128 research outputs found
- conversion in nuclei and Z physics
Together with the existence of new neutral gauge bosons, models based on
extended gauge groups (rank ) often predict also new charged fermions. A
mixing of the known fermions with new states with {\it exotic} weak-isospin
assignments (left-handed singlets and right-handed doublets) will induce tree
level flavour changing neutral interactions mediated by exchange, while if
the mixing is only with new states with {\it ordinary} weak-isospin
assignments, the flavour changing neutral currents are mainly due to the
exchange of the lightest new neutral gauge boson . We show that the
present experimental limits on conversion in nuclei give a
nuclear-model-independent bound on the -- vertex which is twice as
strong as that obtained from . In the case of E models these
limits provide quite stringent constraints on the mass and on the
mixing angle. We point out that the proposed experiments to search
for conversion in nuclei have good chances to find evidence of lepton
flavour violation, either in the case that new exotic fermions are present at
the electroweak scale, or if a new neutral gauge boson of E
origin lighter than a few TeV exists.Comment: Plain Tex, 24 pages, + 2 PostScript figure appended after \bye (and
available upon request), UM-TH 93--08, FTUV 93-1
Nodal domains of Maass forms I
This paper deals with some questions that have received a lot of attention
since they were raised by Hejhal and Rackner in their 1992 numerical
computations of Maass forms. We establish sharp upper and lower bounds for the
-restrictions of these forms to certain curves on the modular surface.
These results, together with the Lindelof Hypothesis and known subconvex
-bounds are applied to prove that locally the number of nodal domains
of such a form goes to infinity with its eigenvalue.Comment: To appear in GAF
Topological solitons in highly anisotropic two dimensional ferromagnets
e study the solitons, stabilized by spin precession in a classical
two--dimensional lattice model of Heisenberg ferromagnets with non-small
easy--axis anisotropy. The properties of such solitons are treated both
analytically using the continuous model including higher then second powers of
magnetization gradients, and numerically for a discrete set of the spins on a
square lattice. The dependence of the soliton energy on the number of spin
deviations (bound magnons) is calculated. We have shown that the
topological solitons are stable if the number exceeds some critical value
. For and the intermediate values of anisotropy
constant ( is an exchange constant), the soliton
properties are similar to those for continuous model; for example, soliton
energy is increasing and the precession frequency is decreasing
monotonously with growth. For high enough anisotropy we found some fundamentally new soliton features absent for continuous
models incorporating even the higher powers of magnetization gradients. For
high anisotropy, the dependence of soliton energy E(N) on the number of bound
magnons become non-monotonic, with the minima at some "magic" numbers of bound
magnons. Soliton frequency have quite irregular behavior with
step-like jumps and negative values of for some regions of . Near
these regions, stable static soliton states, stabilized by the lattice effects,
exist.Comment: 17 page
Theoretical aspects of the CEBAF 89-009 experiment on inclusive scattering of 4.05 GeV electrons from nuclei
We compare recent CEBAF data on inclusive electron scattering on nuclei with
predictions, based on a relation between structure functions (SF) of a nucleus,
a nucleon and a nucleus of point-nucleons. The latter contains nuclear
dynamics, e.g. binary collision contributions in addition to the asymptotic
limit. The agreement with the data is good, except in low-intensity regions.
Computed ternary collsion contributions appear too small for an explanation. We
perform scaling analyses in Gurvitz's scaling variable and found that for
, ratios of scaling functions for pairs of nuclei differ by less
than 15-20% from 1. Scaling functions for are, for increasing ,
shown to approach a plateau from above. We observe only weak -dependence
in FSI, which in the relevant kinematic region is ascribed to the diffractive
nature of the NN amplitudes appearing in FSI. This renders it difficult to
separate asymptotic from FSI parts and seriously hampers the extraction of
from scaling analyses in a model-independnent fashion.Comment: 11 p. Latex file, 2 ps fig
Photo- and Electro-Disintegration of 3He at Threshold and pd Radiative Capture
The present work reports results for: pd radiative capture observables
measured at center-of-mass (c.m.) energies in the range 0--100 keV and at 2 MeV
by the TUNL and Wisconsin groups, respectively; contributions to the
Gerasimov-Drell-Hearn (GDH) integral in 3He from the two- up to the three-body
breakup thresholds, compared to experimental determinations by the TUNL group
in this threshold region; longitudinal, transverse, and interference response
functions measured in inclusive polarized electron scattering off polarized 3He
at excitation energies below the threshold for breakup into ppn, compared to
unpolarized longitudinal and transverse data from the Saskatoon group. The
calculations are based on a realistic Hamiltonian with two- and three-nucleon
interactions and a realistic current operator, including one- and two-body
components. The theoretical predictions obtained by including only one-body
currents are in violent disagreement with data. These differences between
theory and experiment are, to a large extent, removed when two-body currents
are taken into account, although some rather large discrepancies remain in the
c.m. energy range 0--100 keV, particularly for the pd differential cross
section and tensor analyzing power at small angles, and contributions to the
GDH integral. A rather detailed analysis indicates that these discrepancies
have, in large part, a common origin, and can be traced back to an excess
strength obtained in the theoretical calculation of the E1 reduced matrix
element associated with the pd channel having L,S,J=1,1/2,3/2. It is suggested
that this lack of E1 strength observed experimentally might have implications
for the nuclear interaction at very low energies. Finally, the validity of the
long-wavelength approximation for electric dipole transitions is discussed.Comment: 47 pages RevTex file, 10 PostScript figures, submitted to Phys. Rev.
Nucleation of vortex arrays in rotating anisotropic Bose-Einstein condensates
The nucleation of vortices and the resulting structures of vortex arrays in
dilute, trapped, zero-temperature Bose-Einstein condensates are investigated
numerically. Vortices are generated by rotating a three-dimensional,
anisotropic harmonic atom trap. The condensate ground state is obtained by
propagating the Gross-Pitaevskii equation in imaginary time. Vortices first
appear at a rotation frequency significantly larger than the critical frequency
for vortex stabilization. This is consistent with a critical velocity mechanism
for vortex nucleation. At higher frequencies, the structures of the vortex
arrays are strongly influenced by trap geometry.Comment: 5 pages, two embedded figures. To appear in Phys. Rev. A (RC
A Rotating Collapsar and Possible Interpretation of the LSD Neutrino Signal from SN 1987A
We consider an improved rotational mechanism of the explosion of a collapsing
supernova. We show that this mechanism leads to two-stage collapse with a phase
difference of \sim 5 h. Based on this model, we attempt a new interpretation of
the events in underground neutrino detectors on February 23, 1987, related to
the supernova SN 1987A.Comment: 18 pages, 3 figures, 9 table
Single pi+ Electroproduction on the Proton in the First and Second Resonance Regions at 0.25GeV^2 < Q^2 < 0.65GeV^2 Using CLAS
The ep -> e'pi^+n reaction was studied in the first and second nucleon
resonance regions in the 0.25 GeV^2 < Q^2 < 0.65 GeV^2 range using the CLAS
detector at Thomas Jefferson National Accelerator Facility. For the first time
the absolute cross sections were measured covering nearly the full angular
range in the hadronic center-of-mass frame. The structure functions sigma_TL,
sigma_TT and the linear combination sigma_T+epsilon*sigma_L were extracted by
fitting the phi-dependence of the measured cross sections, and were compared to
the MAID and Sato-Lee models.Comment: Accepted for publication in PR
Effect of Manganese in Test Media on In Vitro Susceptibility of Enterobacteriaceae and Acinetobacter baumannii to Tigecycline
Item does not contain fulltextWe assessed the effect of increasing manganese concentrations in test media (0.001 to 1,024 mg/liter) on MICs of tigecycline. For both broth microdilution (BMD) and Etests, this effect was negligible for physiological concentrations, but MICs increased when concentrations exceeded 8 mg/liter. Susceptibility testing should be performed on media with standardized low manganese content.1 september 201
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