15,196 research outputs found
Pi pi scattering lengths at O(p^6) revisited
This article completes a former work where part of the O(p^6) low-energy
constants entering in the pi pi scattering were estimated. Some resonance
contributions were missed in former calculations and slight differences
appeared with respect to our outcome. Here, we provide the full results for all
the contributing O(p^6) couplings. We also perform a reanalysis of the hadronic
inputs used for the estimation (resonance masses, widths...). Their reliability
was checked together with the impact of the input uncertainties on the
determinations of the chiral couplings and the scattering lengths a^I_J. Our
outcome is found in agreement with former works though with slightly larger
errors. However, the effect in the final values of the a^I_J is negligible
after combining them with the other uncertainties. Based on this consistency,
we conclude that the previous scattering length determinations seem to be
rather solid and reliable, with the cO(p^6) low-energy constants quite under
control. Nevertheless, the uncertainties found in the present work point out
the limitation on further improvements unless the precision of the O(p^6)
couplings is properly increased.Comment: 19 pages. Improved treatment of the a0 decay width and update of the
numerical outcomes. Final version published in Phys. Rev. D
(10.1103/PhysRevD.79.096006
Radiative and leptonic decays of the pseudoscalar charmonium state
The radiative and leptonic decays of and are studied. For decay, the second-order
electromagnetic tree-level diagram gives the leading contribution. The decay
rate of is calculated, the prediction is in good
agreement with the experimental data. For \eta_c\to l^+\l^-, both the tree
and loop diagrams are calculated. The analysis shows that the loop contribution
dominates, the contribution of tree diagram with intermediate state can
only modifies the decay rate by less than 1%. The prediction of the branching
ratios of and are very tiny within the standard
model. The smallness of these predictions within the standard model makes the
leptonic decays of sensitive to physics beyond the standard model.
Measurement of the leptonic decay may give information of new physics.Comment: 9 pages, 4 figures, RevTex, small change, version to appear in Phys.
Rev.
Rapid formation of black holes in galaxies: a self-limiting growth mechanism
We present high-quality fluid dynamical simulations of isothermal gas flows
in a rotating barred potential. We show that a large quantity of gas is driven
right into the nucleus of a model galaxy when the potential lacks a central
mass concentration, but the inflow stalls at a nuclear ring in comparison
simulations that include a central massive object. The radius of the nuclear
gas ring increases linearly with the mass of the central object. We argue that
bars drive gas right into the nucleus in the early stages of disk galaxy
formation, where a nuclear star cluster and perhaps a massive black hole could
be created. The process is self-limiting, however, because inflow stalls at a
nuclear ring once the mass of gas and stars in the nucleus exceeds ~1% of the
disk mass, which shuts off rapid growth of the black hole. We briefly discuss
the relevance of these results to the seeding of massive black holes in
galaxies, the merger model for quasar evolution, and the existence of massive
black holes in disk galaxies that lack a significant classical bulge.Comment: 11 pages, 6 figures, accepted to appear in Ap
The global stability of M33: still a puzzle
The inner disc of the local group galaxy M33 appears to be in settled
rotational balance, and near IR images reveal a mild, large-scale, two-arm
spiral pattern with no strong bar. We have constructed N-body models that match
all the extensive observational data on the kinematics and surface density of
stars and gas in the inner part of M33. We find that currently favoured models
are unstable to the formation of a strong bar of semi-major axis 2 < a_B < 3
kpc within 1 Gyr, which changes the dynamical properties of the models to
become inconsistent with the current, apparently well-settled, state. The
formation of a bar is unaffected by how the gas component is modelled, by
increasing the mass of the nuclear star cluster, or by making the dark matter
halo counter-rotate, but it can be prevented by either reducing the
mass-to-light ratio of the stars to Upsilon_V ~ 0.6 or Upsilon_K ~ 0.23 in
solar units or by increasing the random motions of the stars. Also a shorter
and weaker bar results when the halo is rigid and unresponsive. However, all
three near-stable models support multi-arm spirals, and not the observed
large-scale bi-symmetric spiral. A two-arm spiral pattern could perhaps be
tidally induced, but such a model would require an implausibly low mass disc to
avoid a bar and there is no visible culprit. Thus the survival of the current
state of this exceptionally well-studied galaxy is not yet understood. We also
suspect that many other unbarred galaxies present a similar puzzle.Comment: 15 pages, 8 figures, to appear in MNRAS. A nymber of revisions from
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