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Akira Ukawa

Publications and source records attributed to Akira Ukawa.

28 records · Page 2Linked to original sources
arXiv2012

Helium nuclei, deuteron and dineutron in 2+1 flavor lattice QCD

We calculate the binding energies for multi-nucleon bound states with the nuclear mass number less than or equal to 4 in 2+1 flavor QCD at the lattice spacing of a = 0.09 fm employing a relatively heavy quark mass corresponding to m_pi = 0.51 GeV. To distinguish a bound state from attractive scattering states, we investigate the volume dependence of the energy shift between the ground state and the state of free nucleons by changing the spatial extent of the lattice from 2.9 fm to 5.8 fm. We conclude that ^4He, ^3He, deuteron and dineutron are bound at m_pi = 0.51 GeV. We compare their binding energies with those in our quenched studies and also with several previous investigations.

hep-lat↗
arXiv2011

On the phase of quark determinant in lattice QCD with finite chemical potential

We investigate the phase of the quark determinant with finite chemical potential in lattice QCD using both analytic and numerical methods. Applying the winding number expansion and the hopping parameter expansion to the logarithm of the determinant, we show that the absolute value of the phase has an upper bound that grows with the spatial volume but decreases exponentially with an increase in the temporal extent of the lattice. This analytic but approximate result is confirmed with a numerical study in four-flavor QCD in which the phase is calculated exactly. Since the phase is well controlled on lattices with larger time extents, we try the phase reweighting method in a region beyond $μ/T=1$ where the Taylor expansion method cannot be applied. Working in four-flavor QCD, we find a first-order like behavior on a $6^3\times 4$ lattice at $μ/T\approx 0.8$ which was previously observed by Kentucky group with the canonical method. We also show that the winding number expansion has a nice convergence property beyond $μ/T=1$. We expect that this expansion is useful to study the high density region of the QCD phase diagram at low temperatures.

hep-lat↗
arXiv2011

Electromagnetic form factor of pion from N_f=2+1 dynamical flavor QCD

We present a calculation of the electromagnetic form factor of the pion in $N_f=2+1$ flavor lattice QCD. Calculations are made on the PACS-CS gauge field configurations generated using Iwasaki gauge action and Wilson-clover quark action on a $32^3\times64$ lattice volume with the lattice spacing estimated as $a=0.0907(13)$ fm at the physical point. Measurements of the form factor are made using the technique of partially twisted boundary condition to reach small momentum transfer as well as periodic boundary condition with integer momenta. Additional improvements including random wall source techniques and a judicious choice of momenta carried by the incoming and outgoing quarks are employed for error reduction. Analyzing the form factor data for the pion mass at $M_π\approx 411$ MeV and 296 MeV, we find that the NNLO SU(2) chiral perturbation theory fit yields $ =0.441 \pm 0.046 {\rm fm}^2$ for the pion charge radius at the physical pion mass. Albeit the error is quite large, this is consistent with the experimental value of $0.452\pm 0.011 {\rm fm}^2$. Below $M_π\approx 300$ MeV, we find that statistical fluctuations in the pion two- and three-point functions become too large to extract statistically meaningful averages on a $32^3$ spatial volume. We carry out a sample calculation on a $64^4$ lattice with the quark masses close to the physical point, which suggests that form factor calculations at the physical point become feasible by enlarging lattice sizes to $M_πL\approx 4$.

hep-lat↗
arXiv2005

Hadron Spectrum from Lattice QCD

A brief review is given of the lattice QCD calculation of the hadron spectrum. The status of current attempts toward inclusion of dynamical up, down and strange quarks is summarized focusing on our own work. Recent work on the possible existence of pentaquark states are assessed. We touch upon the PACS-CS Project for building our next machine for lattice QCD, and conclude with a near-term physics and machine prospects.

hep-lat↗
arXiv1998

One-loop renormalization factors and mixing coeffecients of bilinear quark operators for improved gluon and quark actions

We calculate one-loop renormalization factors and mixing coefficients of bilinear quark operators for a class of gluon actions with six-link loops and O(a)-improved quark action. The calculation is carried out by evaluating on-shell Green's functions of quarks and gluons in the standard perturbation theory. We find a general trend that finite parts of one-loop coefficients are reduced approximately by a factor two for the renormalization-group improved gluon actions compared with the case of the standard plaquette gluon action.

hep-lat↗
arXiv1998

Perturbative calculation of improvement coefficients to O(g^2a) for bilinear quark operators in lattice QCD

We calculate the O(g^2 a) mixing coefficients of bilinear quark operators in lattice QCD using a standard perturbative evaluation of on-shell Green's functions. Our results for the plaquette gluon action are in agreement with those previously obtained with the Schrödinger functional method. The coefficients are also calculated for a class of improved gluon actions having six-link terms.

hep-lat↗
arXiv1998

Two-dimensional Lattice Gross-Neveu Model with Wilson Fermion Action at Finite Temperature and Chemical Potential

We investigate the phase structure of the two-dimensional lattice Gross-Neveu model formulated with the Wilson fermion action to leading order of 1/N expansion. Structural change of the parity-broken phase under the influence of finite temperature and chemical potential is studied. The connection between the lattice phase structure and the chiral phase transition of the continuum theory is clarified.

hep-lat↗
arXiv1998

Perturbative Renormalization Factors of Bilinear Quark Operators for Improved Gluon and Quark Actions in Lattice QCD

We calculate one-loop renormalization factors of bilinear quark operators for gluon action including six-link loops and $O(a)$-improved quark action in the limit of massless quark. We find that finite parts of one-loop coefficients of renormalization factors diminish monotonically as either of the coefficients $c_1$ or $c_2+c_3$ of the six-link terms are decreased below zero. Detailed numerical results are given, for general values of the clover coefficient, for the tree-level improved gluon action in the Symanzik approach $(c_1=-1/12, c_2=c_3=0)$ and for the choices suggested by Wilson $(c_1=-0.252, c_2=0, c_3=-0.17)$ and by Iwasaki $(c_1=-0.331, c_2=c_3=0$ and $c_1=-0.27, c_2+c_3=-0.04)$ from renormalization-group analyses. Compared with the case of the standard plaquette gluon action, finite parts of one-loop coefficients are reduced by 10--20% for the Symanzik action, and approximately by a factor two for the renormalization-group improved gluon actions.

hep-lat↗
arXiv1996

Structure of Critical Lines in Quenched Lattice QCD with the Wilson Quark Action

The structure of critical lines of vanishing pion mass for the Wilson quark action is examined in quenched lattice QCD. The numerical evidence is presented that critical lines spread into five branches beyond beta=5.6-5.7 at zero temperature. It is also shown that critical lines disappear in the deconfined phase for the case of finite temperatures.

hep-lat↗