1,282 research outputs found
Helicity operators for mesons in flight on the lattice
Motivated by the desire to construct meson-meson operators of definite
relative momentum in order to study resonances in lattice QCD, we present a set
of single-meson interpolating fields at non-zero momentum that respect the
reduced symmetry of a cubic lattice in a finite cubic volume. These operators
follow from the subduction of operators of definite helicity into irreducible
representations of the appropriate little groups. We show their effectiveness
in explicit computations where we find that the spectrum of states interpolated
by these operators is close to diagonal in helicity, admitting a description in
terms of single-meson states of identified J^{PC}. The variationally determined
optimal superpositions of the operators for each state give rapid relaxation in
Euclidean time to that state, ideal for the construction of meson-meson
operators and for the evaluation of matrix elements at finite momentum.Comment: 25 pages, 14 figures; v2: minor changes to reflect journal versio
S and D-wave phase shifts in isospin-2 pi pi scattering from lattice QCD
The isospin-2 pi pi system provides a useful testing ground for determining
elastic hadron scattering parameters from finite-volume spectra obtained using
lattice QCD computations. A reliable determination of the excited state
spectrum of two pions in a cubic box follows from variational analysis of
correlator matrices constructed using a large basis of operators. A general
operator construction is presented which respects the symmetries of a
multi-hadron system in flight. This is applied to the case of pi pi and allows
for the determination of the scattering phase-shifts at a large number of
kinematic points, in both S-wave and D-wave, within the elastic region. The
technique is demonstrated with a calculation at a pion mass of 396 MeV, where
the elastic scattering is found to be well described by a scattering length
parameterisation.Comment: Tables of little-group CGCs in ancillary file; v2: minor changes to
reflect published versio
Excited state baryon spectroscopy from lattice QCD
We present a calculation of the Nucleon and Delta excited state spectrum on
dynamical anisotropic clover lattices. A method for operator construction is
introduced that allows for the reliable identification of the continuum spins
of baryon states, overcoming the reduced symmetry of the cubic lattice. Using
this method, we are able to determine a spectrum of single-particle states for
spins up to and including J = 7/2, of both parities, the first time this has
been achieved in a lattice calculation. We find a spectrum of states
identifiable as admixtures of SU(6) x O(3) representations and a counting of
levels that is consistent with the non-relativistic constituent quark
model. This dense spectrum is incompatible with quark-diquark model solutions
to the "missing resonance problem" and shows no signs of parity doubling of
states.Comment: 29 pages, 18 figure
Diquark effects in light baryon correlators from lattice QCD
We study the role of diquarks in light baryons through point to point baryon
correlators. We contrast results from quenched simulations with ones with two
flavors of dynamical overlap fermions. The scalar, pseudoscalar and axial
vector diquarks are combined with light quarks to form color singlets. The
quenched simulation shows large zero mode effects in correlators containing the
scalar and pseudoscalar diquark. The two scalar diquarks created by gamma_5 and
gamma_0gamma_5 lead to different behavior in baryon correlators, showing that
the interaction of diquarks with the third light quark matters: we do not see
an isolated diquark. In our quark mass range, the scalar diquark created by
gamma_5 seems to play a greater role than the others.Comment: 12 pages, 11 figure
Adaptive multigrid algorithm for the lattice Wilson-Dirac operator
We present an adaptive multigrid solver for application to the non-Hermitian
Wilson-Dirac system of QCD. The key components leading to the success of our
proposed algorithm are the use of an adaptive projection onto coarse grids that
preserves the near null space of the system matrix together with a simplified
form of the correction based on the so-called gamma_5-Hermitian symmetry of the
Dirac operator. We demonstrate that the algorithm nearly eliminates critical
slowing down in the chiral limit and that it has weak dependence on the lattice
volume
The intrinsic strangeness and charm of the nucleon using improved staggered fermions
We calculate the intrinsic strangeness of the nucleon, - ,
using the MILC library of improved staggered gauge configurations using the
Asqtad and HISQ actions. Additionally, we present a preliminary calculation of
the intrinsic charm of the nucleon using the HISQ action with dynamical charm.
The calculation is done with a method which incorporates features of both
commonly-used methods, the direct evaluation of the three-point function and
the application of the Feynman- Hellman theorem. We present an improvement on
this method that further reduces the statistical error, and check the result
from this hybrid method against the other two methods and find that they are
consistent. The values for and found here, together with
perturbative results for heavy quarks, show that dark matter scattering through
Higgs-like exchange receives roughly equal contributions from all heavy quark
flavors.Comment: 17 pages, 14 figure
The Sunyaev-Zeldovich Effect from Quasar Feedback
The observed relationship between X-ray luminosity and temperature of the
diffuse intercluster medium clearly shows the effect of nongravitational
heating on the formation of galaxy clusters. Quasar feedback into the
intergalactic medium can potentially be an important source of heating, and can
have significant impact on structure formation. This feedback process is a
source of thermal Sunyaev-Zel'dovich distortions of the cosmic microwave
background. Using a simple one-dimensional Sedov-Taylor model of energy
outflow, we calculate the angular power spectrum of the temperature distortion,
which has an amplitude on the order of one micro-Kelvin. This signal will be at
the noise limit of upcoming arcminute-scale microwave background experiments,
including the Atacama Cosmology Telescope and the South Pole Telescope, but
will be directly detectable with deep exposures by the Atacama Large Millimeter
Array or by stacking many microwave images.Comment: The discussion of detectability is expanded. Matches the ApJ Letters
accepted versio
Excursion Sets and Non-Gaussian Void Statistics
Primordial non-Gaussianity (NG) affects the large scale structure (LSS) of
the universe by leaving an imprint on the distribution of matter at late times.
Much attention has been focused on using the distribution of collapsed objects
(i.e. dark matter halos and the galaxies and galaxy clusters that reside in
them) to probe primordial NG. An equally interesting and complementary probe
however is the abundance of extended underdense regions or voids in the LSS.
The calculation of the abundance of voids using the excursion set formalism in
the presence of primordial NG is subject to the same technical issues as the
one for halos, which were discussed e.g. in arXiv:1005.1203. However, unlike
the excursion set problem for halos which involved random walks in the presence
of one barrier , the void excursion set problem involves two barriers
and . This leads to a new complication introduced by what
is called the "void-in-cloud" effect discussed in the literature, which is
unique to the case of voids. We explore a path integral approach which allows
us to carefully account for all these issues, leading to a rigorous derivation
of the effects of primordial NG on void abundances. The void-in-cloud issue in
particular makes the calculation conceptually rather different from the one for
halos. However, we show that its final effect can be described by a simple yet
accurate approximation. Our final void abundance function is valid on larger
scales than the expressions of other authors, while being broadly in agreement
with those expressions on smaller scales.Comment: 28 pages (18+appendices), 7 figures; v2 -- minor changes in sec 3.2,
version published in PR
- …