292 research outputs found
Meson decay constants from Nf=2 clover fermions
We present recent results for meson decay constants calculated on
configurations with two flavours of O(a)-improved Wilson fermions.
Non-perturbative renormalisation is applied and quark mass dependencies as well
as finite volume and discretisation effects are investigated. In this work we
also present a computation of the coupling of the light vector mesons to the
tensor current using dynamical fermions.Comment: 6 pages, contribution to Lattice2005(Hadron spectrum and quark
masses
A determination of the strange quark mass for unquenched clover fermions using the AWI
Using the O(a) Symanzik improved action an estimate is given for the strange
quark mass for unquenched (nf=2) QCD. The determination is via the axial Ward
identity (AWI) and includes a non-perturbative evaluation of the
renormalisation constant. Numerical results have been obtained at several
lattice spacings, enabling the continuum limit to be taken. Results indicate a
value for the strange quark mass (in the MSbar-scheme at a scale of 2GeV) in
the range 100 - 130MeV.Comment: 6 pages, contribution to Lattice2005(Hadron spectrum and quark
masses), uses PoS.cl
Progress towards a lattice determination of (moments of) nucleon structure functions
Using unimproved and non-perturbatively O(a) improved Wilson fermions,
results are given for the three lowest moments of unpolarised nucleon structure
functions. Renormalisation, chiral extrapolation and the continuum limit of the
matrix elements are briefly discussed. The simulations are performed for both
quenched and two flavours of unquenched fermions. No obvious sign of deviation
from linearity in the chiral extrapolations are found. (This is most clearly
seen in our quenched unimproved data, which extends to lighter quark mass.)
Possible quenching effects also seem to be small. The lowest moment thus
remains too large, so it seems to be necessary to reach smaller quark masses in
numerical simulations.Comment: 3 pages, Latex, 4 figures, Talk presented at
Lattice2001(matrixelement
Baryonic Flux in Quenched and Two-Flavor Dynamical QCD after Abelian projection
We study the distribution of color electric flux of the three-quark system in
quenched and full QCD (with N_f = 2 flavors of dynamical quarks) at zero and
finite temperature. To reduce ultra-violet fluctuations, the calculations are
done in the abelian projected theory fixed to the maximally abelian gauge. In
the confined phase we find clear evidence for a Y--shape flux tube surrounded
and formed by the solenoidal monopole current, in accordance with the dual
superconductor picture of confinement. In the deconfined, high temperature
phase monopoles cease to condense, and the distribution of the color electric
field becomes Coulomb--like.Comment: 21 pages, 15 figures, Latex, published version (4 figures added
Accelerating Hasenbusch's acceleration of Hybrid Monte Carlo
Hasenbusch has proposed splitting the pseudo-fermionic action into two parts,
in order to speed-up Hybrid Monte Carlo simulations of QCD. We have tested a
different splitting, also using clover-improved Wilson fermions. An additional
speed-up between 5 and 20% over the original proposal was achieved in
production runs.Comment: Poster presented by H. Stueben at Lattice2003, meta-data correcte
A Determination of the Lambda Parameter from Full Lattice QCD
We present a determination of the QCD parameter Lambda in the quenched
approximation (n_f=0) and for two flavours (n_f=2) of light dynamical quarks.
The calculations are performed on the lattice using O(a) improved Wilson
fermions and include taking the continuum limit. We find Lambda_{n_f=0} =
259(1)(20) MeV and Lambda_{n_f=2} = 261(17)(26) MeV}, using r_0 = 0.467 fm to
set the scale. Extrapolating our results to five flavours, we obtain for the
running coupling constant at the mass of the Z boson alpha_s(m_Z) =
0.112(1)(2). All numbers refer to the MSbar scheme.Comment: 25 pages, 9 figure
Isospin splittings of meson and baryon masses from three-flavor lattice QCD + QED
Lattice QCD simulations are now reaching a precision where isospin breaking
effects become important. Previously, we have developed a program to
systematically investigate the pattern of flavor symmetry beaking within QCD
and successfully applied it to meson and baryon masses involving up, down and
strange quarks. In this Letter we extend the calculations to QCD + QED and
present our first results on isospin splittings in the pseudoscalar meson and
baryon octets. In particular, we obtain the nucleon mass difference of
M_n-M_p=1.35(18)(8)\,\mbox{MeV} and the electromagnetic contribution to the
pion splitting M_{\pi^+}-M_{\pi^0}=4.60(20)\,\mbox{MeV}. Further we report
first determination of the separation between strong and electromagnetic
contributions in the scheme.Comment: 14 pages, 10 figures, text and figures added, Journal versio
Connected and disconnected quark contributions to hadron spin
By introducing an external spin operator to the fermion action, the quark
spin fractions of hadrons are determined from the linear response of the hadron
energies using the Feynman-Hellmann (FH) theorem. At our SU(3)-flavour
symmetric point, we find that the connected quark spin fractions are
universally in the range 55-70\% for vector mesons and octet and decuplet
baryons. There is an indication that the amount of spin suppression is quite
sensitive to the strength of SU(3) breaking. We also present first preliminary
results applying the FH technique to calculations of quark-line disconnected
contributions to hadronic matrix elements of axial and tensor operators. At the
SU(3)-flavour symmetric point we find a small negative contribution to the
nucleon spin from disconnected quark diagrams, while the corresponding tensor
matrix elements are consistent with zero.Comment: 7 pages, 5 figures, 32nd International Symposium on Lattice Field
Theor
Investigation of the Second Moment of the Nucleon's g1 and g2 Structure Functions in Two-Flavor Lattice QCD
The reduced matrix elements a_2 and d_2 are computed in lattice QCD with
N_f=2 flavors of light dynamical (sea) quarks. For proton and neutron targets
we obtain as our best estimates d_2^(p)=0.004(5) and d_2^(n)=-0.001(3),
respectively, in the MSbar scheme at Q^2=5 GeV^2, while for a_2 we find
a_2^(p)=0.077(12) and a_2^(n)=-0.005(5), where the errors are purely
statistical.Comment: 9 pages, 9 figures, 5 tables, revised version: estimate of systematic
uncertainty in chiral extrapolation, 2 references added, version to appear in
Phys.Rev.
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