Lattice QCD is a discretized version of Quantum Chromodynamics, the theory of quarks and gluons. In the regime of nuclear matter, the interactions of these basic constituents cannot be calculated analytically, so we have resorted to large scale computer simulations of this highly non-linear theory. These calculations strive to determine the unknown parameters of the Standard Model of particle physics, and ultimately guide our search for the physics which lies beyond. The HPCC Grand Challenges award of resources at the ACL have been essential for the success of this project.
Our results for the average mass of the up and down quarks (ml), and of the strange quark (ms) (two of the nineteen unknown fundamental parameters of the Standard Model), ml ~ 2.5 MeV and ms ~ 60 MeV, have created an excitement in the particle physics community because they are half of previously believed values.
The calculation of the matrix elements of weak interaction Hamiltonian between states involving mesons and baryons are crucial to unraveling a large body of experimental data. Our calculations of decay constants and form-factors will be essential for determining the four fundamental parameters describing the mixing between the various flavors of quarks due to weak interactions. Our new results for the B-parameters for the electromagnetic penguin opertors have lead to significant increase in the estimates of the size of direct CP violation in kaon decays, which will be tested in up coming experiments.
We have developed and optimized lattice QCD codes for the Crays, CM5, T3D and T3E. For example our highly optimized codes attain 100 Mflops per node on the CM5. The stress produced by these codes has consistently helped debug the software and hardware.
The semi-leptonic decay of a D meson into a kaon, electron, and neutrino via
the weak interactions (through the weak W boson) is illustrated below. Strong
interaction corrections to this process (called form factors) have been
calculated and compared to experimental data.