2,833 research outputs found
pairs from a nuclear transition signaling an elusive light neutral boson
Electron-positron pairs have been observed in the 10.95-MeV decay
in O. The branching ratio of the ee pairs compared to the
3.84-MeV decay of the level is deduced to be
. This magnetic monopole (M0) transition cannot proceed by
-ray decay and is, to first order, forbidden for internal pair
creation. However, the transition may also proceed by the emission of a light
neutral or boson. Indeed, we do observe a sharp peak in the
angular correlation with all the characteristics belonging to the
intermediate emission of such a boson with an invariant mass of 8.5(5)
MeV/c. It may play a role in the current quest for light dark matter in the
universe.Comment: 6 page
A Re-evaluation of Evidence for Light Neutral Bosons in Nuclear Emulsions
Electron-positron pair-production data obtained by bombardment of emulsion
detectors with either cosmic rays or projectiles with mass between one and 207
and kinetic energies between 18 GeV and 32 TeV have been re-analysed using a
consistent and conservative model of the background from electromagnetic pair
conversion. The combined data yield a spectrum of putative neutral bosons
decaying to e+e- pairs, with masses between 3 and 20 MeV/c^2 and femtosecond
lifetimes. The statistical significance against background for these "X-bosons"
varies between 2 and 8 sigma. The cross-section for direct production of
X-bosons increases slowly with projectile energy, remaining over 1,000 times
smaller the the pion production cross-section.Comment: major revision with improved figures; accepted by Int J Mod Phys
Pygmy Dipol Resonances as a Manifestation of the Structure of the Neutron-Rich Nuclei
Dipole excitations in neutron-rich nuclei below the neutron threshold are
investigated. The method is based on Hartree-Fock-Bogoliubov (HFB) and
Quasiparticle-Phonon Model (QPM) theory. Of our special interest are the
properties of the low-lying 1- Pygmy Resonance and the two-phonon
quadrupole-octupole 1- states in Sn-isotopes including exploratory
investigations for the experimentally unknown mass regions. In particular we
investigate the evolution of the dipole strength function with the neutron
excess. The use of HFB mean-field potentials and s.p. energies is found to
provide a reliable extrapolation into the region off stability.Comment: 8 pages, 3 figures, Proceedings of the International Conference on
Collective Motion in Nuclei Under Extreme Conditions (COMEX1), Paris, France,
10-13 June 200
Transmission resonance spectroscopy in the third minimum of 232Pa
The fission probability of 232Pa was measured as a function of the excitation
energy in order to search for hyperdeformed (HD) transmission resonances using
the (d,pf) transfer reaction on a radioactive 231Pa target. The experiment was
performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at
Garching using the 231Pa(d,pf) reaction at a bombarding energy of E=12 MeV and
with an energy resolution of dE=5.5 keV. Two groups of transmission resonances
have been observed at excitation energies of E=5.7 and 5.9 MeV. The fine
structure of the resonance group at E=5.7 MeV could be interpreted as
overlapping rotational bands with a rotational parameter characteristic to a HD
nuclear shape. The fission barrier parameters of 232Pa have been determined by
fitting TALYS 1.2 nuclear reaction code calculations to the overall structure
of the fission probability. From the average level spacing of the J=4 states,
the excitation energy of the ground state of the 3rd minimum has been deduced
to be E(III)=5.05 MeV.Comment: 6 pages, 8 figure
Observation of Anomalous Internal Pair Creation in Be: A Possible Signature of a Light, Neutral Boson
Electron-positron angular correlations were measured for the isovector
magnetic dipole 17.6 MeV state (, ) ground state
(, ) and the isoscalar magnetic dipole 18.15 MeV (,
) state ground state transitions in Be. Significant
deviation from the internal pair creation was observed at large angles in the
angular correlation for the isoscalar transition with a confidence level of . This observation might indicate that, in an intermediate step, a
neutral isoscalar particle with a mass of 16.70 (stat)
(sys) MeV and was created.Comment: 5 pages, 5 figure
Investigation of Pygmy Dipole Resonances in the Tin Region
The evolution of the low-energy electromagnetic dipole response with the
neutron excess is investigated along the Sn isotopic chain within an approach
incorporating Hartree-Fock-Bogoljubov (HFB) and multi-phonon
Quasiparticle-Phonon-Model (QPM) theory. General aspects of the relationship of
nuclear skins and dipole sum rules are discussed. Neutron and proton transition
densities serve to identify the Pygmy Dipole Resonance (PDR) as a generic mode
of excitation. The PDR is distinct from the GDR by its own characteristic
pattern given by a mixture of isoscalar and isovector components. Results for
the Sn-Sn isotopes and the several N=82 isotones are presented.
In the heavy Sn-isotopes the PDR excitations are closely related to the
thickness of the neutron skin. Approaching Sn a gradual change from a
neutron to a proton skin is found and the character of the PDR is changed
correspondingly. A delicate balance between Coulomb and strong interaction
effects is found. The fragmentation of the PDR strength in Sn is
investigated by multi-phonon calculations. Recent measurements of the dipole
response in Sn are well reproduced.Comment: 41 pages, 10 figures, PR
Effect of large neutron excess on the dipole response in the region of the Giant Dipole Resonance
The evolution of the Dipole Response in nuclei with strong neutron excess is
studied in the Hartree-Fock plus Random Phase Approximation with Skyrme forces.
We find that the neutron excess increases the fragmentation of the isovector
Giant Dipole Resonance, while pushing the centroid of the distribution to lower
energies beyond the mass dependence predicted by the collective models. The
radial separation of proton and neutron densities associated with a large
neutron excess leads to non vanishing isoscalar transition densities to the GDR
states, which are therefore predicted to be excited also by isoscalar nuclear
probes. The evolution of the isoscalar compression dipole mode as a function of
the neutron excess is finally studied. We find that the large neutron excess
leads to a strong concentration of the strength associated with the isoscalar
dipole operator , that mainly originates from uncorrelated
excitations of the neutrons of the skin.Comment: 11 pages 8 figures, use elsart.sty and graphics packag
Electromagnetic transitions between giant resonances within a continuum-RPA approach
A general continuum-RPA approach is developed to describe electromagnetic
transitions between giant resonances. Using a diagrammatic representation for
the three-point Green's function, an expression for the transition amplitude is
derived which allows one to incorporate effects of mixing of single and double
giant resonances as well as to take the entire basis of particle-hole states
into consideration. The radiative widths for E1 transition between the
charge-exchange spin-dipole giant resonance and Gamow-Teller states are
calculated for ^{90}Nb and ^{208}Bi nuclei. The importance of the mixing is
stressed.Comment: 10 pages, 2 figures, uses elsart.st
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