Emergence of the $rho$ resonance from the HAL QCD potential in lattice QCD

We investigate the $I=1$ $pi pi$ interaction using the HAL QCD method in lattice QCD. We employ the (2+1)-flavor gauge configurations on $32^3 times 64$ lattice at the lattice spacing $a approx 0.0907$ fm and $m_{pi} approx 411$ MeV, in which the $rho$ meson appears as a resonance state. We find that all-to-all propagators necessary in this calculation can be obtained with reasonable precision by a combination of three techniques, the one-end trick, the sequential propagator, and the covariant approximation averaging (CAA). The non-local $I=1$ $pi pi$ potential is determined at the next-to-next-to-leading order (N$^2$LO) of the derivative expansion for the first time, and the resonance parameters of the $rho$ meson are extracted. The obtained $rho$ meson mass is found to be consistent with the value in the literature, while the value of the coupling $g_{rho pi pi}$ turns out to be somewhat larger. The latter observation is most likely attributed to the lack of low-energy information in our lattice setup with the center-of-mass frame. Such a limitation may appear in other P-wave resonant systems and we discuss possible improvement in future. With this caution in mind, we positively conclude that we can reasonably extract the N$^2$LO potential and resonance parameters even in the system requiring the all-to-all propagators in the HAL QCD method, which opens up new possibilities for the study of resonances in lattice QCD.

S-wave kaon-nucleon potentials with all-to-all propagators in the HAL QCD method

In this paper, employing an all-to-all quark propagator technique, we investigate the kaon-nucleon interactions in lattice QCD. We calculate the S-wave kaon-nucleon potentials at the leading order in the derivative expansion in the time-dependent HAL QCD method, using (2+1)-flavor gauge configurations at the lattice spacing $a approx 0.09$ fm on $32^3 times 64$ lattices and the pion mass $m_{pi} approx 570$ MeV. We take the one-end trick for all-to-all propagators, which allows us to put the zero momentum hadron operators at both source and sink and to smear quark operators at the source. We find the stronger repulsive interaction in the $I=1$ channel than in the $I=0$. The phase shifts obtained by solving the Schr{o}dinger equations with the potentials qualitatively reproduce the energy dependence of the experimental phase shifts, and have the similar behavior to the previous results from lattice QCD without all-to-all propagators. Our study demonstrates that the all-to-all quark propagator technique with the one-end trick is useful to study interactions for meson-baryon systems in the HAL QCD method, so that we will apply it to meson-baryon systems which contain quark-antiquark creation/annihilation processes in our future studies.

The HAL QCD potential in $I=1$ $pi pi$ system with the $rho$ meson bound state

In this paper, we investigate the HAL QCD potential in the $I=1$ $pi pi$ scattering using the hybrid method for all-to-all propagators, in which a propagator is approximated by low-eigenmodes and the remaining high-eigenmode part is stochastically estimated. To verify the applicability of the hybrid method to systems containing quark creation$/$annihilation contributions such as the $rho$ meson, we calculate the $I=1$ $pipi$ potential with the 2+1 flavor gauge configurations on $16^3 times 32$ lattice at the lattice spacing $a approx 0.12$ fm and $(m_{pi},m_{rho}) approx (870, 1230)$ MeV, in which the $rho$ meson appears as a deeply-bound state. While we find that the naive stochastic evaluations for quark creation$/$annihilation contributions lead to extremely large statistical fluctuations, additional noise reduction methods enable us to obtain a sufficiently precise potential, which shows a strong attractive force. We also confirm that the binding energy and $k^3 cot delta$ obtained from our potential are roughly consistent with an existing $rho$ meson bound state, within a large systematic error associated with our calculation, whose possible origin is also discussed.

Study of the pion-pion scatterings with a combination of all-to-all propagators and the HAL QCD method

In this paper, we report recent developments of the HAL QCD method for two hadron systems which contain quark annihilation processes using all-to-all quark propagators. We employ the hybrid method for all-to-all propagators, which combines a low-mode spectral decomposition of the quark propagator and stochastic estimators for remaining high modes, to evaluate the HAL QCD potentials for the first time. Using this method, we investigate the $I= 1,2$ $pi pi$ scatterings at $m_{pi} approx 870$ MeV. In the $I=2$ study, we study how statistical fluctuations of the HAL QCD potentials are increased due to stochastic estimators in the hybrid method, compared with the conventional one without them. We find that we can reduce statistical fluctuations by dilutions of stochastic noises in order to obtain sufficiently precise results, which turn out to be consistent with conventional results without all-to-all propagators. In the $I=1$ $pi pi$ case, which contains quark annihilation processes, we find that statistical fluctuations are further enhanced due to noise contaminations in annihilation processes. We, however, confirm that we can also reduce such statistical fluctuations to obtain the potential with a reasonable precision as long as we further increase a degree of dilutions at a price of large numerical costs and take an appropriate scheme for the potential.

Partial wave decomposition on the lattice and its applications to the HAL QCD method

The approximated partial wave decomposition method to the discrete data on a cubic lattice, developed by C. W. Misner, is applied to the calculation of $S$-wave hadron-hadron scatterings by the HAL QCD method in lattice QCD. We consider the Nambu-Bethe-Salpeter (NBS) wave function for the spin-singlet $Lambda_c N$ system calculated in the $(2+1)$-flavor QCD on a $(32a~mathrm{fm})^3$ lattice at the lattice spacing $asimeq0.0907$ fm and $m_pi simeq 700$ MeV. We find that the $l=0$ component can be successfully extracted by Misners method from the NBS wave function projected to $A_1^+$ representation of the cubic group, which contains small $lge 4$ components. Furthermore, while the higher partial wave components are enhanced so as to produce significant comb-like structures in the conventional HAL QCD potential if the Laplacian approximated by the usual second order difference is applied to the NBS wave function, such structures are found to be absent in the potential extracted by Misners method, where the Laplacian can be evaluated analytically for each partial wave component. Despite the difference in the potentials, two methods give almost identical results on the central values and on the magnitude of statistical errors for the fits of the potentials, and consequently on the scattering phase shifts. This indicates not only that Misners method works well in lattice QCD with the HAL QCD method but also that the contaminations from higher partial waves in the study of $S$-wave scatterings are well under control even in the conventional HAL QCD method. It will be of interest to study interactions in higher partial wave channels in the HAL QCD method with Misners decomposition, where the utility of this new technique may become clearer.

$I=2$ $pipi$ potential in the HAL QCD method with all-to-all propagators

In this paper, we perform the first application of the hybrid method (exact low modes plus stochastically estimated high modes) for all-to-all propagators to the HAL QCD method. We calculate the HAL QCD potentials in the $I=2$ $pipi$ scattering in order to see how statistical fluctuations of the potential behave under the hybrid method. All of the calculations are performed with the 2+1 flavor gauge configurations on $16^3 times 32$ lattice at the lattice spacing $a approx 0.12$ fm and $m_{pi} approx 870$ MeV. It is revealed that statistical errors for the potential are enhanced by stochastic noises introduced by the hybrid method, which, however, are shown to be reduced by increasing the level of dilutions, in particular, that of space dilutions. From systematic studies, we obtain a guiding principle for a choice of dilution types/levels and a number of eigenvectors to reduce noise contaminations to the potential while keeping numerical costs reasonable. We also confirm that we can obtain the scattering phase shifts for the $I=2$ $pipi$ system by the hybrid method within a reasonable numerical cost, which are consistent with the result obtained with the conventional method. The knowledge we obtain in this study will become useful to investigate hadron resonances which require quark annihilation diagrams such as the $rho$ meson by the HAL QCD potential with the hybrid method.