5,528 research outputs found
An Interesting Fitting of Quark Masses
In this note we show an empirical formula of quark masses, which is found by
implementing a least squares fit. In this formula the measured QCD coupling is
almost a "best fitting coupling".Comment: 5 pages, 2 figure
Chiral Perturbation Theory in Few-Nucleon Systems
The low-energy effective theory of nuclear physics based on chiral symmetry
is reviewed. Topics discussed include the nucleon-nucleon force, few-body
potentials, isospin violation, pion-deuteron scattering, proton-neutron
radiative capture, pion photoproduction on the deuteron, and pion production in
proton-proton collisions.Comment: 15 pages, 2 figures, Latex, aipproc.sty and epsfig, invited talk at
the 6th Conference on the Intersections of Particle and Nuclear Physics, Big
Sky, May 199
A custodial symmetry for Zbb
We show that a subgroup of the custodial symmetry O(3) that protects delta
rho from radiative corrections can also protect the Zbb coupling. This allows
one to build models of electroweak symmetry breaking, such as Higgsless, Little
Higgs or 5D composite Higgs models, that are safe from corrections to Z-> bb.
We show that when this symmetry protects Zbb it cannot simultaneously protect
Ztt and Wtb. Therefore one can expect to measure sizable deviations from the SM
predictions of these couplings at future collider experiments. We also show
under what circumstances Zb_R b_R can receive corrections in the right
direction to explain the anomaly in the LEP/SLD forward-backward asymmetry
A^b_{FB}.Comment: 11 pages. v2: minor correction
On the motion of particles in covariant Horava-Lifshitz gravity and the meaning of the A-field
We studied the low energy motion of particles in the general covariant
version of Horava-Lifshitz gravity proposed by Horava and Melby-Thompson. Using
a scalar field coupled to gravity according to the minimal substitution recipe
proposed by da Silva and taking the geometrical optics limit, we could write an
effective relativistic metric for a general solution. As a result, we
discovered that the equivalence principle is not in general recovered at low
energies, unless the spatial Laplacian of A vanishes. Finally, we analyzed the
motion on the spherical symmetric solution proposed by Horava and
Melby-Thompson, where we could find its effective line element and compute
spin-0 geodesics. Using standard methods we have shown that such an effective
metric cannot reproduce Newton's gravity law even in the weak gravitational
field approximation.Comment: New result about equivalence principle added, improved discussion and
typos corrected. Version to appear in Phys. Lett.
Nuclear Shell Model Calculations with Fundamental Nucleon-Nucleon Interactions
Some fundamental Nucleon-Nucleon interactions and their applications to
finite nuclei are reviewed. Results for the few-body systems and from
Shell-Model calculations are discussed and compared to point out the advantages
and disadvantages of the different Nucleon-Nucleon interactions. The recently
developed Drexel University Shell Model (DUSM) code is mentioned.Comment: 16 pages, 4 figures. To appear in Phys. Rep. 199
Second quantization approach to composite hadron interactions in quark models
Starting from the Fock space representation of hadron bound states in a quark
model, a change of representation is implemented by a unitary transformation
such that the composite hadrons are redescribed by elementary-particle field
operators. Application of the unitary transformation to the microscopic quark
Hamiltonian gives rise to effective hadron-hadron, hadron-quark, and
quark-quark Hamiltonians. An effective baryon Hamiltonian is derived using a
simple quark model. The baryon Hamiltonian is free of the post-prior
discrepancy which usually plagues composite-particle effective interactions.Comment: Correction on titl
Realistic fluids as source for dynamically accreting black holes in a cosmological background
We show that a single imperfect fluid can be used as a source to obtain the
generalized McVittie metric as an exact solution to Einstein's equations. The
mass parameter in this metric varies with time thanks to a mechanism based on
the presence of a temperature gradient. This fully dynamical solution is
interpreted as an accreting black hole in an expanding universe if the metric
asymptotes to Schwarzschild-de Sitter at temporal infinity. We present a simple
but instructive example for the mass function and briefly discuss the structure
of the apparent horizons and the past singularity.Comment: 5 pages, 2 figures. Updated references and minor changes to match the
version accepted for publishing in PR
A Higher-Order Calculation of Scattering in Cut-Off Effective Field Theory
We report a next-to-leading-order (NLO) chiral perturbation theory
calculation of the neutron-proton scattering cross section in the
channel using a cut-off regularization. The inclusion of two-pion exchanges in
the irreducible diagrams -- or potential -- figuring at NLO is found to be
important in enlarging the domain of validity of the effective field theory. We
are able to reproduce the {\it empirical} scattering phase shift up to p=300
MeV -- which is comparable to the cutoff scale involved -- with an agreement
which is superior to results of other effective field theory approaches. We
also discuss the role of the cutoff as a renormalization prescription and the
importance of the explicit pion degree of freedom in scattering process.Comment: Substantial changes made in texts and Fig.2. To appear in Phys. Lett.
The S-Wave Pion-Nucleon Scattering Lengths from Pionic Atoms using Effective Field Theory
The pion-deuteron scattering length is computed to next-to-next-to-leading
order in baryon chiral perturbation theory. A modified power-counting is then
formulated which properly accounts for infrared enhancements engendered by the
large size of the deuteron, as compared to the pion Compton wavelength. We use
the precise experimental value of the real part of the pion-deuteron scattering
length determined from the decay of pionic deuterium, together with constraints
on pion-nucleon scattering lengths from the decay of pionic hydrogen, to
extract the isovector and isoscalar S-wave pion-nucleon scattering lengths, a^-
and a^+, respectively. We find a^-=(0.0918 \pm 0.0013) M_\pi^{-1} and
a^+=(-0.0034 \pm 0.0007) M_\pi^{-1}.Comment: 19 pages LaTeX, 7 eps fig
The Two-Nucleon Potential from Chiral Lagrangians
Chiral symmetry is consistently implemented in the two-nucleon problem at
low-energy through the general effective chiral lagrangian. The potential is
obtained up to a certain order in chiral perturbation theory both in momentum
and coordinate space. Results of a fit to scattering phase shifts and bound
state data are presented, where satisfactory agreement is found for laboratory
energies up to about 100 Mev.Comment: Postscript file; figures available by reques
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