18,124 research outputs found
On the structure of large N cancellations in baryon chiral perturbation theory
We show how to compute loop graphs in heavy baryon chiral perturbation theory
including the full functional dependence on the ratio of the Delta--nucleon
mass difference to the pion mass, while at the same time automatically
incorporating the 1/N cancellations that follow from the large-N spin-flavor
symmetry of baryons in QCD. The one-loop renormalization of the baryon axial
vector current is studied to demonstrate the procedure. A new cancellation is
identified in the one-loop contribution to the baryon axial vector current. We
show that loop corrections to the axial vector currents are exceptionally
sensitive to deviations of the ratios of baryon-pion axial couplings from SU(6)
values
HIV testing and informed consent - ethical considerations
One of the authors (T.J.) was invited by the AIDS Advisory Group to form a widely representative committee to recommend ethical guidelines concerning the extent to which HIV testing should receive informed consent. This paper presents and argues for the recommended guidelines. The question is considered with regard to a number of distinct purposes of HIV testing: the care of a patient; research; blood, tissue and organ donation; and the protection of third parties, including the health care worker. We contend that in each case there is no good reason for the requirement of informed consent to be significantly waived
Spin-Flavor Structure of Large N Baryons
The spin-flavor structure of large N baryons is described in the 1/N
expansion of QCD using quark operators. The complete set of quark operator
identities is obtained, and used to derive an operator reduction rule which
simplifies the 1/N expansion. The operator reduction rule is applied to the
axial currents, masses, magnetic moments and hyperon non-leptonic decay
amplitudes in the limit, to first order in breaking, and
without assuming symmetry. The connection between the Skyrme and quark
representations is discussed. An explicit formula is given for the quark model
operators in terms of the Skyrme model operators to all orders in for
the two flavor case.Comment: 36 pages, 2 eps figures, uses revte
Renormalization of the baryon axial vector current in large-N_c chiral perturbation theory
The baryon axial vector current is computed at one-loop order in heavy baryon
chiral perturbation theory in the large-N_c limit, where N_c is the number of
colors. Loop graphs with octet and decuplet intermediate states cancel to
various orders in N_c as a consequence of the large-N_c spin-flavor symmetry of
QCD baryons. These cancellations are explicitly shown for the general case of
N_f flavors of light quarks. In particular, a new generic cancellation is
identified in the renormalization of the baryon axial vector current at
one-loop order. A comparison with conventional heavy baryon chiral perturbation
theory is performed at the physical values N_c=3, N_f=3.Comment: REVTex4, 29 pages, 2 figures, 6 tables. Equations (32) and (81)
corrected. Some typos fixed. Results and conclusions remain unchange
Large N_c, Constituent Quarks, and N, Delta Charge Radii
We show how one may define baryon constituent quarks in a rigorous manner,
given physical assumptions that hold in the large-N_c limit of QCD. This
constituent picture gives rise to an operator expansion that has been used to
study large-N_c baryon observables; here we apply it to the case of charge
radii of the N and Delta states, using minimal dynamical assumptions. For
example, one finds the relation r_p^2 - r_{Delta^+}^2 = r_n^2 - r_{Delta^0}^2
to be broken only by three-body, O(1/N_c^2) effects for any N_c.Comment: 15 pages, 1 eps figure. Version to appear in Phys. Rev.
Chiral Perturbation Theory for , , and
We use heavy vector meson chiral perturbation theory
to predict differential decay distributions for and in the kinematic region where
(here or ) is much smaller than the
chiral symmetry breaking scale. Using the large number of colors limit we also
predict the rate for in this region (now
). Comparing our prediction with experimental data, we determine
one of the coupling constants in the heavy vector meson chiral Lagrangian.Comment: 14 pages, latex 2e. We include the decay of the tau into the omega,
pi minus and the tau neutrino, and extract a value for the coupling constant
g2, using experimental dat
Electromagnetic Moments of the Baryon Decuplet
We compute the leading contributions to the magnetic dipole and electric
quadrupole moments of the baryon decuplet in chiral perturbation theory. The
measured value for the magnetic moment of the is used to determine
the local counterterm for the magnetic moments. We compare the chiral
perturbation theory predictions for the magnetic moments of the decuplet with
those of the baryon octet and find reasonable agreement with the predictions of
the large-- limit of QCD. The leading contribution to the quadrupole
moment of the and other members of the decuplet comes from one--loop
graphs. The pionic contribution is shown to be proportional to (and so
will not contribute to the quadrupole moment of nuclei), while the
contribution from kaons has both isovector and isoscalar components. The chiral
logarithmic enhancement of both pion and kaon loops has a coefficient that
vanishes in the limit. The third allowed moment, the magnetic octupole,
is shown to be dominated by a local counterterm with corrections arising at two
loops. We briefly mention the strange counterparts of these moments.Comment: Uses harvmac.tex, 15 pages with 3 PostScript figures packed using
uufiles. UCSD/PTH 93-22, QUSTH-93-05, Duke-TH-93-5
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