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Register a new userProbing the QCD vacuum with flavour singlet objects: $eta$ on the lattice
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We present a study on the direct determination of the $eta $ mass on the full set of SESAM and T$chi$L QCD vacuum configurations with 2 active flavours of Wilson fermions, at $beta = 5.6$. We observe a definite dependency of the two-loop correlator on the topological charge sector.
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We apply spectral methods to compute the OZI-rule suppressed loop-loop correlators in the pseudoscalar meson flavour singlet channel. Using SESAM configurations (obtained with two degenerate sea quark flavours on 16x16x16x32 lattices at beta = 5.6 with standard Wilson action), we find for the first time clear evidence for mass plateau formation in the eta channel of this theory. As a consequence, we observe a clear signal of a mass gap persistent under chiral extrapolation. This sets the stage for a more realistic two-channel approach, where partially quenched strange quarks would be included, in addition to u, d sea quarks.
We determine the non-perturbatively renormalized axial current for O($a$) improved lattice QCD with Wilson quarks. Our strategy is based on the chirally rotated Schrodinger functional and can be generalized to other finite (ratios of) renormalization constants which are traditionally obtained by imposing continuum chiral Ward identities as normalization conditions. Compared to the latter we achieve an error reduction up to one order of magnitude. Our results have already enabled the setting of the scale for the $N_{rm f}=2+1$ CLS ensembles [1] and are thus an essential ingredient for the recent $alpha_s$ determination by the ALPHA collaboration [2]. In this paper we shortly review the strategy and present our results for both $N_{rm f}=2$ and $N_{rm f}=3$ lattice QCD, where we match the $beta$-values of the CLS gauge configurations. In addition to the axial current renormalization, we also present precise results for the renormalized local vector current.
It has been known for a long time that the large experimental singlet-octet mass gap in the pseudoscalar meson mass spectrum originates from the anomaly of the axial vector current, i.e. from nonperturbative effects and the nontrivial topological structure of the QCD vacuum. In the N_colour -> infinity limit of the theory, this connection elucidates in the famous Witten-Veneziano relation between the eta-mass and the topological susceptibility of the quenched QCD vacuum.While lattice QCD has by now produced impressive high precision results on the flavour nonsinglet hadron spectrum, the determination of the pseudoscalar singlet mesons from direct correlator studies is markedly lagging behind, due to the computational complexity in handling observables that include OZI-rule violating diagrams, like the eta propagator. In this article, we report on some recent progress in dealing with the numerical bottleneck problem.
Overlap fermions implement exact chiral symmetry on the lattice and are thus an appropriate tool for investigating the chiral and topological structure of the QCD vacuum. We study various chiral and topological aspects on Luescher-Weisz-type quenched gauge field configurations using overlap fermions as a probe. Particular emphasis is placed upon the analysis of the spectral density and the localisation properties of the eigenmodes as well as on the local structure of topological charge fluctuations.
We perform a lattice mass analysis in the flavour singlet pseudoscalar channel on the SESAM and TXL full QCD vacuum configurations, with 2 active flavours of dynamical Wilson fermions at beta = 5.6. At our inverse lattice spacing, a^-1 = 2.3 GeV, we retrieve by a chiral extrapolation to the physical light quark masses the value m_eta = 3.7(+8)(-4) m_pi. A crude extrapolation from (N_f = 3) phenomenology would suggest m_eta approx 5.1 m_pi for N_f = 2 QCD. we verify that the mass gap between the singlet state eta and the pi flavour triplt state is due to gauge configurations with non-trivial topology.
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