206 research outputs found
The core structure of presolar graphite onions
Of the ``presolar particles'' extracted from carbonaceous chondrite
dissolution residues, i.e. of those particles which show isotopic evidence of
solidification in the neighborhood of other stars prior to the origin of our
solar system, one subset has an interesting concentric
graphite-rim/graphene-core structure. We show here that single graphene sheet
defects in the onion cores (e.g. cyclopentane loops) may be observable edge-on
by HREM. This could allow a closer look at models for their formation, and in
particular strengthen the possibility that growth of these assemblages proceeds
atom-by-atom with the aid of such in-plane defects, under conditions of growth
(e.g. radiation fluxes or grain temperature) which discourage the graphite
layering that dominates subsequent formation of the rim.Comment: 4 pages, 7 figures, 11 refs, see also
http://www.umsl.edu/~fraundor/isocore.htm
Constraints on mixing angles of Majorana neutrinos
By combining the inputs from the neutrinoless double beta decay and the fits
of cosmological models of dark matter with solar and atmospheric neutrino data,
we obtain constraints on two of the mixing angles of Majorana neutrinos, which
become stronger when coupled with the reactor neutrino data. These constraints
are strong enough to rule out Majorana neutrinos if the small angle solution of
solar neutrino puzzle is borne out.Comment: Some corrections and clarifications adde
A Calorimetric Search on Double Beta Decay of 130Te
We report on the final results of a series of experiments on double decay of
130Te carried out with an array of twenty cryogenic detectors. The set-up is
made with crystals of TeO2 with a total mass of 6.8 kg, the largest operating
one for a cryogenic experiment. Four crystals are made with isotopically
enriched materials: two in 128Te and two others in 130Te. The remaining ones
are made with natural tellurium, which contains 31.7 % and 33.8 % 128Te and
130Te, respectively. The array was run under a heavy shield in the Gran Sasso
Underground Laboratory at a depth of about 3500 m.w.e. By recording the pulses
of each detector in anticoincidence with the others a lower limit of 2.1E23
years has been obtained at the 90 % C.L. on the lifetime for neutrinoless
double beta decay of 130Te. In terms of effective neutrino mass this is the
most restrictive limit in direct experiments, after those obtained with Ge
diodes. Limits on other lepton violating decays of 130Te and on the
neutrinoless double beta decay of 128Te to the ground state of 128Xe are also
reported and discussed. An indication is presented for the two neutrino double
beta decay of 130Te. Some consequences of the present results in the
interpretation of geochemical experiments are discussed.Comment: 14 pages, 3 figures, 2 tables; more analysis details. Accepted for
publication on Physics Letters
Activated platelets form protected zones of adhesion on fibrinogen and fibronectin-coated surfaces.
Leukocytes form zones of close apposition when they adhere to ligand-coated surfaces. Because plasma proteins are excluded from these contact zones, we have termed them protected zones of adhesion. To determine whether platelets form similar protected zones of adhesion, gel-filtered platelets stimulated with thrombin or ADP were allowed to adhere to fibrinogen- or fibronectin-coated surfaces. The protein-coated surfaces with platelets attached were stained with either fluorochrome-conjugated goat anti-human fibrinogen or anti-human fibronectin antibodies, or with rhodamine-conjugated polyethylene glycol polymers. Fluorescence microscopy revealed that F(ab')2 anti-fibrinogen (100 kD) did not penetrate into the contact zones between stimulated platelets and the underlying fibrinogen-coated surface, while Fab antifibrinogen (50 kD) and 10 kD polyethylene glycol readily penetrated and stained the substrate beneath the platelets. Thrombin- or ADP-stimulated platelets also formed protected zones of adhesion on fibronectin-coated surfaces. F(ab')2 anti-fibronectin and 10 kD polyethylene glycol were excluded from these adhesion zones, indicating that they are much less permeable than those formed by platelets on fibrinogen-coated surfaces. The permeability properties of protected zones of adhesion formed by stimulated platelets on surfaces coated with both fibrinogen and fibronectin were similar to the zones of adhesion formed on fibronectin alone. mAb 7E3, directed against the alpha IIb beta 3 integrin blocked the formation of protected adhesion zones between thrombin-stimulated platelets and fibrinogen or fibronectin coated surfaces. mAb C13 is directed against the alpha 5 beta 1 integrin on platelets. Stimulated platelets treated with this mAb formed protected zones of adhesion on surfaces coated with fibronectin. These protected zones were impermeable to F(ab')2 antifibronectin but were permeable to 10 kD polyethylene glycol. These results show that activated platelets form protected zones of adhesion and that the size of molecules excluded from these zones depends upon the composition of the matrix proteins to which the platelets adhere. They also show that formation of protected zones of adhesion by platelets requires alpha IIb beta 3 integrins while the permeability properties of these zones of adhesion are regulated by both alpha IIb beta 3 and alpha 5 beta 1 integrins
A New Class of Majoron-Emitting Double-Beta Decays
Motivated by the excess events that have recently been found near the
endpoints of the double beta decay spectra of several elements, we re-examine
models in which double beta decay can proceed through the neutrinoless emission
of massless Nambu-Goldstone bosons (majorons). Noting that models proposed to
date for this process must fine-tune either a scalar mass or a VEV to be less
than 10 keV, we introduce a new kind of majoron which avoids this difficulty by
carrying lepton number . We analyze in detail the requirements that
models of both the conventional and our new type must satisfy if they are to
account for the observed excess events. We find: (1) the electron sum-energy
spectrum can be used to distinguish the two classes of models from one another;
(2) the decay rate for the new models depends on different nuclear matrix
elements than for ordinary majorons; and (3) all models require a (pseudo)
Dirac neutrino, having a mass of a several hundred MeV, which mixes with
.Comment: 43 pages, 10 figures (included), [figure captions are now included
Generalized Bounds on Majoron-neutrino couplings
We discuss limits on neutrino-Majoron couplings both from laboratory
experiments as well as from astrophysics. They apply to the simplest class of
Majoron models which covers a variety of possibilities where neutrinos acquire
mass either via a seesaw-type scheme or via radiative corrections. By adopting
a general framework including CP phases we generalize bounds obtained
previously. The combination of complementary bounds enables us to obtain a
highly non-trivial exclusion region in the parameter space. We find that the
future double beta project GENIUS, together with constraints based on supernova
energy release arguments, could restrict neutrino-Majoron couplings down to the
10^{-7} level.Comment: 17 pages, LateX, 7 figures, version to be published in Phys. Rev.
Pions in Nuclei and Manifestations of Supersymmetry in Neutrinoless Double Beta Decay
We examine the pion realization of the short ranged supersymmetric (SUSY)
mechanism of neutrinoless double beta decay. It originates from the R-parity
violating quark-lepton interactions of the SUSY extensions of the standard
model of the electroweak interactions. We argue that pions are dominant SUSY
mediators in neutrinoless double beta decay. The corresponding nuclear matrix
elements for various isotopes are calculated within the proton-neutron
renormalized quasiparticle random phase approximation. We define those isotopes
which are most sensitive to the SUSY signal and outlook the present
experimental situation with the double beta decay searches for the SUSY. Upper
limits on the R-parity violating 1st generation Yukawa coupling are derived
from various double beta decay experiments.Comment: 15 pages, Latex, 3 Postscript figure
Neutrinoless Double Beta Decay within QRPA with Proton-Neutron Pairing
We have investigated the role of proton-neutron pairing in the context of the
Quasiparticle Random Phase approximation formalism. This way the neutrinoless
double beta decay matrix elements of the experimentally interesting A= 48, 76,
82, 96, 100, 116, 128, 130 and 136 systems have been calculated. We have found
that the inclusion of proton-neutron pairing influences the neutrinoless double
beta decay rates significantly, in all cases allowing for larger values of the
expectation value of light neutrino masses. Using the best presently available
experimental limits on the half life-time of neutrinoless double beta decay we
have extracted the limits on lepton number violating parameters.Comment: 16 RevTex page
Neutrinoless double beta decay in seesaw models
We study the general phenomenology of neutrinoless double beta decay in
seesaw models. In particular, we focus on the dependence of the neutrinoless
double beta decay rate on the mass of the extra states introduced to account
for the Majorana masses of light neutrinos. For this purpose, we compute the
nuclear matrix elements as functions of the mass of the mediating fermions and
estimate the associated uncertainties. We then discuss what can be inferred on
the seesaw model parameters in the different mass regimes and clarify how the
contribution of the light neutrinos should always be taken into account when
deriving bounds on the extra parameters. Conversely, the extra states can also
have a significant impact, cancelling the Standard Model neutrino contribution
for masses lighter than the nuclear scale and leading to vanishing neutrinoless
double beta decay amplitudes even if neutrinos are Majorana particles. We also
discuss how seesaw models could reconcile large rates of neutrinoless double
beta decay with more stringent cosmological bounds on neutrino masses.Comment: 34 pages, 5 eps figures and 1 axodraw figure. Final version published
in JHEP. NME results available in Appendi
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