Lattice QCD Study of the σ Meson

We study the sigma meson in 2+1 flavor full quantum chromodynamics (QCD) with sufficiently light u and d quarks. The point-to-point correlator is measured for the a channel in the Asqtad improved staggered fermion formulation. Particular attention is paid to its disconnected diagram and bubble contribution. After chiral extrapolation, we obtain the a mass with 706 +/- 53 MeV, which is close to the observed a meson within error, and is heavier than the pi-pi threshold. The simulations are carried out with MILC 2+1 flavor gauge configurations at lattice spacing of a approximate to 0.15 fm.