780 research outputs found
Double Beta Decay, Nuclear Structure and Physics beyond the Standard Model
Neutrinoless Double Beta Decay () is presently the only known
experiment to distinguisch between Dirac neutrinos, different from their
antiparticles, and Majorana neutrinos, identical with their antiparticles. In
addition allows to determine the absolute scale of the
neutrino masses. This is not possible with neutrino oscillations. To determine
the neutrino masses one must assume, that the light Majorana neutrino exchange
is the leading mechanism for and that the matrix element of
this transition can ba calculated reliably. The experimental
transition amplitude in this mechanism is a product of the light left handed
effective Majorana neutrino mass and of this transition matrix element. The
different methods, Quasi-particle Random Phase Approximation (QRPA), Shell
Model (SM), Projected Hartree-Fock-Bogoliubov (PHFB) and Interacting Boson
Model (IBM2) used in the literature and the reliability of the matrix elements
in these approaches are reviewed. In the second part it is investigated how one
can determine the leading mechanism or mechanisms from the data of the
decay in different nuclei. Explicite expressions are given for
the transition matrix elements. is shown, that possible interference terms
allow to test CP (Charge and Parity conjugation) violation.Comment: Contribution to the EPS conference in Eilath: "Nuclear Physics in
Astrophysics 5." April 3rd to 8th. 201
Nucleon Properties in the Perturbative Chiral Quark Model
We apply the perturbative chiral quark model (PCQM) to analyse low-energy
nucleon properties: electromagnetic form factors, meson-nucleon sigma-terms and
pion-nucleon scattering. Baryons are described as bound states of valence
quarks surrounded by a cloud of Goldstone bosons (pi, K, eta) as required by
chiral symmetry. The model is based on the following guide lines: chiral
symmetry constraints, fulfilment of low-energy theorems and proper treatment of
sea-quarks, that is meson cloud contributions. Analytic expressions for nucleon
observables are obtained in terms of fundamental parameters of low-energy
pion-nucleon physics (weak pion decay constant, axial nucleon coupling
constant, strong pion-nucleon form factor) and of only one model parameter
(radius of the nucleonic three-quark core). Our results are in good agreement
with experimental data and results of other theoretical approaches.Comment: 8 pages. Invited talk given at Workshop on the Physics of Excited
Nucleons "NSTAR2001", Mainz, Germany, March 7-10, 200
Once more on electromagnetic form factors of nucleons in extended vector meson dominance model
Extended vector meson dominance model, that allows to describe the
electromagnetic form factors of nucleons obeying the asymptotic quark counting
rule prescriptions and contains the minimal number of free parameters, is
presented. We get a reasonable fit of form factors over experimentally
available space-like region of momentum transfer and get also reasonable
results in the time-like region.Comment: 7 pages, 2 figure
Charge-exchange reaction cross sections and the Gamow-Teller strength for double beta decay
The proportionality between single charge-exchange reaction cross sections in
the forward direction as found, for example from and He, and
from and He) reactions, and the Gamow-Teller (GT) strength into
the same final nuclear states has been studied and/or assumed often in the
past. Using the most physically justified theory we have at our disposal and
for the specific example of the Ge-Se system that may undergo
double beta-decay, we demonstrate that the proportionality is a relative good
assumption for reactions changing a neutron into a proton, i.e.
GeAs. In this channel, the main contribution to the GT
strengths comes from the removal of a neutron from an occupied single-particle
(SP) state and putting a proton into an unoccupied SP state having either the
same state quantum numbers or those of the spin-orbit partner. In contrast to
this, in the second leg of the double beta decay a single proton must be taken
from an occupied SP state and a neutron placed in an unoccupied one. This
second process often is Pauli forbidden in medium-heavy nuclei and only can be
effected if the Fermi surface is smeared out. Such is the case for
SeAs. Our results suggest that one may not always assume
a proportionality between the forward-angle cross sections of the
charge-exchange reactions and the GT strength in any such medium-heavy nuclei.
The discrepancy originates from a pronounced effect of the radial dependence of
the nucleon-nucleon () interaction in connection with the Pauli principle
on the cross sections in the reaction channel. Such a radial dependence
is completely absent in the GT transition operator.Comment: 14 p., 7 fig
Magnetic moments of S-shell pentaquarks in the constituent quark model
We discuss the magnetic moments of the recently discovered exotic baryons
Theta(+) and Xi(--) and their multiplet partners in the framework of a naive
additive quark model. These baryons are set up as pentaquark states where four
quarks and a single antiquark are located in their ground state orbit. The
pentaquark states are classified as members of the flavor SU(3) octet and
antidecuplet with spin-parity J^P = (1/2)^- and (3/2)^-, where we determine the
magnetic moments of these pentaquark states.Comment: 13 pages, 2 figures, elsart; publication version, 8 pages, 1 figure,
ptptex; upload miss correcte
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