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Shoichi Sasaki

Publications and source records attributed to Shoichi Sasaki.

At least 73 records · Page 4Linked to original sources
arXiv2005

Excited Baryon Spectroscopy from Lattice QCD: Finite Size Effect and Hyperfine Mass Splitting

A study of the finite-size effect is carried out for spectra of both ground-state and excited-state baryons in quenched lattice QCD using Wilson fermions. Our simulations are performed at beta=6.2 with three different spatial sizes, La~1.6, 2.2 and 3.2 fm. It is found that the physical lattice size larger than 3 fm is required for delta states in all spin-parity (J^{P}=1/2^{+/-}, 3/2^{+/-}) channels and also negative-parity nucleon state (J^{P}=1/2^{-}) even in the rather heavy quark mass region. We also find a peculiar behavior of the finite-size effect on the hyperfine mass splittings between the nucleon and the delta in both parity channels.

hep-lat↗
arXiv2005

Strange Tribaryons as Nona-quark States

Strange tribaryons as nona-quark (9 quark) states is studied to describe the S=-1 resonance S^0(3115) recently discovered in the reaction K- + ^4 He -> S^0 + p. We have identified S^0(3115) as a member of the flavor 27-plet, in particular, (F_{flavor},I_{isospin},J_{spin})=(27,1,1/2) or (27,1,3/2). The color-magnetic interaction between quarks favors small multiplets in flavor and spin, which leads to a natural explanation that I=1 is the lowest state among the S=-1 tribaryons with J=1/2. Classification of the S^+ -state recently reported as well as possible locations of other light strange tribaryons such as (10*,0,3/2) with S=-1, (8,1/2,1/2) with S=-2 and (1,0,3/2) with S=-3 are also discussed.

hep-ph↗
arXiv2004

Lattice study of exotic S=+1 baryon

We propose S=+1 baryon interpolating operators, which are based on an exotic description of the antidecuplet baryon like diquark-diqaurk-antiquark. By using one of the new operators, the mass spectrum of the spin-1/2 pentaquark states is calculated in quenched lattice QCD at $β=6/g^2=6.2$ on a $32^3\times48$ lattice. It is found that the $J^P$ assignment of the lowest $Θ(uudd \bar s)$ state is most likely $(1/2)^-$. We also calculate the mass of the charm analog of the $Θ$ and find that the $Θ_c(uudd \bar c)$ state lies much higher than the $DN$ threshold, in contrast to several model predictions.

hep-lat↗
arXiv2003

Nucleon axial charge from quenched lattice QCD with domain wall fermions

We present a quenched lattice calculation of the nucleon isovector vector and axial-vector charges gV and gA. The chiral symmetry of domain wall fermions makes the calculation of the nucleon axial charge particularly easy since the Ward-Takahashi identity requires the vector and axial-vector currents to have the same renormalization, up to lattice spacing errors of order O(a^2). The DBW2 gauge action provides enhancement of the good chiral symmetry properties of domain wall fermions at larger lattice spacing than the conventional Wilson gauge action. Taking advantage of these methods and performing a high statistics simulation, we find a significant finite volume effect between the nucleon axial charges calculated on lattices with (1.2 fm)^3 and (2.4 fm)^3 volumes (with lattice spacing, a, of about 0.15 fm). On the large volume we find gA = 1.212 +/- 0.027(statistical error) +/- 0.024(normalization error). The quoted systematic error is the dominant (known) one, corresponding to current renormalization. We discuss other possible remaining sources of error. This theoretical first principles calculation, which does not yet include isospin breaking effects, yields a value of gA only a little bit below the experimental one, 1.2670 +/- 0.0030.

hep-lat↗
arXiv2003

Latest results from lattice QCD for the Roper resonance

The present status of the Roper resonance in lattice QCD is reviewed. Some of the latest lattice results are discussed with particular emphasis on a large systematic error stemming from the finite size effect. These results suggest that the Roper resonance can be described by the simple three quark excitation of sizable extent.

nucl-th↗
arXiv2002

A Lattice Study of the Nucleon Excited States with Domain Wall Fermions

We present results of our numerical calculation of the mass spectrum for isospin one-half and spin one-half non-strange baryons, i.e. the ground and excited states of the nucleon, in quenched lattice QCD. We use a new lattice discretization scheme for fermions, domain wall fermions, which possess almost exact chiral symmetry at non-zero lattice spacing. We make a systematic investigation of the negative-parity $N^*$ spectrum by using two distinct interpolating operators at $β=6/g^2=6.0$ on a $16^3 \times 32 \times 16$ lattice. The mass estimates extracted from the two operators are consistent with each other. The observed large mass splitting between this state, $N^*(1535)$, and the positive-parity ground state, the nucleon N(939), is well reproduced by our calculations. We have also calculated the mass of the first positive-parity excited state and found that it is heavier than the negative-parity excited state for the quark masses studied.

hep-lat↗
arXiv2001

Nucleon axial charge from quenched lattice QCD with domain wall fermions and improved gauge action

In our previous DWF calculation with the Wilson gauge action at $β=6.0$ ($a^{-1}\simeq$ 1.9 GeV) on a $16^3 \times 32 \times 16$ lattice, we found that $\Ga$ had a fairly strong dependence on the quark mass. A simple linear extrapolation of $\Ga$ to the chiral limit yielded a value that was almost a factor of two smaller than the experimental one. Here we report our recent study of this issue. In particular, we investigate possible errors arising from finite lattice volume, especially in the lighter quark mass region. We employ a RG-improved gauge action (DBW2), which maintains very good chiral behavior even on a coarse lattice ($a^{-1}\simeq$ 1.3 GeV), in order to perform simulations at large physical volume ($> (2{\rm fm})^3$). Our preliminary results suggest that the finite volume effect is significant.

hep-lat↗
arXiv2001

Lattice study of nucleon properties with domain wall fermions

Domain wall fermions (DWF) are a new fermion discretization scheme with greatly improved chiral symmetry. Our final goal is to study the nucleon spin structure through lattice simulation using DWF. In this paper, we present our current progress on two topics toward this goal: 1) the mass spectrum of the nucleon excited states and 2) the iso-vector vector and axial charges, $g_{V}$ and $g_{A}$, of the nucleon.

hep-lat↗
arXiv2000

Domain wall fermion calculation of nucleon g_A/g_V

We present a preliminary domain-wall fermion lattice-QCD calculation of isovector vector and axial charges, g_V and g_A, of the nucleon. Since the lattice renormalizations, Z_V and Z_A, of the currents are identical with DWF, the lattice ratio (g_A/g_V)^{lattice} directly yields the continuum value. Indeed Z_V determined from the matrix element of the vector current agrees closely with Z_A from a non-perturbative renormalization study of quark bilinears. We also obtain spin related quantities Delta-q/g_V and delta-q/g_V.

hep-lat↗
arXiv2000

N* Spectrum in Lattice QCD

We investigate the mass of the parity partner $N^* (1/2^{-})$ of the nucleon $N(1/2^{+})$, in lattice QCD using a new lattice discretization scheme for fermions, domain wall fermions (DWF). DWF possess exact chiral symmetry and flavor symmetry, both of which are required for this problem, even at finite lattice spacing. Our calculation reproduces the large mass splitting between those two states, in good agreement with experiment. We also present preliminary results for the mass of the positive-parity excited state $N'(1/2^{+})$.

hep-ph↗
arXiv1999

The parity partner of the nucleon in quenched QCD with domain wall fermions

We present preliminary results for the mass spectrum of the nucleon and its low-lying excited states from quenched lattice QCD using the domain wall fermion method which preserves the chiral symmetry at finite lattice cutoff. Definite mass splitting is observed between the nucleon and its parity partner. This splitting grows with decreasing valence quark mass. We also present preliminary data regarding the first positive-parity excited state.

hep-lat↗
arXiv1999

Topological Aspect of Abelian Projected SU(2) Lattice Gauge Theory

We show that the hypothesis of abelian dominance allows QCD-monopoles to preserve the topological feature of the QCD vacuum within SU(2) lattice gauge theory. An analytical study is made to find the relationship between the topological charge and QCD-monopoles in the lattice formulation. The topological charge is explicitly represented in terms of the monopole current and the abelian component of gauge fields in the abelian dominated system. We numerically examine the relation and demonstrate the abelian dominance in the topological structure by using Monte Carlo simulation.

hep-lat↗
arXiv1998

Lattice Study of $U_{A}(1)$ Anomaly: The Role of QCD-Monopoles

We investigate the role of QCD-monopoles for the $U_{A}(1)$ anomaly in the maximally abelian gauge within the SU(2) lattice gauge theory. The existence of the strong correlation between instantons and QCD-monopoles in the abelian gauge was already shown by both analytic and numerical works including the Monte Carlo simulation. Their interrelation brings us a conjecture that the presence of QCD-monopoles plays a considerable role on the $U_{A}(1)$ anomaly. We find an evidence for our conjecture on a determination of the fermionic zero modes of the Dirac operator in both the ``monopole removed'' gauge configuration and the ``photon removed'' gauge configuration.

hep-lat↗
arXiv1998

A Conclusive Test of Abelian Dominance Hypothesis for Topological Charge in the QCD Vacuum

We study the topological feature in the QCD vacuum based on the hypothesis of abelian dominance. The topological charge $Q_{\rm SU(2)}$ can be explicitly represented in terms of the monopole current in the abelian dominated system. To appreciate its justification, we directly measure the corresponding topological charge $Q_{\rm Mono}$, which is reconstructed only from the monopole current and the abelian component of gauge fields, by using the Monte Carlo simulation on SU(2) lattice. We find that there exists a one-to-one correspondence between $Q_{\rm SU(2)}$ and $Q_{\rm Mono}$ in the maximally abelian gauge. Furthermore, $Q_{\rm Mono}$ is classified by approximately discrete values.

hep-lat↗
arXiv1997

Existence of Chiral-Asymmetric Zero Modes in the Background of QCD-Monopoles

We study topological aspects of the QCD vacuum structure in SU(2) lattice gauge theory with the abelian gauge fixing. The index of the Dirac operator is measured by using the Wilson fermion in the quenched approximation. We find chiral-asymmetric zero modes in background fields dominated by QCD-monopoles without any cooling.

hep-lat↗
arXiv1997

$U_A$(1) Anomaly in Background Fields Dominated by QCD-Monopoles on SU(2) Lattice

We study $U_A$(1) anomaly of non-perturbative QCD in the maximally abelian gauge on SU(2) lattice. The existence of the strong correlation between QCD-monopoles and instantons in the abelian gauge is shown by both analytic and numerical works including lattice simulations. These results bring us a conjecture that the $U_A$(1) symmetry would be explicitly broken in the background fields dominated by QCD-monopoles. We find an evidence for our conjecture by measuring the chiral-asymmetric zero modes of the Dirac operator in the backgrounds of QCD-monopoles.

hep-lat↗
arXiv1996

Finite Temperature SD Equation for Chiral Symmetry Restoration in Dual Ginzburg-Landau Theory

We study the chiral phase transition at $T\neq 0$ in the dual Ginzburg-Landau theory using the Schwinger-Dyson (SD) equation. In order to solve the SD equation at $T\neq 0$, we provide a new ansatz for the quark self-energy in the imaginary-time formalism. The recovery of the chiral symmetry is found at $T_{_{C}}\sim 100 MeV$ with realistic parameters, which are set by reproducing the values of the string tension $\sqrtσ\simeq 0.44 GeV$ and the chiral condensate $< {\bar q}q > \simeq -(250 MeV}^{3}$ at $T=0$.

hep-ph↗