arXiv subjects

Shoichi Sasaki

Publications and source records attributed to Shoichi Sasaki.

At least 19 recordsLinked to original sources

arXiv2026

Recent update of nucleon axial-vector charge with the PACS10 superfine lattice

We update the results of the nucleon axial-vector charge with the third ensemble of the PACS10 gauge configurations, which are generated by the PACS Collaboration at the physical point with lattice volume larger than $(10\;{\rm fm})^4$ and three different lattice spacings, 0.085 fm (coarse), 0.063 fm (fine) and 0.041 fm (superfine). Although the results of the first two ensembles generated at the coarse and fine lattice spacings are published, our study using the third one generated at the superfine lattice spacing is still underway. In this work, the low-energy relations arising from the partially conserved axial-vector current (PCAC) relation are also examined in terms of the nucleon three-point functions to verify whether the lattice QCD data correctly reproduces the physics in the continuum within the statistical accuracy.

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arXiv2025

Lattice study of scattering phase shifts for $DD^$ and $BB^$ systems using twisted boundary conditions: Search for bound state formation

We investigate the $S$- and $P$-wave phase shifts for the $DD^\ast$ and $BB^\ast$ scatterings using Lüscher's finite-size method under twisted boundary conditions to search for doubly charmed tetraquaks, $T_{cc}^+$, and doubly bottomed tetraquarks, $T_{bb}^-$ as the hadronic bound states. The $T_{cc}^+$ state was observed as a peak just bellow the $DD^*$ threshold by LHCb Collaboration, while the $T_{bb}^-$ state is a theoretically predicted tetraquark state having heavier quark flavors $bb\bar u \bar d$. Lüscher's finite-size method is one of the well established methods for calculating the scattering phase shifts between two hadrons in lattice QCD simulations. Several studies have used simulations under the periodic boundary condition to determine the scattering phase shifts at a few discrete momenta for the $DD^*$ system. However, the scattering phase shift has not been investigated for the $BB^*$ system. In this study, $S$- and $P$-wave scattering phase shifts for the $DD^*$ and $BB^*$ systems in both $I=0$ and $I=1$ channels under several types of partially twisted boundary conditions. The use of the partially twisted boundary conditions enables us to obtain the scattering phase shift at any momentum by continuously varying the twisting angle. It also allows us to easily access the $P$-wave scattering phase shifts through the mixing of $S$- and $P$-waves, which is induced by the imposed boundary conditions. The 2+1 flavor PACS-CS gauge ensembles at $m_π=295$, 411 and 569 MeV are used. For charm and bottom quarks, the relativistic heavy quark action is adopted to reduce the lattice discretization artifacts due to the heavy quark mass. We discuss the emergence of a shallow bound state with a binding energy of $O(100)$ keV at the physical pion mass in the $BB^*$ system, which has the quantum number $I(J^P)=0(1^+)$.

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arXiv2025

Triad representation for the anisotropic tensor renormalization group in four dimensions

The development of tensor renormalization group (TRG) algorithm in higher dimensions is an important and urgent task, as the TRG is expected to provide a way to overcome the sign problem in lattice quantum chromodynamics (QCD) calculations at finite density. One possible approach that enables faster computations in four-dimensional lattice theories is the anisotropic tensor renormalization group (ATRG). However, the computational cost remains substantial and requires significant computational resources. In this paper, we propose a novel algorithm, called the triad-ATRG, which is based on the ATRG and other improved TRG variants with triad network representation. This method achieves lower scaling with respect to the bond dimension, while minimizing the loss of accuracy in the free energy and other physical quantities. We also present parallel implementations of both the ATRG and triad-ATRG on multiple GPUs, which significantly improve performance compared to CPU-based calculations for the four-dimensional system.

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arXiv2025

Method for high-precision determination of the nucleon axial structure using lattice QCD: Removing $πN$-state contamination

We performed a precise calculation of physical quantities related to the axial structure of the nucleon using 2+1 flavor lattice QCD gauge configuration (PACS10 configuration) generated at the physical point with lattice volume larger than $(10\;{\mathrm{fm}})^4$ by the PACS Collaboration. The nucleon matrix element of the axial-vector current has two types of the nucleon form factors, the axial-vector ($F_A$) form factor and the induced pseudoscalar ($F_P$) form factor. Recently lattice QCD simulations have succeeded in reproducing the experimental value of the axial-vector coupling, $g_A$, determined from $F_A(q^2)$ at zero momentum transfer $q^2=0$, at a percent level of statistical accuracy. However, the $F_P$ form factor so far has not reproduced the experimental values well due to strong $πN$ excited-state contamination. Therefore, we proposed a simple subtraction method for removing the so-called leading $πN$-state contribution, and succeeded in reproducing the values obtained by two experiments of muon capture on the proton and pion electro-production for $F_P(q^2)$. The novel approach can also be applied to the nucleon pseudoscalar matrix element to determine the pseudoscalar ($G_P$) form factor with the help of the axial Ward-Takahashi identity. The resulting form factors, $F_P(q^2)$ and $G_P(q^2)$, are in good agreement with the prediction of the pion-pole dominance model. In the new analysis, the induced pseudoscalar coupling $g_P^\ast$ and the pion-nucleon coupling $g_{πNN}$ can be evaluated with a few percent accuracy including systematic uncertainties using existing data calculated at two lattice spacings.

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arXiv2025

Glueball mass spectrum at finite temperature revisited: Constant contribution in glueball correlators in the deconfinement phase

We study the glueball properties at finite temperature from the temporal correlation in $SU(3)$ Yang-Mills theory using anisotropic lattice QCD. Although the existence of a constant contribution to the meson correlation function appearing in the deconfinement phase is known, its effect on the glueball correlation function at finite temperature has not been considered in previous studies. The present study reveals that the constant contribution to the glueball correlation function actually occurs in the three lowest-lying glueball states, corresponding to the $0^{++}$, $2^{++}$, and $0^{-+}$ glueballs, from near the critical temperature of the deconfinement phase transition $T_C$ to the high temperature side. If the existence of constant terms is taken into account in the standard pole-mass analysis, it is observed that the pole-mass of the glueball ground state remains unchanged below $T_c$ and then increases with temperature above $T_C$. The result indicates that the true temperature dependence of the glueball mass above $T_C$ is opposite to the results of the previous studies.

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arXiv2025

Investigating the axial structure of the nucleon based on large-volume lattice QCD at the physical point

We present a short summary for the calculations of the nucleon $\textit{isovector}$ form factors, which are relevant to improving the accuracy of the current neutrino oscillation experiments. The calculations are carried out with two of three sets of the $2+1$ flavor lattice QCD configurations generated at the physical point in large spatial volumes by the PACS Collaboration. The two gauge configurations are generated with the six stout-smeared $O(a)$ improved Wilson quark action and Iwasaki gauge action at the lattice spacing of $0.09$ fm and $0.06$ fm. We summarize the results for three form factors as well as the nucleon axial-vector ($g_A$), induced pseudoscalar ($g_P^*$) and pion-nucleon ($g_{πNN}$) couplings. Although our couplings agree with the experimental data, a firm conclusion should be drawn only after a continuum limit extrapolation is taken. We investigate the partially conserved axial-vector current (PCAC) relation in the context of the nucleon correlation functions. The low-energy relations arising from the PCAC relation can be used to verify whether the lattice QCD data correctly reproduce the physics in the continuum within the statistical accuracy. It is demonstrated that our $\textit{new analysis}$ reduces the systematic uncertainty for the induced pseudoscalar and pseudoscalar form factors to a greater extent than the $\textit{traditional analysis}$, and the results offer a theoretical insight into the pion-pole dominance model. Finally, we examine the applicable $q^2$ region for the low-energy relations.

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arXiv2025

A proposal for removing $πN$-state contamination from the nucleon induced pseudoscalar form factor in lattice QCD

In the PACS10 project, the PACS collaboration has generated three sets of the PACS10 gauge configurations at the physical point with lattice volume larger than $(10\;{\rm fm})^4$ and three different lattice spacings. The isovector nucleon form factors had been already calculated by using two sets of the PACS10 gauge configurations. In our strategy, the smearing parameters of the nucleon interpolation operator were highly optimized to eliminate as much as possible the contribution of excited states in the nucleon two-point function. This strategy was quite successful in calculations of the electric ($G_E$), magnetic ($G_M$) and axial-vector ($F_A$) form factors, while the induced pseudoscalar ($F_P$) and pseudoscalar ($G_P$) form factors remained strongly affected by residual contamination of $πN$-state contribution. In this work, we propose a simple method to remove the $πN$-state contamination from the $F_P$ form factor, and then evaluate the induced pseudoscalar charge $g_P^\ast$ and the pion-nucleon coupling $g_{πNN}$ from existing data in a new analysis. Applying this method to the $G_P$ form factor is also considered with a help of the axial Ward-Takahashi identity.

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arXiv2025

Applying the Triad network representation to four-dimensional ATRG method

Anisotropic Tensor Renormalization Group (ATRG) is a powerful algorithm for four-dimensional tensor network calculations. However, the larger bond dimensions are known to be difficult to achieve in practice due to the higher computational cost. Adopting the methods of the minimally decomposed TRG and its triad prescriptions, we construct a triad representation of the four-dimensional ATRG by decomposing the unit-cell tensor. We observe that this combining approach can significantly improve the computational cost even with maintaining the convergence accuracy of the free energy in the four-dimensional Ising model. In addition, we also show that a further improvement can be achieved in terms of the computational cost when our proposed approach is implemented in parallel on GPUs.

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arXiv2025

Studies of nucleon isovector structure with the PACS10 superfine lattice

We present the results for the nucleon axial-vector, induced pseudoscalar and pion-nucleon couplings obtained from 2+1 flavor lattice QCD at the physical point with a large spatial extent of about 10 fm. Our calculations are performed with the PACS10 gauge configurations generated by the PACS Collaboration with the six stout-smeared $O(a)$ improved Wilson-clover quark action and Iwasaki gauge action at $β$ = 1.82, 2.00 and 2.20 corresponding to lattice spacings of 0.09 fm (coarse), 0.06 fm (fine) and 0.04 fm (superfine), respectively. We first evaluate the value of the nucleon axial-vector coupling. In addition, the induced pseudoscalar and pion-nucleon couplings from the induced pseudoscalar form factor are also investigated. Combining the results obtained from the all of our coarse, fine and superfine lattices, we finally discuss the systematic uncertainties in our calculation based on the comparison with both of the experimental values and lattice QCD results provided by the other collaborations.

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arXiv2024

Extraction of the $S$-wave and $P$-wave $DD^*$ scattering phase shifts using twisted boundary conditions

We present results of a lattice study of the $S$-wave and $P$-wave $DD^*$ scattering phase shifts using Lüscher's method under the twisted boundary conditions to investigate the doubly charmed tetraquark $T_{cc}^+$ observed by the LHCb collaboration. Although the scattering phase shift at zero momentum gives information about the number of bound states according to Levinson's theorem, Lüscher's method under the periodic boundary condition only accesses the scattering phase shifts at some discrete momenta and is not suitable for watching the signal of bound state formation. On the other hand, the twisted boundary condition has the advantage that the scattering phase shift at any momentum can be calculated and that not only the $S$-wave scattering phase shift but also the $P$-wave scattering phase shift can be obtained simultaneously. In this study, we perform the simulation for the $DD^*$ and $BB^*$ systems in the $I=0$ channel using 2+1 flavor PACS-CS gauge ensembles simulated at $m_π=295$ and 411 $\mathrm{MeV}$.

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arXiv2024

Nucleon form factors in $N_f=2+1$ lattice QCD at the physical point : finite lattice spacing effect on the root-mean-square radii

We present results for the nucleon form factors: electric ($G_E$), magnetic ($G_M$), axial ($F_A$), induced pseudoscalar ($F_P$) and pseudoscalar ($G_P$) form factors, using the second PACS10 ensemble that is one of three sets of $2+1$ flavor lattice QCD configurations at physical quark masses in large spatial volumes (exceeding $(10\ \mathrm{fm})^3$). The second PACS10 gauge configurations are generated by the PACS Collaboration with the six stout-smeared $O(a)$ improved Wilson quark action and Iwasaki gauge action at the second gauge coupling $β=2.00$ corresponding to the lattice spacing of $a=0.063$ fm. We determine the isovector electric, magnetic and axial radii and magnetic moment from the corresponding form factors, as well as the axial-vector coupling $g_A$. Combining our previous results for the coarser lattice spacing [E. Shintani et al., Phys. Rev. D99 (2019) 014510; Phys. Rev. D102 (2020) 019902 (erattum)], the finite lattice spacing effects on the isovector radii, magnetic moment and axial-vector coupling are investigated using the difference between the two results. It was found that the effect on $g_A$ is kept smaller than the statistical error of 2% while the effect on the isovector radii was observed as a possible discretization error of about 10%, regardless of the channel. We also report the partially conserved axial vector current (PCAC) relation using a set of nucleon three-point correlation functions in order to verify the effect by $O(a)$-improvement of the axial-vector current.

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arXiv2023

Discretization effects on nucleon root-mean-square radii from lattice QCD at the physical point

We present results for the axial-vector coupling and root-mean-square (RMS) radii of the nucleon obtained from 2+1 flavor lattice QCD at the physical point with a large spatial extent of about 10 fm. Our calculations are performed with the PACS10 gauge configurations generated by the PACS Collaboration with the six stout-smeared $O(a)$ improved Wilson-clover quark action and Iwasaki gauge action at $β$ = 1.82 and 2.00 corresponding to lattice spacings of 0.085 fm and 0.063 fm, respectively. We first evaluate the value of the axial-vector coupling of the nucleon ($g_A$). In addition, the isovector electric, magnetic and axial radii and magnetic moment from the corresponding form factors are also determined. Combining the results at $β=1.82$ and $2.00$, we finally discuss the finite lattice spacing effect. It was found that the effect on $g_A$ is kept smaller than the statistical error of 2% while the effect on the isovector radii was observed as a possible discretization error of about 10%, regardless of the channel.

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arXiv2023

Prospects for the stout smearing as an equivalent approach to the Wilson flow

We present the equivalence between the Wilson flow and the stout smearing. The similarity between these two methods was first pointed out by Lüscher's original paper on the Wilson flow. We first show the analytical equivalence of two methods, which indicates that the finite stout smearing parameter induces ${\cal O}(a^2)$ correction. We secondly show that they remain equivalent in numerical simulations within some numerical precision even with finite cutoffs and stout smearing parameters by directly comparing the expectation values of the action density and we shortly mention the use of the equivalence.

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arXiv2023

On the equivalence between the Wilson flow and stout-link smearing

We present the numerical equivalence between the Wilson flow and stout-link smearing, both of which are known to be a relatively new technique for smoothing the gauge fields on the lattice. Although the conceptional correspondence between two methods was first pointed out by Lüscher in his original paper [J. High Energy Phys.~08 (2010) 071], we provide a direct analytical proof of the equivalence between the two methods at finite lattice spacing $a$ in the zero limit of the stout-smearing parameter $ρ$. The leading order corrections start at ${\cal O}(ρ)$, which would induce ${\cal O}(a^2)$ corrections. It is, therefore, not obvious that they remain equivalent even with finite parameters ($a\neq 0$ and $ρ\neq0$) within some numerical precision. In this paper, we demonstrate the equivalence of both methods by directly comparing the expectation value of the action density, which is measured in actual numerical simulations.

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arXiv2023

Glueball spectroscopy in lattice QCD using gradient flow

Removing ultraviolet noise from the gauge fields is necessary for glueball spectroscopy in lattice QCD. It is known that the Yang-Mills gradient flow method is an alternative approach instead of link smearing or link fuzzing in various aspects. In this work we study the application of the gradient flow technique to the construction of the extended glueball operators. We examine a simple application of the gradient flow method, which has some problems in glueball mass calculations at large flow time because of its nature of diffusion in space-time. To avoid this problem, the spatial links are evolved by the ``spatial gradient flow'', that is defined to restrict the diffusion to spatial directions only. We test the spatial gradient flow in calculations of glueball two-point functions and Wilson loops as a new smearing method, and then discuss its efficiency in comparison with the original gradient flow method and the conventional method. Furthermore, to demonstrate the feasibility of our proposed method, we determine the masses of the three lowest-lying glueball states, corresponding to the $0^{++}$, $2^{++}$ and $0^{-+}$ glueballs, in the continuum limit in the pure Yang-Mills theory.

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arXiv2022

Towards the continuum limit of nucleon form factors at the physical point using lattice QCD

We present results for the axial charge and root-mean-square (RMS) radii of the nucleon obtained from 2+1 flavor lattice QCD at the physical point with a large spatial extent of about 10 fm. Our calculations are performed with the PACS10 gauge configurations generated by the PACS Collaboration with the six stout-smeared $O(a)$ improved Wilson-clover quark action and Iwasaki gauge action at $β$ = 1.82 and 2.00 corresponding to lattice spacings of 0.085 fm and 0.063 fm respectively. We first evaluate the value of $g_A/g_V$ , which is not renormalized in the continuum limit and thus ends up with the renormalized axial charge. Moreover, we also calculate the nucleon elastic form factors and determine three kinds of isovector RMS radii such as electric, magnetic and axial ones at the two lattice spacings. We finally discuss the discretization uncertainties on renormalized axial charge and isovector RMS radii towards the continuum limit.

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arXiv2022

Nucleon isovector couplings in Nf = 2 + 1 lattice QCD at the physical point

We present results for the scalar and tensor isovector-couplings ($g_S$ and $g_T$) of the nucleon measured at the physical point ($M_π=135$ MeV) with a single lattice spacing of $0.085\ \mathrm{fm}$ in 2+1 flavor QCD. Our calculations are carried out with two ensembles of gauge configurations generated by the PACS Collaboration with nonperturbatively ${\cal O}(a)$ improved Wilson quark action and Iwasaki gauge action on $(10.9\ {\rm fm})^4$ and $(5.5\ {\rm fm})^4$ lattices, where the finite-size effect on the nucleon mass was not shown at the level of the statistical precision less than 0.5%. We compute the nucleon three-point correlation functions in the vector, axial, scalar, and tensor channels. We confirm that our previous result of the nucleon axial coupling on the large spatial volume of $(10.9\ {\rm fm})^4$ has no finite-size effect at the level of the statistical precision of 1.9%. For the renormalization, we first renormalize $g_S$ and $g_T$ nonperturbatively using the RI/SMOM$_{(γ_μ)}$ scheme, a variant of Rome-Southampton RI/MOM scheme with reduced systematic errors, as the intermediate scheme. We evaluate our final results at the renormalization scale of 2 GeV in the $\overline{\rm MS}$ scheme through matching procedure between the RI/SMOM$_{(γ_μ)}$ and $\overline{\rm MS}$ schemes with the help of perturbation theory, and then obtain $g_S=0.927(71)_{\rm stat}(22)_{\rm syst}$ and $g_T=1.036(6)_{\rm stat}(20)_{\rm syst}$.

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arXiv2022

Nucleon isovector tensor charge from lattice QCD with physical light quarks

We present preliminary results for the axial, scalar and tensor charges of the nucleon measured in 2+1 flavor QCD with the physical light quarks ($m_π=135$ MeV). Our simulations are carried out with gauge configurations generated by the PACS Collaboration with the stout-smeared $O(a)$ improved Wilson fermions and Iwasaki gauge action at a single lattice spacing of $0.085\ (\mathrm{fm})$. There are two lattice ensembles of the PACS gauge configurations, which have physical lattice sizes over $(10\ \mathrm{fm})^4$ and $(5\ \mathrm{fm})^4$, respectively. We compute the nucleon three-point correlation functions in the axial, scalar, and tensor channels. For the renormalization, we use the Rome-Southampton method as the intermediate scheme in order to evaluate the renormalization constants for the scalar and tensor currents in fully nonperturbative manner. We then evaluate the renormalized values of the scalar and tensor charges ($g_S$ and $g_T$) in the $\overline{\rm MS}$ scheme at the renormalization scale of 2 GeV with a help of the continuum perturbation theory for the matching between two schemes. We compare our preliminary results of $g_S$ and $g_T$ with those of other collaboration results.

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