Browsing by Author "Alexandru, A"

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    Critical point of Nf =3 QCD from lattice simulations in the canonical ensemble
    (2011) Li, A; Alexandru, A; Liu, KF
    A canonical ensemble algorithm is employed to study the phase diagram of Nf ¼ 3 QCD using lattice simulations. We lock in the desired quark number sector using an exact Fourier transform of the fermion determinant. We scan the phase space below Tc and look for an S-shape structure in the chemical potential, which signals the coexistence phase of a first order phase transition in finite volume. Applying Maxwell construction, we determine the boundaries of the coexistence phase at three temperatures and extrapolate them to locate the critical point. Using an improved gauge action and improved Wilson fermions on lattices with a spatial extent of 1.8 fm and quark masses close to that of the strange, we find the critical point at TE ¼ 0:925ð5ÞTc and baryon chemical potential EB ¼ 2:60ð8ÞTc.
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    Finite density phase transition of QCD with N-f=4 and N-f=2 using canonical ensemble method
    (2010) Li, A; Alexandru, A; Liu, KF; Meng, X
    In a progress toward searching for the QCD critical point, we study the finite density phase transition of N-f = 4 and 2 lattice QCD at finite temperature with the canonical ensemble approach. We develop a winding number expansion method to accurately project out the particle number from the fermion determinant which greatly extends the applicable range of baryon number sectors to make the study feasible. Our lattice simulation was carried out with the clover fermions and improved gauge action. For a given temperature, we calculate the baryon-chemical potential from the canonical approach to look for the mixed phase as a signal for the first-order phase transition. In the case of N-f = 4, we observe an "S-shape'' structure in the chemical potential-density plane due to the surface tension of the mixed phase in a finite volume which is a signal for the first-order phase transition. We use the Maxwell construction to determine the phase boundaries for three temperatures below T-c. The intersecting point of the two extrapolated boundaries turns out to be at the expected first-order transition point at T-c with mu = 0. This serves as a check for our method of identifying the critical point. We also studied the N-f = 2 case, but do not see a signal of the mixed phase for temperature as low as 0.83T(c).
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    Overlap valence on 2 + 1 flavor domain wall fermion configurations with deflation and low-mode substitution
    (2010-12-01) Li, A; Alexandru, A; Chen, Y; Doi, T; Dong, SJ; Draper, T; Gong, M; Hasenfratz, A; Horváth, I; Lee, FX; Liu, KF; Mathur, N; Streuer, T; Zhang, JB
    The overlap fermion propagator is calculated on 2 þ 1 flavor domain-wall fermion gauge configurations on 163 32, 243 64 and 323 64 lattices. With hyper-cubic (HYP) smearing and low eigenmode deflation, it is shown that the inversion of the overlap operator can be expedited by 20 times for the 163 32 lattice and 80 times for the 323 64 lattice. The overhead cost for calculating eigenmodes ranges from 4.5 to 7.9 propagators for the above lattices. Through the study of hyperfine splitting, we found that the Oðm2a2Þ error is small and these dynamical fermion lattices can adequately accommodate quark mass up to the charm quark. A preliminary calculation of the low-energy constant mix which characterizes the discretization error of the pion made up of a pair of sea and valence quarks in this mixedaction approach is carried out via the scalar correlator with periodic and antiperiodic boundary conditions. It is found to be small which shifts a 300 MeV pion mass by 10 to 19 MeVon these sets of lattices. We have studied the signal-to-noise issue of the noise source for the meson and baryon. We introduce a new algorithm with Z3 grid source and low eigenmode substitution to study the many-to-all meson and baryon correlators. It is found to be efficient in reducing errors for the correlators of both mesons and baryons. With 64-point Z3 grid source and low-mode substitution, it can reduce the statistical errors of the light quark (m 200–300 MeV) meson and nucleon correlators by a factor of 3–4 as compared to the point source. The Z3 grid source itself can reduce the errors of the charmonium correlators by a factor of 3.