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K-I. Ishikawa

Publications and source records attributed to K-I. Ishikawa.

At least 73 records · Page 4Linked to original sources
arXiv1999

Pion decay constant for the Kogut-Susskind quark action in quenched lattice QCD

We present a study for the pion decay constant $f_π$ in the quenched approximation to lattice QCD with the Kogut-Susskind (KS) quark action, with the emphasis given to the renormalization problems. Numerical simulations are carried out at the couplings $β= 6.0$ and 6.2 on $32^3\times 64$ and $48^3\times 64$ lattices, respectively. The pion decay constant is evaluated for all KS flavors via gauge invariant and non-invariant axial vector currents with the renormalization constants calculated by both non-perturbative method and perturbation theory. We obtain $f_π= 89(6)$ MeV in the continuum limit as the best value using the partially conserved axial vector current, which requires no renormalization. From a study for the other KS flavors we find that the results obtained with the non-perturbative renormalization constants are well convergent among the KS flavors in the continuum limit, confirming restoration of $\rm SU(4)_A$ flavor symmetry, while perturbative renormalization still leaves an apparent flavor breaking effect even in the continuum limit.

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arXiv1999

$B \to πl\barν$ Form Factors with NRQCD Heavy Quark and Clover Light Quark Actions

We report results on semileptonic $B\toπl\barν$ decay form factors near $q^2_{\rm max}$ using NRQCD heavy quark and clover light quark actions and currents improved through $O(αa)$. An inconsistency with the soft pion relation $f^0(q^2_{\rm max})= f_B/f_π$ found in a previous work is confirmed, and a possible solution with nonperturbative renormalization is discussed. We find that $f^+(q^2)$ is well described by the $B^*$ pole near $q^2_{\rm max}$, and its $1/M_B$ scaling is also consistent with the prediction of the pole dominance model.

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arXiv1999

Nucleon Decay Matrix Elements from Lattice QCD

We present a model-independent calculation of hadron matrix elements for all dimension-six operators associated with baryon number violating processes using lattice QCD. The calculation is performed with the Wilson quark action in the quenched approximation at $β=6/g^2=6.0$ on a $28^2\times 48\times 80$ lattice. Our results cover all the matrix elements required to estimate the partial lifetimes of (proton,neutron)$\to$($π,K,η$) +(${\bar ν},e^+,μ^+$) decay modes. We point out the necessity of disentangling two form factors that contribute to the matrix element; previous calculations did not make the separation, which led to an underestimate of the physical matrix elements. With a correct separation, we find that the matrix elements have values 3-5 times larger than the smallest estimates employed in phenomenological analyses of the nucleon decays, which could give strong constraints on several GUT models. We also find that the values of the matrix elements are comparable with the tree-level predictions of chiral lagrangian.

hep-lat↗
arXiv1999

I=2 Pion Scattering Length with Wilson Fermions

We present results for I=2 pion scattering length with the Wilson fermions in the quenched approximation. The finite size method presented by Lüscher is employed, and calculations are carried out at $β=5.9$, 6.1, and 6.3. In the continuum limit, we obtain a result in reasonable agreement with the experimental value.

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arXiv1999

Pion decay constant in quenched QCD with Kogut-Susskind quarks

We present a non-perturbative calculation for the pion decay constant with quenched Kogut-Susskind quarks. Numerical simulations are carried out at $β= 6.0$ and 6.2 with various operators extending over all flavors. The renormalization correction is applied for each flavor by computing non-perturbative renormalization constants, and it is compared with a perturbative calculation. We also study the behavior of $f_π$ in the continuum limits for both non-perturbative and perturbative calculations. The results in the continuum limit is also discussed.

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arXiv1999

Nucleon decay matrix elements with the Wilson quark action: an update

We present preliminary results of a new lattice computation of hadronic matrix elements of baryon number violating operators which appear in the low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The contribution of irrelevant form factor which has caused an underestimate of the matrix elements in previous studies is subtracted in this calculation. Our results are 2$\sim$4 times larger than the most conservative values often employed in phenomenological analyses of nucleon decay with specific GUT models.

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arXiv1999

$O(α_{s}a)$ matching coefficients for axial vector current and $ΔB$$=$2 operator

We present a calculation of the perturbative matching coefficients including mixing with higher dimensional operators for the temporal component of the heavy-light axial current, $A_{4}$, and the $ΔB=2$ operator, $O_S$. For $A_{4}^{\scriptsize static, NRQCD}$, calculations with various RG-improved gauge actions are peformed. Matching coefficients with NRQCD and heavy-clover actions are also compared.

hep-lat↗
arXiv1999

B meson leptonic decay constant with quenched lattice NRQCD

We present a lattice NRQCD study of the B meson decay constant in the quenched approximation with emphasis given to the scaling behavior. The NRQCD action and the heavy-light axial current we use include all terms of order 1/M and the perturbative $O(α_s a)$ and $O(α_s/M)$ corrections. Using simulations at three value of couplings $β$=5.7, 5.9 and 6.1 on lattices of size $12^3\times 32, 16^3\times 48$ and $24^3\times 64$, we find no significant $a$ dependence in $f_B$ if the $O(α_s a)$ correction is included in the axial current. We obtain $f_B = 167(7)(15)$ MeV, $f_{B_s}= 191(4)(17)(^{+4}_{-0})$ MeV and $f_{B_s}/f_B =1.15(3)(1)(^{+3}_{-0})$, with the first error being statistical, the second systematic, and the third due to uncertainty of strange quark mass, while quenching errors being not included.

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arXiv1999

A lattice NRQCD calculation of the $B^0-\bar{B}^0$ mixing parameter B_B

We present a lattice calculation of the B meson B-parameter B_B using the NRQCD action. The heavy quark mass dependence is explicitly studied over a mass range between m_b and 4m_b with the $O(1/m_Q)$ and $O(1/m_Q^2)$ actions. We find that the ratios of lattice matrix elements $ / ^2$ and $ / ^2$, which contribute to B_B through mixing, have significant $1/m_Q$ dependence while that of the leading operator $ / ^2$ has little $1/m_Q$ effect. The combined result for B_B(m_b) has small but non-zero mass dependence, and the B_B(m_b) becomes smaller by 10% with the 1/m_Q correction compared to the static result. Our result in the quenched approximation at β=5.9 is B_{B_d}(5 GeV) = 0.75(3)(12), where the first error is statistical and the second is a systematic uncertainty.

hep-lat↗
arXiv1999

Non-perturbative determination of quark masses in quenched lattice QCD with the Kogut-Susskind fermion action

We report results of quark masses in quenched lattice QCD with the Kogut-Susskind fermion action, employing the Reguralization Independent scheme (RI) of Martinelli et al. to non-perturbatively evaluate the renormalization factor relating the bare quark mass on the lattice to that in the continuum. Calculations are carried out at β=6.0, 6.2, and 6.4, from which we find $m^{\bar{MS}}_{ud} (2 GeV)= 4.23(29) MeV$ for the average up and down quark mass and, with the $ϕ$ meson mass as input, $m^{\bar{MS}}_{s} (2 GeV)= 129(12) MeV$ for the strange mass in the continuum limit. These values are about 20% larger than those obtained with the one-loop perturbative renormalization factor.

hep-lat↗
arXiv1998

$O(aα_s)$ matching coefficients for the $ΔB$=2 operators in the lattice static theory

We present the perturbative matching coefficient to $O(aα_s)$ which relates the $ΔB$=2 operator in the continuum to that of the lattice static theory, which is important in the accurate extraction of the continuum value of the $B_B$ from lattice simulations. The coefficients are obtained by the one-loop calculations in both of the continuum and lattice theory. We find that two new dimension seven operators appear at the $O(aα_s)$ with the O(1) coefficients. We also discuss the possible cancellation of $O(aα_s)$ correction in the ratio $B_B=<\bar{B}|\Op_L|B>/ ((8/3)(f_{B}M_{B})^2)$ qualitatively.

hep-lat↗
arXiv1998

Preliminary study of $B_B$ parameter using Lattice NRQCD

We present the preliminary result of the calculation of $B_B$ parameter using lattice NRQCD. The calculation is performed on a $16^3 \times 48$ quenched lattice at $β=5.9$ with clover light quark. The use of lattice NRQCD enables us to investigate the $1/m_Q$ correction from the static limit. The observed mass dependence is well described by the vacuum saturation.

hep-lat↗
arXiv1998

Hadron matrix elements for nucleon decay with the Wilson quark action

We report preliminary results of our study of matrix elements of baryon number violating operators which appear in the low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The calculation is performed on a $32^{3}\times80$ lattice at $β=6.1$ using Wilson fermions in the quenched approximation. Our calculation is independent of details of (SUSY-)GUT models and covers all interesting decay modes.

hep-lat↗
arXiv1998

Scaling behavior of $f_{B}$ with NRQCD

We investigate the scaling behavior of the $B$ meson decay constant $f_B$ and $f_{B_{s}}$ at $β$$=$$5.7, 5.9, 6.1$, employing the NRQCD heavy quark action and the clover light quark action. Mixing effect from dimension-4 operator in the heavy-light axial-vector current is studied, and we find that the $a$ dependence of $f_B$ is significantly reduced. Our preliminary result for the decay constants in the quenched approximation is $f_{B}$$=$$162(^{+35}_{-18})$ MeV, $f_{B_{s}}$$=$$190(^{+40}_{-19})$ MeV, and $f_{B_{s}}/f_{B}$$=$$1.18(^{+6}_{-6})$.

hep-lat↗
arXiv1998

Non-perturbative renormalization factors of bilinear quark operators for Kogut-Susskind fermions and light quark masses in quenched QCD

Light quark masses are computed for Kogut-Susskind fermions by evaluating non-perturbatively the renormalization factor for bilinear quark operators. Calculations are carried out in the quenched approximation at β=6.0, 6.2, and 6.4. For the average up and down quark mass we find $m_{\bar MS}(2 GeV)= 4.15(27) MeV$ in the continuum limit, which is significantly larger than $3.51(20) MeV$ ($q^*=1/a$) or $3.40(21) MeV$ ($q^*=π/a$) obtained with the one-loop perturbative renormalization factor.

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arXiv1998

Phase structure of lattice QCD at finite temperature for 2+1 flavors of Kogut-Susskind quarks

We report on a study of the finite-temperature chiral transition on an $N_t=4$ lattice for 2+1 flavors of Kogut-Susskind quarks. We find the point of physical quark masses to lie in the region of crossover, in agreement with results of previous studies. Results of a detailed examination of the $m_{u,d}=m_s$ case indicate vanishing of the screening mass of $σ$ meson at the end point of the first-order transition.

hep-lat↗
arXiv1998

Lattice study of $B\to π$ semileptonic decay using nonrelativistic lattice QCD

We present an exploratory lattice study of $B\toπ$ semileptonic decay form factors using the nonrelativistic lattice QCD for heavy quark with Wilson light quark on a $16^3 \times 32$ quenched lattice at $β=5.8$. The matrix elements are calculated at eight values of heavy quark mass in a range of $1.5-8$ GeV and with three values of light quark mass. The $1/m_B$ corrections to the matrix elements are found to be fairly small except for the spatial component proportional to the $B$ meson momentum. We find that the $q^2$ dependence of the form factor $f^+(q^2)$ near $q_{max}^2$ becomes much stronger for larger heavy quark mass, which may suggest the increase of a pole contribution. We perform a model independent fit of the form factors and study whether the $q^2$ dependence is consistent with the pole contribution. Although the soft pion theorem predicts $f^0(q_{max}^2)=f_B/f_π$ in the chiral limit, we observe a significant violation of this relation.

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