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Register a new userIs there still any Tc mystery in lattice QCD? Results with physical masses in the continuum limit III
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The present paper concludes our investigations on the QCD cross-over transition temperatures with 2+1 staggered flavours and one-link stout improvement. We extend our previous two studies [Phys. Lett. B643 (2006) 46, JHEP 0906:088 (2009)] by choosing even finer lattices ($N_t$=16) and we work again with physical quark masses. The new results on this broad cross-over are in complete agreement with our earlier ones. We compare our findings with the published results of the hotQCD collaboration. All these results are confronted with the predictions of the Hadron Resonance Gas model and Chiral Perturbation Theory for temperatures below the transition region. Our results can be reproduced by using the physical spectrum in these analytic calculations. The findings of the hotQCD collaboration can be recovered by using a distorted spectrum which takes into account lattice discretization artifacts and heavier than physical quark masses. This analysis provides a simple explanation for the observed discrepancy in the transition temperatures between our and the hotQCD collaborations.
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The equation of state of QCD at vanishing chemical potential as a function of temperature is determined for two sets of lattice spacings. Coarser lattices with temporal extension of N_t=4 and finer lattices of N_t=6 are used. Symanzik improved gauge and stout-link improved staggered fermionic actions are applied. The results are given for physical quark masses both for the light quarks and for the strange quark. Pressure, energy density, entropy density, quark number susceptibilities and the speed of sound are presented.
We compute charm and bottom quark masses in the quenched approximation and in the continuum limit of lattice QCD. We make use of a step scaling method, previously introduced to deal with two scale problems, that allows to take the continuum limit of the lattice data. We determine the RGI quark masses and make the connection to the MSbar scheme. The continuum extrapolation gives us a value m_b^{RGI} = 6.73(16) GeV for the b-quark and m_c^{RGI} = 1.681(36) GeV for the c-quark, corresponding respectively to m_b^{MSbar}(m_b^{MSbar}) = 4.33(10) GeV and m_c^{MSbar}(m_c^{MSbar}) = 1.319(28) GeV. The latter result, in agreement with current estimates, is for us a check of the method. Using our results on the heavy quark masses we compute the mass of the Bc meson, M_{Bc} = 6.46(15) GeV.
We present the first lattice-QCD calculation of the nucleon isovector unpolarized parton distribution functions (PDFs) at the physical-continuum limit using Large-Momentum Effective Theory (LaMET). The lattice results are calculated using ensembles with multiple sea pion masses with the lightest one around 135~MeV, 3 lattice spacings $ain[0.06,0.12]$~fm, and multiple volumes with $M_pi L$ ranging 3.3 to 5.5. We perform a simultaneous chiral-continuum extrapolation to obtain RI/MOM renormalized nucleon matrix elements with various Wilson-link displacements in the continuum limit at physical pion mass. Then, we apply one-loop perturbative matching to the quasi-PDFs to obtain the lightcone PDFs. We find the lattice-spacing dependence to be much larger than the dependence on pion mass and lattice volume for these LaMET matrix elements. Our physical-continuum limit unpolarized isovector nucleon PDFs are found to be consistent with global-PDF results.
We present the first continuum extrapolation of the hyperon octet axial couplings ($g_{Sigma Sigma}$ and $g_{Xi Xi}$) from $N_f=2+1+1$ lattice QCD. These couplings are important parameters in the low-energy effective field theory description of the octet baryons and fundamental to the nonleptonic decays of hyperons and to hyperon-hyperon and hyperon-nucleon scattering with application to neutron stars. We use clover lattice fermion action for the valence quarks with sea quarks coming from configurations of $N_f=2+1+1$ highly improved staggered quarks (HISQ) generated by MILC Collaboration. Our work includes the first calculation of $g_{Sigma Sigma}$ and $g_{Xi Xi}$ directly at the physical pion mass on the lattice, and a full account of systematic uncertainty, including excited-state contamination, finite-volume effects and continuum extrapolation, all addressed for the first time. We find the continuum-limit hyperon coupling constants to be $g_{Sigma Sigma}=0.4455(55)_text{stat}(65)_text{sys}$ and $g_{Xi Xi} =-0.2703(47)_text{stat}(13)_text{sys}$, which correspond to low-energy constants of $D = 0.708(10)_text{stat}(6)_text{sys}$ and $F = 0.438(7)_text{stat}(6)_text{sys}$. The corresponding SU(3) symmetry breaking is 9% which is about a factor of 2 smaller than the earlier lattice estimate.
We present a quenched lattice QCD calculation of the alpha and beta parameters of the proton decay matrix element. The simulation is carried out using the Wilson quark action at three values of the lattice spacing in the range aapprox 0.1-0.064 fm to study the scaling violation effect. We find only mild scaling violation when the lattice scale is determined by the nucleon mass. We obtain in the continuum limit, |alpha(NDR,2GeV)|=0.0090(09)(^{+5}_{-19})GeV^3 and |beta(NDR,2GeV)|=0.0096(09)(^{+6}_{-20})GeV^3 with alpha and beta in a relatively opposite sign, where the first error is statistical and the second is due to the uncertainty in the determination of the physical scale.
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