5,344 research outputs found
Nonelastic nuclear reactions and accompanying gamma radiation
Several aspects of nonelastic nuclear reactions which proceed through the formation of a compound nucleus are dealt with. The full statistical model and the partial statistical model are described and computer programs based on these models are presented along with operating instructions and input and output for sample problems. A theoretical development of the expression for the reaction cross section for the hybrid case which involves a combination of the continuum aspects of the full statistical model with the discrete level aspects of the partial statistical model is presented. Cross sections for level excitation and gamma production by neutron inelastic scattering from the nuclei Al-27, Fe-56, Si-28, and Pb-208 are calculated and compared with avaliable experimental data
Determining the Quark Mixing Matrix From CP-Violating Asymmetries
If the Standard Model explanation of CP violation is correct, then
measurements of CP-violating asymmetries in meson decays can in principle
determine the entire quark mixing matrix.Comment: 8 pages (plain TeX), 1 figure (postscript file appended), DAPNIA/SPP
94-06, NSF-PT-94-2, UdeM-LPN-TH-94-18
Forward-Backward Asymmetries in Hadronically Produced Lepton Pairs
It has now become possible to observe appreciable numbers of hadronically
produced lepton pairs in mass ranges where the contributions of the photon and
are comparable. Consequently, in the reaction , substantial forward-backward asymmetries can be seen. These
asymmetries provide a test of the electroweak theory in a new regime of
energies, and can serve as diagnostics for any new neutral vector bosons
coupling both to quarks and to charged lepton pairs.Comment: 11 pages, latex, 4 uuencoded figures sent separately, Fig. 2 revise
The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study
We investigate the ground-state magnetic order of the spin-1/2 J1-J2
Heisenberg model on the square lattice with ferromagnetic nearest-neighbor
exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange
J2>0. We use the coupled-cluster method to high orders of approximation and
Lanczos exact diagonalization of finite lattices of up to N=40 sites in order
to calculate the ground-state energy, the spin-spin correlation functions, and
the magnetic order parameter. We find that the transition point at which the
ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact
diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our
results for ferromagnetic J1 with established results for the spin-1/2 J1-J2
Heisenberg model with antiferromagnetic J1. We find that both models (i.e.,
ferro- and antiferromagnetic J1) behave similarly for large J2, although
significant differences between them are observed for J2/|J1| \lesssim 0.6.
Although the semiclassical collinear magnetic long-range order breaks down at
J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar
breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model
with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an
intermediate disordered phase does occur between the phases exhibiting
semiclassical collinear stripe order and ferromagnetic order for ferromagnetic
J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.Comment: 15 pages, 7 figures, 2 table
Large-wavelength instabilities in free-surface Hartmann flow at low magnetic Prandtl numbers
We study the linear stability of the flow of a viscous electrically
conducting capillary fluid on a planar fixed plate in the presence of gravity
and a uniform magnetic field. We first confirm that the Squire transformation
for MHD is compatible with the stress and insulating boundary conditions at the
free surface, but argue that unless the flow is driven at fixed Galilei and
capillary numbers, the critical mode is not necessarily two-dimensional. We
then investigate numerically how a flow-normal magnetic field, and the
associated Hartmann steady state, affect the soft and hard instability modes of
free surface flow, working in the low magnetic Prandtl number regime of
laboratory fluids. Because it is a critical layer instability, the hard mode is
found to exhibit similar behaviour to the even unstable mode in channel
Hartmann flow, in terms of both the weak influence of Pm on its neutral
stability curve, and the dependence of its critical Reynolds number Re_c on the
Hartmann number Ha. In contrast, the structure of the soft mode's growth rate
contours in the (Re, alpha) plane, where alpha is the wavenumber, differs
markedly between problems with small, but nonzero, Pm, and their counterparts
in the inductionless limit. As derived from large wavelength approximations,
and confirmed numerically, the soft mode's critical Reynolds number grows
exponentially with Ha in inductionless problems. However, when Pm is nonzero
the Lorentz force originating from the steady state current leads to a
modification of Re_c(Ha) to either a sublinearly increasing, or decreasing
function of Ha, respectively for problems with insulating and conducting walls.
In the former, we also observe pairs of Alfven waves, the upstream propagating
wave undergoing an instability at large Alfven numbers.Comment: 58 pages, 16 figure
Differential cross section analysis in kaon photoproduction using associated legendre polynomials
Angular distributions of differential cross sections from the latest CLAS
data sets \cite{bradford}, for the reaction have been analyzed using associated Legendre polynomials. This
analysis is based upon theoretical calculations in Ref. \cite{fasano} where all
sixteen observables in kaon photoproduction can be classified into four
Legendre classes. Each observable can be described by an expansion of
associated Legendre polynomial functions. One of the questions to be addressed
is how many associated Legendre polynomials are required to describe the data.
In this preliminary analysis, we used data models with different numbers of
associated Legendre polynomials. We then compared these models by calculating
posterior probabilities of the models. We found that the CLAS data set needs no
more than four associated Legendre polynomials to describe the differential
cross section data. In addition, we also show the extracted coefficients of the
best model.Comment: Talk given at APFB08, Depok, Indonesia, August, 19-23, 200
Solving the Cooling Flow Problem of Galaxy Clusters by Dark Matter Neutralino Annihilation
Recent X-ray observations revealed that strong cooling flow of intracluster
gas is not present in galaxy clusters, even though predicted theoretically if
there is no additional heating source. I show that relativistic particles
produced by dark matter neutralino annihilation in cluster cores provide a
sufficient heating source to suppress the cooling flow, under reasonable
astrophysical circumstances including adiabatic growth of central density
profile, with appropriate particle physics parameters for dark matter
neutralinos. In contrast to other astrophysical heat sources such as AGNs, this
process is a steady and stable feedback over cosmological time scales after
turned on.Comment: 4 pages, no figure. Accepted to Phys. Rev. Lett. A few minor
revisions and references adde
Meson Decay Constants from Isospin Mass Splittings in the Quark Model
Decay constants of and mesons are estimated within the framework of a
heavy-quark approach using measured isospin mass splittings in the , ,
and states to isolate the electromagnetic hyperfine interaction between
quarks. The values MeV and MeV are
obtained. Only experimental errors are given; possible theoretical ambiguities,
and suggestions for reducing them, are noted.Comment: 7 pages, LaTeX, EFI-92-3
Parametrisations of the D -> K l nu form factor and the determination of \hat{g}
The vector form factor f_+(t) of the semileptonic decay D -> K l nu, measured
recently with a high accuracy, can be used to determine the strong coupling
constant g_{D_s^* D K}. The latter is related to the normalised coupling
\hat{g} releveant in heavy-meson chiral perturbation theory. This determination
relies on the estimation of the residue of the form factor at the D_s^* pole
and thus on an extrapolation of the form factor in the unphysical region
(m_D-m_K)^2<t<(m_D+m_K)^2. We test this extrapolation for several
parametrisations of the form factors by determining the value of \hat{g}, whose
value can be compared to other (experimental and theoretical) estimates.
Several unsophisticated parametrisations, differing by the amount of physical
information that they embed, are shown to pass this test. An apparently more
elaborated parametrisation of form factors, the so-called z-expansion, is at
variance with the other models, and we point out some significant shortcomings
of this parametrisation for the problem under consideration.Comment: 16 pages, 2 figures. A few references added. Accepted for publication
in JoP
Catalysis study for space shuttle vehicle thermal protection systems
Experimental results on the problem of reducing aerodynamic heating on space shuttle orbiter surfaces are presented. Data include: (1) development of a laboratory flow reactor technique for measuring gamma sub O and gamma sub N on candidate materials at surfaces, T sub w, in the nominal range 1000 to 2000, (2) measurements of gamma sub O and gamma sub N above 1000 K for both the glass coating of a reusable surface insulation material and the siliconized surface of a reinforced pyrolyzed plastic material, (3) measurement of the ablation behavior of the coated RPP material at T sub w is greater than or equal to 2150 K, (4) X-ray photoelectron spectral studies of the chemical constituents on these surfaces before and after dissociated gas exposure, (5) scanning electron micrograph examination of as-received and reacted specimens, and (6) development and exploitation of a method of predicting the aerodynamic heating consquences of these gamma sub O(T sub w) and gamma sub N(T sub w) measurements for critical locations on a radiation cooled orbiter vehicle
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