Study of Lattice QCD by numerical simulations including dynamical quark effects

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Including dynamical quark loop effects in lattice QCD simulations is one of the most important problem to make quantitative study of strong interaction and to obtain phenomenologically important hadron matrix elements. In fact it has been shown that in the quenched approximation where the dynamical quark loop effect is neglected the evaluated hadron mass spectrums deviate from experimental values by about 10%. For N_f = 2 QCD where the dynamical u and d quark effects are taken into account we have made large scale simulations with the exact hybrid Monte Carlo (HMC) simulation algorithm. It has been shown that a large part of the deviation in hadron mass spectrum observed in the quenched approximation disappears with the inclusion of N_f = 2 dynamical quark effect. In the real world, however, there are three light quarks and it is important to make simulations including the s quark loop effect. For this purpose we have developed efficient exact simulation algorithm for odd number of O(a) improved Wilson fermions. We have made first realistic N_f = 3 QCD simulation with O(a) improved Wilson fermions and estimated mass spectrum for various hadrons, from which fundamental lattice QCD parameters such as lattice spacing and quark masses are determined.