2,199 research outputs found
Neutrino dispersion relation in a magnetized multi-stream matter background
We study the propagation of a neutrino in a medium that consists of two or
more thermal backgrounds of electrons and nucleons moving with some relative
velocity, in the presence of a static and homogeneous electromagnetic field. We
calculate the neutrino self-energy and dispersion relation using the linear
thermal Schwinger propagator, we give the formulas for the dispersion relation
and discuss general features of the results obtained, in particular the effects
of the stream contributions. As a specific example we discuss in some detail
the case of a magnetized two-stream electron, i.e., two electron backgrounds
with a relative velocity in the presence of a magnetic field. For a
neutrino propagating with momentum , in the presence of the stream the
neutrino dispersion relation acquires an anisotropic contribution of the form
in addition to the well known term , as
well as an additional contribution proportional to . We
consider the contribution from a nucleon stream background as an example of
other possible stream backgrounds, and comment on possible generalizations to
take into account the effects of inhomogeneous fields. We explain why a term of
the form does not appear in the dispersion
relation in the constant field case, while a term of similar form can appear in
the presence of an inhomogeneous field involving its gradient.Comment: Title changed, 21 pages, 1 figur
Couplings in coupled channels versus wave functions: application to the X(3872) resonance
We perform an analytical study of the scattering matrix and bound states in
problems with many physical coupled channels. We establish the relationship of
the couplings of the states to the different channels, obtained from the
residues of the scattering matrix at the poles, with the wave functions for the
different channels. The couplings basically reflect the value of the wave
functions around the origin in coordinate space. In the concrete case of the
X(3872) resonance, understood as a bound state of \ddn and \ddc (and
), with the \ddn loosely bound, we find that the couplings to the two
channels are essentially equal leading to a state of good isospin I=0
character. This is in spite of having a probability for finding the \ddn
state much larger than for \ddc since the loosely bound channel extends
further in space. The analytical results, obtained with exact solutions of the
Schr\"odinger equation for the wave functions, can be useful in general to
interpret results found numerically in the study of problems with unitary
coupled channels methods.Comment: 14 pages, 4 figure
Isospin breaking effects in the dynamical generation of the X(3872)
We have studied isospin breaking effects in the X(3872) resonance and found a
natural explanation for the branching fraction of the X decaying to
with two and three pions being close to unit. Within our framework the X(3872)
is a dynamically generated resonance in coupled channels. We also study the
relationship between the couplings of the resonance to the coupled channels
with its wave function, which further helps us to understand the isospin
structure of the resonance.Comment: 5 pages, 1 figure. To appear in the Proceedings of XIII International
Conference on Hadron Spectroscopy, November 29 - December 4, 2009, Florida
State Universit
Ultrafast relaxation rates and reversal time in disordered ferrimagnets
In response to ultrafast laser pulses, single-phase metals have been classified as âfastâ (with magnetization quenching on the time scale of the order of 100 fs and recovery in the time scale of several picoseconds and below) and âslowâ (with longer characteristic time scales). Disordered ferrimagnetic alloys consisting of a combination of âfastâ transition (TM) and âslowâ rare-earth (RE) metals have been shown to exhibit an ultrafast all-optical switching mediated by the heat mechanism. The behavior of the characteristic time scales of coupled alloys is more complicated and is influenced by many parameters such as the intersublattice exchange, doping (RE) concentration, and the temperature. Here, the longitudinal relaxation times of each sublattice are analyzed within the Landau-Lifshitz-Bloch framework. We show that for moderate intersublattice coupling strength both materials slow down as a function of slow (RE) material concentration. For larger coupling, the fast (TM) material may become faster, while the slow (RE) one is still slower. These conclusions may have important implications in the switching time of disordered ferrimagnets such as GdFeCo with partial clustering. Using atomistic modeling, we show that in the moderately coupled case, the reversal would start in the Gd-rich region, while the situation may be reversed if the coupling strength is larger
Momentum-dependent contributions to the gravitational coupling of neutrinos in a medium
When neutrinos travel through a normal matter medium, the electron neutrinos
couple differently to gravity compared to the other neutrinos, due to the
presence of electrons in the medium and the absence of the other charged
leptons. We calculate the momentum-dependent part of the matter-induced
gravitational couplings of the neutrinos under such conditions, which arise at
order , and determine their contribution to the neutrino dispersion
relation in the presence of a gravitational potential .
These new contributions vanish for the muon and tau neutrinos. For electron
neutrinos with momentum , they are of the order of the usual Wolfenstein
term times the factor , for high energy
neutrinos. In environments where the gravitational potential is substantial,
such as those in the vicinity of Active Galactic Nuclei, they could be the
dominant term in the neutrino dispersion relation. They must also be taken into
account in the analysis of possible violations of the Equivalence Principle in
the neutrino sector, in experimental settings involving high energy neutrinos
traveling through a matter background.Comment: Minor corrections in the references; one reference adde
Gravitational coupling of neutrinos in a medium
In a medium that contains electrons but not the other charged leptons, such
as normal matter, the gravitational interactions of neutrinos are not the same
for all the neutrino flavors. We calculate the leading order matter-induced
corrections to the neutrino gravitational interactions in such a medium and
consider some of their physical implications.Comment: 21 pages, Latex, uses axodraw.sty (typos corrected; two references
added. To appear in Phys. Rev. D
Gravitational decay of the Z-boson
We study the decay process of the Z boson to a photon and a graviton. The
most general form of the on-shell amplitude, subject to the constraints due to
the conservation of the electromagnetic and the energy-momentum tensor, is
determined. The amplitude is expressed in terms of three form factors, two of
which are CP-odd while one is CP-even. The latter, which is the only non-zero
form factor at the one-loop level, is computed in the standard model and the
decay rate is determined.Comment: 30 pages, Latex, uses Axodraw. (Some typographical errors corrected,
and some references added in the new version.
Enhanced stability of hydrogen atoms at the graphene/graphane interface of nanoribbons
The thermal stability of graphene/graphane nanoribbons (GGNRs) is
investigated using density functional theory. It is found that the energy
barriers for the diffusion of hydrogen atoms on the zigzag and armchair
interfaces of GGNRs are 2.86 and 3.17 eV, respectively, while the diffusion
barrier of an isolated H atom on pristine graphene was only ~0.3 eV. These
results unambiguously demonstrate that the thermal stability of GGNRs can be
enhanced significantly by increasing the hydrogen diffusion barriers through
graphene/graphane interface engineering. This may provide new insights for
viable applications of GGNRs.Comment: 13 pages, 1 figure, 2 tables to appear in Appl. Phys. Let
- âŠ