3,974 research outputs found
Creation of Dirac neutrinos in a dense medium with time-dependent effective potential
We consider Dirac neutrinos interacting with background fermions in the frame
of the standard model. We demonstrate that a time-dependent effective potential
is quite possible in a protoneutron star (PNS) at certain stages of its
evolution. For the first time, we formulate a nonperturbative treatment of
neutrino processes in a matter with arbitrary time-dependent effective
potential. Using linearly growing effective potential, we study the typical
case of a slowly varying matter interaction potential. We calculate
differential mean numbers of pairs created from the vacuum by
this potential and find that they crucially depend on the magnitude of masses
of the lightest neutrino eigenstate. These distributions uniformly span up to
eV energies for muon and tau neutrinos created in PNS core due to the
compression just before the hydrodynamic bounce and up to eV
energies for all three active neutrino flavors created in the neutronization.
Considering different stages of the PNS evolution, we derive constraints on
neutrino masses, eV corresponding to the
nonvanishing pairs flux produced by this mechanism. We show
that one can distinguish such coherent flux from chaotic fluxes of any other
origin. Part of these neutrinos, depending on the flavor and helicity, are
bounded in the PNS, while antineutrinos of any flavor escape the PNS. If the
created pairs are , then a part of the corresponding
neutrinos also escape the PNS. The detection of and with
such low energies is beyond current experimental techniques.Comment: 18 pages, Revtex4.1, 1 eps figure, 2 columns; minimal changes,
version to be published in Phys. Rev.
Neutrino spin oscillations in gravitational fields
We study neutrino spin oscillations in gravitational fields. The
quasi-classical approach is used to describe the neutrino spin evolution. First
we examine the case of a weak gravitational field. We obtain the effective
Hamiltonian for the description of neutrino spin oscillations. We also receive
the neutrino transition probability when a particle propagates in the
gravitational field of a rotating massive object. Then we apply the general
technique to the description of neutrino spin oscillations in the Schwarzschild
metric. The neutrino spin evolution equation for the case of the neutrino
motion in the vicinity of a black hole is obtained. The effective Hamiltonian
and the transition probability are also derived. We examine the neutrino
oscillations process on different circular orbits and analyze the frequencies
of spin transitions. The validity of the quasi-classical approach is also
considered.Comment: RevTeX4, 9 pages, 1 esp figure; article was revised, some misprints
were corrected, 6 references added; accepted for publication in
Int.J.Mod.Phys.
Leading Infrared Logarithms from Unitarity, Analyticity and Crossing
We derive non-linear recursion equations for the leading infrared logarithms
in massless non-renormalizable effective field theories. The derivation is
based solely on the requirements of the unitarity, analyticity and crossing
symmetry of the amplitudes. That emphasizes the general nature of the
corresponding equations. The derived equations allow one to compute leading
infrared logarithms to essentially unlimited loop order without performing a
loop calculation. For the implementation of the recursion equation one needs to
calculate tree diagrams only. The application of the equation is demonstrated
on several examples of effective field theories in four and higher space-time
dimensions.Comment: 12 page
Neutrino Physics with Dark Matter Experiments and the Signature of New Baryonic Neutral Currents
New neutrino states \nu_b, sterile under the Standard Model interactions, can
be coupled to baryons via the isoscalar vector currents that are much stronger
than the Standard Model weak interactions. If some fraction of solar neutrinos
oscillate into \nu_b on their way to Earth, the coherently enhanced elastic
\nu_b-nucleus scattering can generate a strong signal in the dark matter
detectors. For the interaction strength a few hundred times stronger than the
weak force, the elastic \nu_b-nucleus scattering via new baryonic currents may
account for the existing anomalies in the direct detection dark matter
experiments at low recoil. We point out that for solar neutrino energies the
baryon-current-induced inelastic scattering is suppressed, so that the possible
enhancement of new force is not in conflict with signals at dedicated neutrino
detectors. We check this explicitly by calculating the \nu_b-induced deuteron
breakup, and the excitation of 4.4 MeV \gamma-line in ^{12}C.
Stronger-than-weak force coupled to baryonic current implies the existence of
new abelian gauge group U(1)_B with a relatively light gauge boson.Comment: 20 pages, 5 figures. References added, inconsistent treatment of
neutrino oscillations corrected, conclusions unchange
Electron-microscopic observation of BaTiO3 prepared by additive assisted aqueous synthesis
Bulk barium titanate (BaTiO3) has found widespread applications especially in multi-layered ceramic capacitors (MLCCs) and embedded decoupling capacitors (EDC). In the last years, the interest in one-dimensional (1D) nanostructured ferroelectric systems (nanotubes, nanowires, nanorods, nanobelts, nanofibers) is increasing. Recently theoretical studies reported an enhancement of ferroelectricity in 1D systems. Although the hydrothermal and aqueous synthesis of equiaxed barium titanate powders have been thoroughly investigated the growth of barium titanate anisotropic nanoparticles still less known. Indeed it is particularly dificult to modify the crystal habit by hydrothermal and related methods. However it is expected that the presence of some additives during the synthesis will change the growth kinetic
Pion photoproduction off the proton in a gauge-invariant chiral unitary framework
We investigate pion photoproduction off the proton in a manifestly
gauge-invariant chiral unitary extension of chiral perturbation theory. In a
first step, we consider meson-baryon scattering taking into account all
next-to-leading order contact interactions. The resulting low-energy constants
are determined by a fit to s-wave pion-nucleon scattering and the low-energy
data for the reaction pi- p --> eta n. To assess the theoretical uncertainty,
we perform two different fit strategies. Having determined the low-energy
constants, we then analyse the data on the s-wave multipole amplitudes E0+ of
pion and eta photoproduction. These are parameter-free predictions, as the two
new low-energy constants are determined by the neutron and proton magnetic
moments.Comment: 23 pages, 17 figure
Electric Dipole Moments of Leptons in the Presence of Majorana Neutrinos
We calculate the two-loop diagrams that give a non-zero contribution to the
electric dipole moment d_l of a charged lepton l due to possible Majorana
masses of neutrinos. Using the example with one generation of the Standard
Model leptons and two heavy right-handed neutrinos, we demonstrate that the
non-vanishing result for d_l first appears in order O(m_l m_\nu^2 G_F^2), where
m_\nu is the mass of the light neutrino and the see-saw type relation is
imposed. This effect is beyond the reach of presently planned experiments.Comment: 13 page
Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co
The magnetic properties and the spin dynamics of two molecular magnets have
been investigated by magnetization and d.c. susceptibility measurements,
Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance
(NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields,
H=0.5 and 1.5 Tesla. The two molecular magnets consist of
CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog
compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is
constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak
ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion
with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the
weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show
the presence of non equivalent groups of protons with a measurable contact
hyperfine interaction consistent with a small admixture of s-wave function with
the d-function of the magnetic ion. The NMR relaxation results are explained in
terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an
almost temperature independent correlation time due to the weak magnetic
exchange interaction. We conclude that the two molecular magnets studied here
behave as single molecule paramagnets with a very weak intramolecular
interaction, almost of the order of the dipolar intermolecular interaction.
Thus they represent a new class of molecular magnets which differ from the
single molecule magnets investigated up to now, where the intramolecular
interaction is much larger than the intermolecular one
- …