48 research outputs found
Chiral fermions in lattice QCD and random matrix theory
In this thesis I present numerical results from quantum chromodynamics with chiral fermions in the quenched approximation. In particular, the thesis is divided into three topics: 1) We investigated the chiral phase transition in the complex and real sector of the Polyakov loop separately. Despite claims in the literature we have found no dependence of the critical temperature of the chiral phase transition on the Polyakov loop sector. 2) Calorons are supposed to be responsible for the spontaneous breaking of the chiral symmetry. We found evidence for caloron states on the lattice by looking at the localization properties of the low-lying eigenmodes of the Dirac operator. 3) Normal modes represent a specific basis of the probability density of the Dirac eigenvalues in chiral random matrix theory. We have compared numerical data from lattice QCD to predictions of chiral random matrix theory and found good agreement
Lattice simulations with improved Wilson fermions at a fixed strange quark mass
The explicit breaking of chiral symmetry of the Wilson fermion action results
in additive quark mass renormalization. Moreover, flavour singlet and
non-singlet scalar currents acquire different renormalization constants with
respect to continuum regularization schemes. This complicates keeping the
renormalized strange quark mass fixed when varying the light quark mass in
simulations with sea quark flavours. Here we present and validate our
strategy within the CLS (Coordinated Lattice Simulations) effort to achieve
this in simulations with non-perturbatively order- improved Wilson fermions.
We also determine various combinations of renormalization constants and
improvement coefficients.Comment: 18 pages, 11 Figures, V2: References added/updated, all fits rerun
with improved statistics for ensemble N204, also using the final values for
the improvement coefficients A and b_P-b_A (very minor impact), The figures
have been replotted accordingly. (The differences with respect to V1 are
invisible to the human eye). Minor change
Direct determinations of the nucleon and pion terms at nearly physical quark masses
We present a high statistics study of the pion and nucleon light and strange
quark sigma terms using dynamical non-perturbatively improved clover
fermions with a range of pion masses down to MeV and several
volumes, up to , and lattice spacings, fm,
enabling a study of finite volume and discretisation effects for MeV. Systematics are found to be reasonably under control. For the nucleon
we obtain MeV and MeV, or equivalently
in terms of the quark fractions, , and
, where the errors include estimates of both the systematic
and statistical uncertainties. These values, together with perturbative
matching in the heavy quark limit, lead to ,
and . In addition, through the use of the
(inverse) Feynman-Hellmann theorem our results for are shown
to be consistent with the nucleon masses determined in the analysis. For the
pion we implement a method which greatly reduces excited state contamination to
the scalar matrix elements from states travelling across the temporal boundary.
This enables us to demonstrate the Gell-Mann-Oakes-Renner expectation
over our range of pion masses.Comment: 31 pages, 18 figures, v2, small changes to text and figure
Thermal mass and dispersion relations of quarks in the deconfined phase of quenched QCD
Temporal quark correlation functions are analyzed in quenched lattice QCD for
two values of temperature above the critical temperature (Tc) for
deconfinement, T=1.5Tc and 3Tc. A two-pole ansatz for the quark spectral
function is used to determine the bare quark mass and the momentum dependence
of excitation spectra on large lattices of size up to 128^3x16. The dependence
of the quark correlator on these parameters as well as the finite volume
dependence of the excitation energies are analyzed in detail in order to
examine the reliability of our analysis. Our results suggest the existence of
quasi-particle peaks in the quark spectrum. We furthermore find evidence that
the dispersion relation of the plasmino mode has a minimum at non-zero momentum
even in the non-perturbative region near Tc. We also elaborate on the
enhancement of the quark correlator near the chiral limit which is observed at
T=1.5Tc on about half of the gauge configurations. We attribute this to the
presence of near zero-modes of the fermion matrix that are associated with
non-trivial topology of the gauge configurations.Comment: 12pages, 7 figure
Leading order mesonic and baryonic SU(3) low energy constants from lattice QCD
We determine the leading order mesonic~( and ) and baryonic~(,
and ) SU(3) chiral perturbation theory low energy constants from lattice
QCD. We employ gauge ensembles with (i.e., )
non-perturbatively improved Wilson fermions at six distinct values of the
lattice spacing in the range fm, which constitute a
subset of the Coordinated Lattice Simulations (CLS) gauge ensembles. The
pseudoscalar meson mass ranges from around MeV down to MeV
and the linear spatial lattice extent from to
, where for the majority of the ensembles.
This allows us to perform a controlled extrapolation of all the low energy
constants to the chiral, infinite volume and continuum limits. We find the
SU(3) chiral condensate and to be smaller than their SU(2) counterparts
while the Gell-Mann--Oakes--Renner parameters are similar.
Regarding baryonic LECs, we obtain .Comment: 17 pages, 12 figures, minor typos corrected, references added, 2
figures update