Flavor dependence of quantum chromodynamics

We study the dependence of Quantum Chromodynamics (QCD) on the number of light dynamical quarks. We conduct simulations on N3s × 32 (Ns = 16 or 24) lattices with two and four flavors of dynamical staggered quarks of mass 0.01 and 0.02. Most of our simulations have at least 5000 trajectories, which is significantly longer than those in previous studies by other research groups. We reproduce some of our previous 16 3 x 32 runs using our new supercomputer QCDSP. We complete our four-flavor 16 3 x 32 simulations. We perform simulations with two and four flavors on 24 3 x 32 lattices. We study the dependence of the nucleon mass on the lattice operators. We measure the pion decay constant using two different methods. We also extract the π − N σ-term. We find that, for four flavors, the masses of most hadrons show strong finite size effects, changing by as much as 10%–15% when the lattice spatial volume increases from 16 3 to 24 3 . We find, in contrast, that the finite size effects on two-flavor hadron masses are less drastic: In this case all meson masses change by only a few percent; the nucleon mass changes by 10%. We find only quantitative differences in the ratios of physical quantities for the two-flavor and four-flavor QCD. The nucleon to the rho mass ratio for four flavors is about 13% higher than that for two flavors and this is significant to three standard deviations. In addition, we find that the pion decay constant for two flavors agrees well with the experimental value.