8,500 research outputs found
Pion Decay Constant, and Chiral Log from Overlap Fermions
We report our calculation of the pion decay constant , the axial
renormalization constant , and the quenched chiral logarithms from the
overlap fermions. The calculation is done on a quenched lattice at
fm using tree level tadpole improved gauge action. The smallest pion
mass we reach is about 280 MeV. The lattice size is about 4 times the Compton
wavelength of the lowest mass pion.Comment: Lattice2001(Hadronic Matrix Elements), 3pages, 5figure
On the Origin of the Dark Gamma-Ray Bursts
The origin of dark bursts - i.e. that have no observed afterglows in X-ray,
optical/NIR and radio ranges - is unclear yet. Different possibilities -
instrumental biases, very high redshifts, extinction in the host galaxies - are
discussed and shown to be important. On the other hand, the dark bursts should
not form a new subgroup of long gamma-ray bursts themselves.Comment: published in Nuovo Ciment
Chiral Properties of Pseudoscalar Mesons on a Quenched Lattice with Overlap Fermions
The chiral properties of the pseudoscalar mesons are studied numerically on a
quenched lattice with the overlap fermion. We elucidate the role of the
zero modes in the meson propagators, particularly that of the pseudoscalar
meson. The non-perturbative renormalization constant is determined from
the axial Ward identity and is found to be almost independent of the quark mass
for the range of quark masses we study; this implies that the error is
small. The pion decay constant, , is calculated from which we
determine the lattice spacing to be 0.148 fm. We look for quenched chiral log
in the pseudoscalar decay constants and the pseudoscalar masses and we find
clear evidence for its presence. The chiral log parameter is
determined to be in the range 0.15 -- 0.4 which is consistent with that
predicted from quenched chiral perturbation theory.Comment: Version accepted for publication by PRD. A few minor typographical
errors have been corrected. 24 pages, 11 figure
Cosmological quintessence accretion onto primordial black holes : conditions for their growth to the supermassive scale
In this work we revisit the growth of small primordial black holes (PBHs)
immersed in a quintessential field and/or radiation to the supermassive black
hole (SMBHs) scale. We show the difficulties of scenarios in which such huge
growth is possible. For that purpose we evaluated analytical solutions of the
differential equations (describing mass evolution) and point out the strong
fine tuning for that conclusions. The timescale for growth in a model with a
constant quintessence flux is calculated and we show that it is much bigger
than the Hubble time.The fractional gain of the mass is further evaluated in
other forms, including quintessence and/or radiation. We calculate the
cosmological density due to quintessence necessary to grow BHs to the
supermassive range and show it to be much bigger than one. We also describe the
set of complete equations analyzing the evolution of the BH+quintessence
universe, showing some interesting effects such the quenching of the BH mass
growth due to the evolution of the background energy. Additional constraints
obtained by using the Holographic Bound are also described. The general
equilibrium conditions for evaporating/accreting black holes evolving in a
quintessence/radiation universe are discussed in the Appendix.Comment: 21 pp., 2 Figures, To appear in IJMP
The Negativity of the Overlap-Based Topological Charge Density Correlator in Pure-Glue QCD and the Non-Integrable Nature of its Contact Part
We calculate the lattice two-point function of topological charge density in
pure-glue QCD using the discretization of the operator based on the overlap
Dirac matrix. Utilizing data at three lattice spacings it is shown that the
continuum limit of the correlator complies with the requirement of
non-positivity at non-zero distances. For our choice of the overlap operator
and the Iwasaki gauge action we find that the size of the positive core is ~2a
(with a being the lattice spacing) sufficiently close to the continuum limit.
This result confirms that the overlap-based topological charge density is a
valid local operator over realistic backgrounds contributing to the QCD path
integral, and is important for the consistency of recent results indicating the
existence of a low-dimensional global brane-like topological structure in the
QCD vacuum. We also confirm the divergent short-distance behavior of the
correlator, and the non-integrable nature of the associated contact part.Comment: 13 pages, 5 figure
Uncovering Low-Dimensional Topological Structure in the QCD Vacuum
Recently, we have pointed out that sign-coherent 4-dimensional structures can
not dominate topological charge fluctuations in QCD vacuum at all scales. Here
we show that an enhanced lower-dimensional coherence is possible. In pure SU(3)
lattice gauge theory we find that in a typical equilibrium configuration about
80% of space-time points are covered by two oppositely-charged connected
structures built of elementary 3-dimensional coherent hypercubes. The
hypercubes within the structure are connected through 2-dimensional common
faces. We suggest that this coherence is a manifestation of a low-dimensional
order present in the QCD vacuum. The use of a topological charge density
associated with Ginsparg-Wilson fermions ("chiral smoothing") is crucial for
observing this structure.Comment: 3 pages, 1 figure; Proceedings of the "Confinement V" Conference,
Gargnano, Italy, Sep 10-14, 200
Inherently Global Nature of Topological Charge Fluctuations in QCD
We have recently presented evidence that in configurations dominating the
regularized pure-glue QCD path integral, the topological charge density
constructed from overlap Dirac operator organizes into an ordered space-time
structure. It was pointed out that, among other properties, this structure
exhibits two important features: it is low-dimensional and geometrically
global, i.e. consisting of connected sign-coherent regions with local
dimensions 1<= d < 4, and spreading over arbitrarily large space--time
distances. Here we show that the space-time structure that is responsible for
the origin of topological susceptibility indeed exhibits global behavior. In
particular, we show numerically that topological fluctuations are not saturated
by localized concentrations of most intense topological charge density. To the
contrary, the susceptibility saturates only after the space-time regions with
most intense fields are included, such that geometrically global structure is
already formed. We demonstrate this result both at the fundamental level (full
topological density) and at low energy (effective density). The drastic
mismatch between the point of fluctuation saturation (~ 50% of space-time at
low energy) and that of global structure formation (<4% of space-time at low
energy) indicates that the ordered space-time structure in topological charge
is inherently global and that topological charge fluctuations in QCD cannot be
understood in terms of individual localized pieces. Description in terms of
global brane-like objects should be sought instead.Comment: 10 pages, 3 figures; v2: typos corrected, minor modifications; v3:
misprint in Eqs. (2,3) fixe
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