17,993 research outputs found
Flavor Doubling and the Nature of Asymptopia
We consider the possibility that QCD with N flavors has a useful low-energy
description with 2N flavors. Specifically, we investigate a free theory of 2N
quarks. Although the free theory is U(N)_L X U(N)_R invariant, it admits a
larger U(2N) invariance. However, when the axial anomaly is accounted for in
the effective theory by a 't Hooft interaction, only SU(N)_L X SU(N)_R X U(1)_B
\subset U(2N) survives. There is however a residual discrete symmetry that is
not a symmetry of the QCD lagrangian. This S_2 subgroup of U(2N) has many
interesting properties. For instance, when explicit chiral symmetry breaking
effects are present, S_2 is broken unless \bar\theta=0 or pi. By expressing the
free theory on the light-front, we show that flavor doubling implies several
superconvergence relations in pion-hadron scattering. Implicit in the 2N-flavor
effective theory is a Regge trajectory with vacuum quantum numbers and unit
intercept whose behavior is constrained by S_2. In particular, S_2 implies that
forward pion-hadron scattering becomes purely elastic at high-energies, in good
agreement with experiment.Comment: 26 pages TeX, uses mtexsis.te
Analytical Approach for the Determination of the Luminosity Distance in a Flat Universe with Dark Energy
Recent cosmological observations indicate that the present universe is flat
and dark energy dominated. In such a universe, the calculation of the
luminosity distance, d_L, involve repeated numerical calculations. In this
paper, it is shown that a quite efficient approximate analytical expression,
having very small uncertainties, can be obtained for d_L. The analytical
calculation is shown to be exceedingly efficient, as compared to the
traditional numerical methods and is potentially useful for Monte-Carlo
simulations involving luminosity distances.Comment: 3 pages, 4 figures, Accepted for publication in MNRA
The UV behavior of Gravity at Large N
A first step in the analysis of the renormalizability of gravity at Large N
is carried on. Suitable resummations of planar diagrams give rise to a theory
in which there is only a finite number of primitive superficially divergent
Feynman diagrams. The mechanism is similar to the the one which makes
renormalizable the 3D Gross-Neveu model at large N. Some potential problems in
fulfilling the Slavnov-Taylor and the Zinn-Justin equations are also pointed
out.Comment: 17 pages, 9 figures. To appear on Phys. Rev. D. Two more references,
further technical details and the discussion of the KLT relations at large N
have been include
The fate of cannibalized fundamental-plane ellipticals
Evolution and disruption of galaxies orbiting in the gravitational field of a
larger cluster galaxy are driven by three coupled mechanisms: 1) the heating
due to its time dependent motion in the primary; 2) mass loss due to the tidal
strain field; and 3) orbital decay. Previous work demonstrated that tidal
heating is effective well inside the impulse approximation limit. Not only does
the overall energy increase over previous predictions, but the work is done
deep inside the secondary galaxy, e.g. at or inside the half mass radius in
most cases. Here, these ideas applied to cannibalization of elliptical galaxies
with fundamental-plane parameters. In summary, satellites which can fall to the
center of a cluster giant by dynamical friction are evaporated by internal
heating by the time they reach the center. This suggests that true
merger-produced multiple nuclei giants should be rare. Specifically,
secondaries with mass ratios as small as 1\% on any initial orbit evaporate and
those on eccentric orbits with mass ratios as small as 0.1\% evolve
significantly and nearly evaporate in a galaxian age. Captured satellites with
mass ratios smaller than roughly 1\% have insufficient time to decay to the
center. After many accretion events, the model predicts that the merged system
has a profile similar to that of the original primary with a weak increase in
concentration.Comment: 19 pages, 10 Postscript figures, uses aaspp4.sty. Submitted to
Astrophysical Journa
On Local Dilatation Invariance
The relationship between local Weyl scaling invariant models and local
dilatation invariant actions is critically scrutinized. While actions invariant
under local Weyl scalings can be constructed in a straightforward manner,
actions invariant under local dilatation transformations can only be achieved
in a very restrictive case. The invariant couplings of matter fields to an
Abelian vector field carrying a non-trivial scaling weight can be easily built,
but an invariant Abelian vector kinetic term can only be realized when the
local scale symmetry is spontaneously broken.Comment: 3 page
The Solar Proton Burning Process Revisited In Chiral Perturbation Theory
The proton burning process p + p -> d + e(+) + \nu(e), important for the
stellar evolution of main-sequence stars of mass equal to or less than that of
the Sun, is computed in effective field theory using chiral perturbation
expansion to the next-to-next-to leading chiral order. This represents a
model-independent calculation consistent with low-energy effective theory of
QCD comparable in accuracy to the radiative np capture at thermal energy
previously calculated by first using very accurate two-nucleon wavefunctions
backed up by an effective field theory technique with a finite cut-off. The
result obtained thereby is found to support within theoretical uncertainties
the previous calculation of the same process by Bahcall and his co-workers.Comment: 30 pages, 2 eps files, aaspp4.sty needed, slightly modified, to be
published in Ap.
Static spherically symmetric perfect fluid solutions in theories of gravity
Static spherically symmetric perfect fluid solutions are studied in metric
theories of gravity. We show that pressure and density do not uniquely
determine ie. given a matter distribution and an equation state, one
cannot determine the functional form of . However, we also show that
matching the outside Schwarzschild-de Sitter-metric to the metric inside the
mass distribution leads to additional constraints that severely limit the
allowed fluid configurations.Comment: 5 page
Cosmic Acceleration from Causal Backreaction with Recursive Nonlinearities
We revisit the causal backreaction paradigm, in which the need for Dark
Energy is eliminated via the generation of an apparent cosmic acceleration from
the causal flow of inhomogeneity information coming in towards each observer
from distant structure-forming regions. This second-generation formalism
incorporates "recursive nonlinearities": the process by which
already-established metric perturbations will then act to slow down all future
flows of inhomogeneity information. Here, the long-range effects of causal
backreaction are now damped, weakening its impact for models that were
previously best-fit cosmologies. Nevertheless, we find that causal backreaction
can be recovered as a replacement for Dark Energy via the adoption of larger
values for the dimensionless `strength' of the clustering evolution functions
being modeled -- a change justified by the hierarchical nature of clustering
and virialization in the universe, occurring on multiple cosmic length scales
simultaneously. With this, and with one new model parameter representing the
slowdown of clustering due to astrophysical feedback processes, an alternative
cosmic concordance can once again be achieved for a matter-only universe in
which the apparent acceleration is generated entirely by causal backreaction
effects. One drawback is a new degeneracy which broadens our predicted range
for the observed jerk parameter , thus removing what had
appeared to be a clear signature for distinguishing causal backreaction from
Cosmological Constant CDM. As for the long-term fate of the universe,
incorporating recursive nonlinearities appears to make the possibility of an
`eternal' acceleration due to causal backreaction far less likely; though this
does not take into account gravitational nonlinearities or the large-scale
breakdown of cosmological isotropy, effects not easily modeled within this
formalism.Comment: 53 pages, 7 figures, 3 tables. This paper is an advancement of
previous research on Causal Backreaction; the earlier work is available at
arXiv:1109.4686 and arXiv:1109.515
Effective Field Theory of pure Gravity and the Renormalization Group
The general structure of the renormalization group equations for the low
energy effective field theory formulation of pure gravity is presented. The
solution of these equations takes a particular simple form if the mass scale of
the effective theory is much smaller than the Planck mass (a possibility
compatible with the renormalization of the effective theory). A theory with
just one free renormalized parameter is obtained when contributions suppressed
by inverse powers of the Planck mass are neglected.Comment: latex, 9 pages, minor correction, version publishe
Effective Nonlocal Euclidean Gravity
A nonlocal form of the effective gravitational action could cure the
unboundedness of euclidean gravity with Einstein action. On sub-horizon length
scales the modified gravitational field equations seem compatible with all
present tests of general relativity and post-Newtonian gravity. They induce a
difference in the effective Newton's constant between regions of space with
vanishing or nonvanishing curvature scalar (or Ricci tensor). In cosmology they
may lead to a value for the critical density after inflation. The
simplest model considered here appears to be in conflict with nucleosynthesis,
but generalizations consistent with all cosmological observations seem
conceivable.Comment: 12 pages, LaTe
- âŠ