect that reproduces neutron-star mass measurements and the neutron-matter equation of state calculated from two- and three-nucleon forces based on the chiral effective field theory. The lower bound of the critical mu_B of the quark-hadron transition at zero T is mu_B = 1.6 GeV. The interplay between the heavy-ion collision physics around mu_B/T =6 and the neutron-star physics where mu_B/T is infinity is discussed.

hep-ph↗

arXiv2013

Differences and similarities between fundamental and adjoint matters in SU(N) gauge theories

We investigate differences and similarities between fundamental fermions and adjoint fermions in SU(N) gauge theories. The gauge theory with fundamental fermions possesses ZN symmetry only in the limit of infinite fermion mass, whereas the gauge theory with adjoint fermions does have the symmetry for any fermion mass. The flavor-dependent twisted boundary condition (FTBC) is then imposed on fundamental fermions so that the theory with fundamental fermions can possess ZN symmetry for any fermion mass. We show similarities between FTBC fundamental fermions and adjoint fermions, using the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. In the mean-field level, the PNJL model with FTBC fundamental fermions has dynamics similar to the PNJL model with adjoint fermions for the confinement/deconfinement transition related to ZN symmetry. The chiral property is somewhat different between the two models, but there is a simple relation between chiral condensates in the two models. As an interesting high-energy phenomenon, a possibility of the gauge symmetry breaking is studied for FTBC fundamental fermions.

hep-ph↗

arXiv2013

Two-color QCD at imaginary chemical potential and its impact on real chemical potential

We study properties of two-color QCD at imaginary chemical potential ($μ$) from the viewpoint of the Roberge-Weiss (RW) periodicity, the charge conjugation and the pseudo-reality. At $μ=\pm iπT/2$, where $T$ is temperature, the system is symmetric under the combination of the charge conjugation ${\cal C}$ and the ${\mathbb Z}_{2}$ transformation. The symmetry, called ${\cal C} {\mathbb Z}_{2}$ symmetry, is preserved at lower $T$ but broken at higher $T$. The Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model has the same properties as two-color QCD for ${\cal C} {\mathbb Z}_{2}$ symmetry and the pseudo-reality. The nontrivial correlation between the chiral restoration and the deconfinement are investigated by introducing the entanglement vertex in the PNJL model. The order of ${\cal C} {\mathbb Z}_{2}$ symmetry breaking at the RW endpoint is second-order when the correlation is weak, but becomes first-order when the correlation is strong. We also investigate the impact of the correlation on the phase diagram at real $μ$.

hep-ph↗

arXiv2013

Confinement and Z_3 symmetry in three-flavor QCD

We investigate the confinement mechanism in three-flavor QCD with imaginary isospin chemical potentials $(μ_u,μ_d,μ_s)=(iθT,-iθT,0)$, using the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model, where $T$ is temperature. As for three degenerate flavors, the system has $\mathbb{Z}_{3}$ symmetry at $θ=2π/3$ and hence the Polyakov loop $Φ$ vanishes there for small $T$. As for 2+1 flavors, the symmetry is not preserved for any $θ$, but $Φ$ becomes zero at $θ=θ_{\rm conf} < 2π/3$ for small $T$. The confinement phase defined by $Φ=0$ is realized, even if the system does not have $\mathbb{Z}_{3}$ symmetry exactly. In the $θ$-$T$ plane, there is a critical endpoint of deconfinement transition. The deconfinement crossover at zero chemical potential is a remnant of the first-order deconfinement transition at $θ=θ_{\rm conf}$. The relation between the non-diagonal element $χ_{us}$ of quark number susceptibilities and the deconfinement transition is studied. The present results can be checked by lattice QCD simulations directly, since the simulations are free from the sign problem for any $θ$.

hep-ph↗

arXiv2012

The quarkynic phase and the Z_{Nc} symmetry

We investigate the interplay between the Z_{Nc} symmetry and the emergence of the quarkyonic phase, adding the flavor-dependent complex chemical potentials μ_f=μ+iTθ_f with (θ_f)=(0, θ, -θ) to the Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model. When θ=0, the PNJL model with the μ_f agrees with the standard PNJL model with the real chemical potential μ. When θ=2π/3, meanwhile, the PNJL model with the μ_f has the Z_{Nc} symmetry exactly for any real μ, so that the quarkyonic phase exists at small T and large μ. Once θvaries from 2π/3, the quarkyonic phase exists only on a line of T=0 and μlarger than the dynamical quark mass, and the region at small T and large μis dominated by the quarkyonic-like phase in which the Polyakov loop is small but finite.

hep-ph↗

arXiv2012

Model approach to the sign problem on lattice QCD with theta vacuum

We propose a practical way of circumventing the sign problem in lattice QCD simulations with a theta-vacuum term. This method is the reweighting method for the QCD Lagrangian after the chiral transformation. In the Lagrangian, the P-odd mass term as a cause of the sign problem is minimized. Additionally, we investigate theta-vacuum effects on the QCD phase diagram for the realistic 2+1 flavor system, using the three-flavor Polyakov-extended Nambu-Jona-Lasinio (PNJL) model and the entanglement PNJL model as an extension of the PNJL model. The theta-vacuum effects make the chiral transition sharper. We finally investigate theta dependence of the transition temperature and compare with the result of the pure gauge lattice simulation with imaginary theta parameter.

hep-ph↗

arXiv2012

A simple model with Z_N symmetry

We propose a simple model with the Z_N symmetry in order to answer whether the symmetry is a good concept in QCD with light quark mass. The model is constructed by imposing the flavor-dependent twisted boundary condition (TBC) on the three-flavor Polyakov-loop extended Nambu-Jona-Lasinio model. In the model, the Z_N symmetry is preserved below some temperature T_c, but spontaneously broken above T_c. Dynamics of the simple model is similar to that of the original PNJL model without the TBC, indicating that the Z_N symmetry is a good concept. We also investigate the interplay between the Z_N symmetry and the emergence of the quarkyonic phase.

hep-ph↗

arXiv2012

A practical solution to the sign problem at finite theta-vacuum angle

hep-ph↗

arXiv2012

A QCD-like theory with the ZNc symmetry

We propose a QCD-like theory with the ZNc symmetry. The flavor-dependent twisted boundary condition (TBC) is imposed on Nc degenerate flavor quarksin the SU(Nc) gauge theory. The QCD-like theory is useful to understand the mechanism of color confinement. Dynamics of the QCD-like theory is studied by imposing the TBC on the Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model. The TBC model is applied to two- and three-color cases. The ZNc symmetry is preserved below some temperature Tc, but spontaneously broken above Tc. The color confinement below Tc preserves the flavor symmetry. Above Tc, the flavor symmetry is broken, but the breaking is suppressed by the entanglement between the Polyakov loop and the chiral condensate. Particularly at low temperature, dynamics of the TBC model is similar to that of the PNJL model with the standard fermion boundary condition, indicating that the ZNc symmetry is a good approximate concept in the latter model even if the current quark mass is small. The present prediction can be tested in future by lattice QCD, since the QCD-like theory has no sign problem.

hep-ph↗

arXiv2012

Theta vacuum and entanglement interaction in the three-flavor Polyakov-loop extended Nambu-Jona-Lasinio model

We investigate theta-vacuum effects on the QCD phase diagram for the realistic 2+1 flavor system, using the three-flavor Polyakov-extended Nambu-Jona-Lasinio (PNJL) model and the entanglement PNJL model as an extension of the PNJL model. The theta-vacuum effects make the chiral transition sharper. For large theta-vacuum angle the chiral transition becomes first order even if the quark number chemical potential is zero, when the entanglement coupling between the chiral condensate and the Polyakov loop is taken into account. We finally propose a way of circumventing the sign problem on lattice QCD with finite theta.

hep-ph↗