Exploring QCD at small sea quark masses with improved Wilson-type quarks

We explore the region of small sea quark masses below $m_{PS}/m_V=0.5$ in two-flavor QCD using a mean-field improved clover quark action and an RG-improved gauge action at $a \simeq 0.2$ fm on $12^3 \times 24$ and $16^3 \times 24$ lattices. We find that instability of the standard BiCGStab algorithm at small quark masses can be mostly removed by the BiCGStab(DS-$L$) algorithm, which employs $L$-th minimal residual polynomials with a dynamical selection of $L$. We also find singular spikes of $ΔH$ in the HMC algorithm at moderate values of $Δτ$. Nature of the spike is studied. We also study finite-size effects and chiral properties of meson masses.

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