304 research outputs found
Improving the Dirac Operator in Lattice QCD
Recently various new concepts for the construction of Dirac operators in
lattice Quantum Chromodynamics (QCD) have been introduced. These operators
satisfy the so-called Ginsparg-Wilson condition (GWC), thus obeying the
Atiyah-Singer index theorem and violating chiral symmetry only in a modest and
local form. Here we present studies in 4-d for SU(3) gauge configurations with
non-trivial topological content. We study the flow of eigenvalues and we
compare the numerical stability and efficiency of a recently suggested chirally
improved operator with that of others in this respect.Comment: Contrib. to Conf. on Comp. Physics, Sept. 2001 (Aachen); 4 pages, 4
figures, (LaTeX style files cpauth.cls, elsart.cls
Renormalization of bilinear quark operators for the chirally improved lattice Dirac operator
We compute non-perturbative renormalization constants of fermionic bilinears
for the chirally improved lattice fermions in the quenched approximation of
QCD. We address finite size effects and the influence of Gribov copies. Our
results are presented in the RI' and MSbar schemes as well as in RGI form and
we discuss relations between the renormalization constants implied by chiral
symmetry. After publication we corrected the numerator of the first coefficient
of \alpha_s^3 in (24) from 3696847 to 3890527, which yields a 0.2% higher value
of the conversion coefficient at \mu=2 GeV.Comment: 24 pages, 7 figures, final version, After publication we corrected
the numerator of the first coefficient of \alpha_s^3 in (24) from 3696847 to
3890527, which yields a 0.2% higher value of the conversion coefficient at
\mu=2 Ge
Staggered versus overlap fermions: a study in the Schwinger model with
We study the scalar condensate and the topological susceptibility for a
continuous range of quark masses in the Schwinger model with
dynamical flavors, using both the overlap and the staggered discretization. At
finite lattice spacing the differences between the two formulations become
rather dramatic near the chiral limit, but they get severely reduced, at the
coupling considered, after a few smearing steps.Comment: 15 pages, 7 figures, v2: 1 ref corrected, minor change
Quenched QCD with fixed-point and chirally improved fermion
In this contribution we present results from quenched QCD simulations with
the parameterized fixed-point (FP) and the chirally improved (CI) Dirac
operator. Both these operators are approximate solutions of the Ginsparg-Wilson
equation and have good chiral properties. We focus our discussion on
observables sensitive to chirality. In particular we explore pion masses down
to 210 MeV in light hadron spectroscopy, quenched chiral logs, the pion decay
constant and the pion scattering length. We discuss finite volume effects,
scaling properties of the FP and CI operators and performance issues in their
numerical implementation.Comment: Lattice2002(chiral), 17 pages, 21 figures, (LaTeX style file
espcrc2.sty and AMS style files
Gauge-ball spectrum of the four-dimensional pure U(1) gauge theory
We investigate the continuum limit of the gauge-ball spectrum in the
four-dimensional pure U(1) lattice gauge theory. In the confinement phase we
identify various states scaling with the correlation length exponent . The square root of the string tension also scales with this
exponent, which agrees with the non-Gaussian fixed point exponent recently
found in the finite size studies of this theory. Possible scenarios for
constructing a non-Gaussian continuum theory with the observed gauge-ball
spectrum are discussed. The state, however, scales with a Gaussian
value . This suggests the existence of a second, Gaussian
continuum limit in the confinement phase and also the presence of a light or
possibly massless scalar in the non-Gaussian continuum theory. In the Coulomb
phase we find evidence for a few gauge-balls, being resonances in multi-photon
channels; they seem to approach the continuum limit with as yet unknown
critical exponents. The maximal value of the renormalized coupling in this
phase is determined and its universality confirmed.Comment: 46 pages, 12 figure
Simple Observables from Fat Link Fermion Actions
A comparison is made of the (quenched) light hadron spectrum and of simple
matrix elements for a hypercubic fermion action (based on a fixed point action)
and the clover action, both using fat links, at a lattice spacing a= 0.18 fm.
Renormalization constants for the naive and improved vector current and the
naive axial current are computed using Ward identities. The renormalization
factors are very close to unity, and the spectroscopy of light hadrons and the
pseudoscalar and vector decay constants agree well with simulations at smaller
lattice spacings (and with experiment).Comment: 22 pages, 12 postscript figures, Revtex plus eps
Quenched spectroscopy with fixed-point and chirally improved fermions
We present results from quenched spectroscopy calculations with the
parametrized fixed-point and the chirally improved Dirac operators. Both these
operators are approximate solutions of the Ginsparg-Wilson equation and have
good chiral properties. This allows us to work at small quark masses and we
explore pseudoscalar-mass to vector-mass ratios down to 0.28. We discuss meson
and baryon masses, their scaling properties, finite volume effects and compare
our results with recent large scale simulations. We find that the size of
quenching artifacts of the masses is strongly correlated with their
experimentally observed widths and that the gauge and hadronic scales are
consistent.Comment: 66 pages, 33 figures. Published version: minor modifications in the
text, references adde
Small, Dense Quark Stars from Perturbative QCD
As a model for nonideal behavior in the equation of state of QCD at high
density, we consider cold quark matter in perturbation theory. To second order
in the strong coupling constant, , the results depend sensitively on
the choice of the renormalization mass scale. Certain choices of this scale
correspond to a strongly first order chiral transition, and generate quark
stars with maximum masses and radii approximately half that of ordinary neutron
stars. At the center of these stars, quarks are essentially massless.Comment: ReVTeX, 5 pages, 3 figure
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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