Research Objectives
To determine the structure of the proton and neutron from the first principle calculation of the fundamental strong interaction theory --- quantum chromodynamics of quarks and gluons.
Computational Approach
The gauge field configurations are generated with Monte Carlo algorithm, and the quark matrices are inverted with conjugate gradient algorithm. Recently, we have developed a Pade-Z2 method to estimate the fermion determinants with high precision. We employ this method in hybrid Monte Carlo updating for the gauge configurations. Our production program is parallelized on T3E, and the data analysis is done on C90.
Accomplishments
We have calculated various form factors of the nucleon. They include the electromagnetic, axial, pseudoscalar, scalar, and tensor form factors. From these form factors, one can deduce the shapes of the densities associated with the probing currents. As shown in the figure, the longest range one (plotted in red) is the pseudoscalar density, which depicts the pion cloud distribution in the nucleon. The next one, plotted in yellow, is the scalar s-bar s density which depicts the k-bar k meson cloud. The green one is the density of the electric charge in the proton, which reflects the vector meson cloud. Finally, the blue one is the axial-vector density which shows the a1 meson cloud distribution.
Significance
These ab initio calculations from the fundamental theory of the strong interaction have revealed nucleon structures which were not achievable using the model approach. In fact, the calculation on the flavor-singlet axial coupling constant has resolved the "proton spin crisis" and made it in agreement with the high energy experiment. Similarly, our calculation on the scalar density and the strangeness content in the nucleon has reconciled a long-standing puzzle associated with the pion nucleon sigma term. Future calculation of the orbital angular momentum and the gluon spin will predict how much the proton spin is divided in its components.
Publications
S.J. Dong, J.-F. Lagae, and K.-F. Liu, 1995. Phys. Rev. Lett. 75:2096.
S.J. Dong, J.-F. Lagae, and K.-F. Liu, 1996. Phys. Rev. D 54:5496.
C. Thron, S. J. Dong, K. F. Liu, and H. P. Ying, N. d. Phys. Rev. D., in press.
URL
http://www.pa.uky.edu/~fu/liu/group.html
Probing current densities deduced from form factors of the nucleon.