arXiv subjects

JLQCD Collaboration

Publications and source records attributed to JLQCD Collaboration.

At least 55 records · Page 3Linked to original sources

arXiv2001

Non-trivial phase structure of $N_f=3$ QCD with $O(a)$-improved Wilson fermion at zero temperature

JLQCD collaboration recently started the $N_f=3$ QCD simulations with the $O(a)$-improved Wilson fermion action employing an exact fermion algorithm developed for odd number of quark flavors. It is found that this theory has an unexpected non-trivial phase structure in the $(β,κ)$ plane even at zero temperature. A detailed study is made to understand the nature of the observed phase transitions and to find the way of avoiding untolerably large lattice artifacts associated with the phase transition.

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arXiv2001

An exact algorithm for three-flavor QCD with $O(a)$-improved Wilson fermions

We present an exact dynamical QCD simulation algorithm for the $O(a)$-improved Wilson fermion with odd number of flavors. Our algorithm is an extension of the non-Hermitian polynomials HMC algorithm proposed by Takaishi and de Forcrand previously. In our algorithm, the systematic errors caused by the polynomial approximation of the inverse of Dirac operator is removed by a noisy-Metropolis test. For one flavor quark it is achieved by taking the square root of the correction matrix explicitly. We test our algorithm for the case of $N_f=1+1$ on a moderately large lattice size ($16^3\times48$). The $N_f=2+1$ case is also investigated.

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arXiv2001

B meson B-parameters and the decay constant in two-flavor dynamical QCD

We present a two-flavor dynamical QCD calculation of the B meson B parameters and decay constant. We use NRQCD for heavy quark and the nonperturbatively O(a)-improved Wilson action for light quark at $β$=5.2 on a $20^3\times 48$ lattice. We confirm that the sea quark effect increases the heavy-light decay constant, while estimate of its magnitude depends significantly on the fitting form in the chiral extrapolation. For the B parameters, on the other hand, we do not find a significant sea quark effect. The chiral extrapolation with logarithmic term is examined for both quantities and compared with the prediction of ChPT.

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arXiv2000

Differential decay rate for $B \to πl ν$ semileptonic decays

We present our study on $B \to πl ν$ semileptonic decay form factors with NRQCD action for heavy quark from a quenched lattice QCD simulation at $β$=5.9 on a $16^3\times 48$ lattice. We obtain form factors defined in the context of heavy quark effective theory by Burdman et al. and find that their $1/m_B$ correction is small. The limit of physical heavy and light quark masses can be performed without introducing any model function, and we obtain a prediction for the differential decay rate $dΓ/dq^2$. We also discuss the soft pion limit of the form factors.

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arXiv2000

B^0-\bar{B}^0 mixing with quenched lattice NRQCD

We present our recent results for the B-parameters, which parameterize the ΔB=2 transition amplitudes. Calculations are made in quenched QCD at β=5.7, 5.9, and 6.1, using NRQCD for heavy quark and the $O(a)$-improved action for light quark. The operators are perturbatively renormalized including corrections of O(α_s/am_Q). We examine scaling behavior of the B-parameters in detail, and discuss the systematic uncertainties using scatter of results with different analysis procedures adopted. As a result, we find B_{B_d}(m_b)=0.84(2)(8), B_{B_s}/B_{B_d}=1.017(10)(^{+4}_{-0}) and B_{S_s}(m_b)=0.87(1)(9)(^{+1}_{-0}) in the quenched approximation.

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arXiv2000

Light hadron spectroscopy with two flavors of O(a) improved dynamical quarks

We report on our study of light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Simulations are made with the plaquette gauge action and the non-perturbatively $O(a)$ improved Wilson quark action. We simulate 5 sea qaurk masses corresponding to $m_{PS}/m_{V} \simeq 0.8$--0.6 at $β=5.2$ on $12^3 \times 48$, $16^3 \times 48$ and $20^3 \times 48$ lattices. A comparison with previous calculations in quenched QCD indicates sea quark effects in meson and quark masses.

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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.

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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

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

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.

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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.

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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})$.

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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.

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arXiv1998

$K^{+}\toπ^{+}π^{0}$ Decay Amplitude with the Wilson Quark Action in Quenched Lattice QCD

We present a calculation for the $K^+\toπ^+ π^0 $ decay amplitude using a quenched simulation of lattice QCD with the Wilson quark action at $β=6/g^2=6.1$. The decay amplitude is extracted from the ratio, the $K\toππ$ three-point function divided by either $K$ and $π$ meson two-point functions or $K$ meson two-point function and I=2 $ππ$ four-point function; the two different methods yield consistent results. Finite size effects are examined with calculations made on $24^3\times 64$ and $32^3\times 64$ lattices, and are shown that they are explained by one-loop effects of chiral perturbation theory. The lattice amplitude is converted to the continuum value by employing a one-loop calculation of chiral perturbation theory, yielding a value in agreement with experiment if extrapolated to the chiral limit. We also report on the $K$ meson $B$ parameter $B_K$ obtained from the $K^+\toπ^+π^0$ amplitude using chiral perturbation theory.

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