313 research outputs found
Spectral density of the Dirac operator in two-flavour QCD
We compute the spectral density of the (Hermitean) Dirac operator in Quantum
Chromodynamics with two light degenerate quarks near the origin. We use
CLS/ALPHA lattices generated with two flavours of O(a)-improved Wilson fermions
corresponding to pseudoscalar meson masses down to 190 MeV, and with spacings
in the range 0.05-0.08 fm. Thanks to the coverage of parameter space, we can
extrapolate our data to the chiral and continuum limits with confidence. The
results show that the spectral density at the origin is non-zero because the
low modes of the Dirac operator do condense as expected in the Banks-Casher
mechanism. Within errors, the spectral density turns out to be a constant
function up to eigenvalues of approximately 80 MeV. Its value agrees with the
one extracted from the Gell-Mann-Oakes-Renner relation
Chiral symmetry breaking in QCD Lite
A distinctive feature of the presence of spontaneous chiral symmetry breaking
in QCD is the condensation of low modes of the Dirac operator near the origin.
The rate of condensation must be equal to the slope of (Mpi^2 Fpi^2)/2 with
respect to the quark mass m in the chiral limit, where Mpi and Fpi are the mass
and the decay constant of the Nambu-Goldstone bosons. We compute the spectral
density of the (Hermitian) Dirac operator, the quark mass, the pseudoscalar
meson mass and decay constant by numerical simulations of lattice QCD with two
light degenerate Wilson quarks. We use CLS lattices at three values of the
lattice spacing in the range 0.05-0.08 fm, and for several quark masses
corresponding to pseudoscalar mesons masses down to 190 MeV. Thanks to this
coverage of parameters space, we can extrapolate all quantities to the chiral
and continuum limits with confidence. The results show that the low quark modes
do condense in the continuum as expected by the Banks-Casher mechanism, and the
rate of condensation agrees with the Gell-Mann-Oakes-Renner (GMOR) relation.
For the renormalisation-group-invariant ratios we obtain [\Sigma^RGI]^(1/3)/F
=2.77(2)(4) and Lambda^MSbar/F = 3.6(2), which correspond to [\Sigma^\MSbar(2
GeV)]^(1/3) =263(3)(4) MeV and F=85.8(7)(20) MeV if FK is used to set the scale
by supplementing the theory with a quenched strange quark.Comment: 4 pages, 3 figures, 1 tabl
Excited light and strange hadrons from the lattice with two Chirally Improved quarks
Results for excited light and strange hadrons from the lattice with two
flavors of Chirally Improved sea quarks are presented. We perform simulations
at several values of the pion mass ranging from 250 to 600 MeV and extrapolate
to the physical pion mass. The variational method is applied to extract excited
energy levels but also to discuss the content of the states. Among others, we
explore the flavor singlet/octet content of Lambda states. In general, our
results agree well with experiment, in particular we confirm the Lambda(1405)
and its dominant flavor singlet structure.Comment: Contribution to the XV International Conference on Hadron
Spectroscopy "Hadron 2013", 4-8 November 2013, Nara, Japa
Meson and baryon spectrum for QCD with two light dynamical quarks
We present results of meson and baryon spectroscopy using the Chirally
Improved Dirac operator on lattices of size 16**3 x 32 with two mass-degenerate
light sea quarks. Three ensembles with pion masses of 322(5), 470(4) and 525(7)
MeV and lattice spacings close to 0.15 fm are investigated. Results on ground
and excited states for several channels are given, including spin two mesons
and hadrons with strange valence quarks. The analysis of the states is done
with the variational method, including two kinds of Gaussian sources and
derivative sources. We obtain several ground states fairly precisely and find
radial excitations in various channels. Excited baryon results seem to suffer
from finite size effects, in particular at small pion masses. We discuss the
possible appearance of scattering states in various channels, considering
masses and eigenvectors. Partially quenched results in the scalar channel
suggest the presence of a 2-particle state, however, in most channels we cannot
identify them. Where available, we compare our results to results of quenched
simulations using the same action.Comment: 27 pages, 29 figures, 11 table
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