2,464 research outputs found
The number of colors in the decays pi^0, eta, eta-prime -> gamma gamma
The decays pi^0, eta, eta-prime -> gamma gamma are investigated up to
next-to-next-to-leading order in the framework of the combined 1/N_c and chiral
expansions. Without mixing of the pseudoscalar mesons the N_c independence of
the pi^0 and eta decay amplitudes is shown to persist at the one-loop level,
although the contribution of the Wess-Zumino-Witten term to the pertinent
vertices is not canceled by the N_c dependent part of a Goldstone-Wilczek term.
The decay amplitude of the singlet field, on the other hand, depends strongly
on N_c and yields under the inclusion of mixing also a strong N_c dependence
for the eta decay. Both the eta and eta-prime decay are suited to confirm the
number of colors to be N_c=3.Comment: 18 pages, 1 figur
Constraints on the variability of quark masses from nuclear binding
Based on recent work on nuclear binding, we update and extend the anthropic
constraints on the light quark masses, with results that are more tightly
constrained than previously obtained. We find that heavy nuclei would fall
apart (because the attractive nuclear central potential becomes too weak) if
the sum of the light quark masses m_u+m_d would exceed their physical values by
64% (at 95% confidence level). We summarize the anthropic constraints that
follow from requiring the existence both of heavy atoms and of hydrogen. With
the additional assumption that the quark Yukawa couplings do not vary, these
constraints provide a remarkably tight anthropic window for the Higgs vacuum
expectation value: 0.39 < v/v_physical < 1.64.Comment: 21 pages, 7 figure
Can one see the number of colors in eta, eta-prime --> pi^+ pi^- gamma?
We investigate the decays eta, eta-prime --> pi^+ pi^- gamma up to
next-to-leading order in the framework of the combined 1/N_c and chiral
expansions. Counter terms of unnatural parity at next-to-leading order with
unknown couplings are important to acommodate the results both to the
experimental decay width and the photon spectrum. The presence of these
coefficients does not allow for a determination of the number of colors from
these decays.Comment: 8 pages, 2 figure
The Reaction in Generalized Chiral Perturbation Theory
The cross section for and the pion
polarizabilities are computed, within generalized chiral perturbation theory,
in the full one loop approximation, {\it i.e.} up to and including order
. The result depends on the parameter defining
the tree level scattering amplitude and on an additional low energy
constant. The latter is shown to be related by an exact sum-rule to the data. The parameter is related to the quark mass ratio
via the expansion of pseudoscalar meson masses. The
generalized one loop amplitude agrees with the
experimental data in the threshold region provided r= {m_s}/{\hat m}\lapprox
10. Higher order corrections are estimated comparing our calculation with the
dispersive approach.Comment: 21 pages (plain latex)+1 fig (using feynman.tex)+ 6 figs (ps file
appended), IPNO/TH 94-0
On the power counting of loop diagrams in general relativity
A class of loop diagrams in general relativity appears to have a behavior
which would upset the utility of the energy expansion for quantum effects. We
show through the study of specific diagrams that cancellations occur which
restore the expected behaviour of the energy expansion. By considering the
power counting in a physical gauge we show that the apparent bad behavior is a
gauge artifact, and that the quantum loops enter with a well behaved energy
expansion.Comment: 29 pages, uses axodraw and epsfig.tex, one small .eps file is
included. The full PostScript version is also available as
http://het.phast.umass.edu/students/kakukk/powercount_hepth.p
Baryon Masses in Chiral Perturbation Theory with Infrared Regularization
The baryon masses are examined in SU(3) chiral perturbation theory to third
order using the recently proposed infrared regularization scheme. Fourth order
is estimated by evaluating the dominant diagram. With this regularization the
magnitude of the loop integrals is reduced so that the convergence of the
series appears to be better than in the heavy baryon approach.Comment: The original third order calculation is supplemented by an estimate
of fourth order using just the dominant diagram. The convergence still
appears to be better than in the heavy baryon approach. To be published in
Phys. Rev. C. 15 pages latex, 2 postscript figure
Quantum power correction to the Newton law
We have found the graviton contribution to the one-loop quantum correction to
the Newton law. This correction results in interaction decreasing with distance
as 1/r^3 and is dominated numerically by the graviton contribution. The
previous calculations of this contribution to the discussed effect are
demonstrated to be incorrect.Comment: 10 pages, 5 figures; numerical error corrected, few references adde
Final state rescattering as a contribution to
We provide a new estimate of the long-distance component to the radiative
transition . Our mechanism involves the soft-scattering of
on-shell hadronic products of nonleptonic decay, as in the chain . We employ a phenomenological fit to scattering data
to estimate the effect. The specific intermediate states considered here modify
the decay rate at roughly the level, although
the underlying effect has the potential to be larger. Contrary to other
mechanisms of long distance physics which have been discussed in the
literature, this yields a non-negligible modification of the channel and hence will provide an uncertainty in the extraction of
. This mechanism also affects the isospin relation between the rates
for and and may generate CP
asymmetries at experimentally observable levels.Comment: 15 pages, RevTex, 3 figure
A sum rule approach to the violation of Dashen's theorem
A classic sum rule by Das et al. is extended to seven of the low-energy
constants , introduced by Urech, which parameterize electromagnetic
corrections at chiral order . Using the spurion formalism, a simple
convolution representation is shown to hold and the structure in terms of the
chiral renormalization scale, the QCD renormalization scale and the QED gauge
parameter is displayed. The role of the resonances is studied as providing
rational interpolants to relevant QCD n-point functions in the euclidian
domain. A variety of asymptotic constraints must be implemented which have
phenomenological consequences. A current assumption concerning the dominance of
the lowest-lying resonances is shown clearly to fail in some cases.Comment: A few corrections and improvements made, the list of references is
more complet
On the extraction of the quark mass ratio (m_d - m_u)/m_s from Gamma(eta' -> pi^0 pi^+ pi^-)/Gamma(eta' -> eta pi^+ pi^-)
The claim that the light quark mass ratio (m_d - m_u)/m_s can be extracted
from the decay width ratio Gamma(eta' -> pi^0 pi^+ pi^-)/Gamma(eta' -> eta pi^+
pi^-) is critically investigated within a U(3) chiral unitary framework. The
influence of the recent VES data on the eta' -> eta pi^+ pi^- decay is also
discussed.Comment: 9 pages, 1 figure, 6 table
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