2,041 research outputs found
Supernova bound on keV-mass sterile neutrinos reexamined
Active-sterile neutrino mixing is strongly constrained for m_s > 100 keV to
avoid excessive energy losses from supernova cores. For smaller m_s, matter
effects suppress the effective mixing angle except for a resonant range of
energies where it is enhanced. We study the case of \nu_tau-\nu_s-mixing where
a \nu_tau-\bar\nu_tau asymmetry builds up due to the strong excess of \nu_s
over \bar\nu_s emission or vice versa, reducing the overall emission rate. In
the warm dark matter range m_s < 10 keV the mixing angle is essentially
unconstrained.Comment: 6 pages, 4 figures; minor changes, references updated, matches the
published versio
Spin and orbital valence bond solids in a one-dimensional spin-orbital system: Schwinger boson mean field theory
A generalized one-dimensional spin-orbital model is
studied by Schwinger boson mean-field theory (SBMFT). We explore mainly the
dimer phases and clarify how to capture properly the low temperature properties
of such a system by SBMFT. The phase diagrams are exemplified. The three dimer
phases, orbital valence bond solid (OVB) state, spin valence bond solid (SVB)
state and spin-orbital valence bond solid (SOVB) state, are found to be favored
in respectively proper parameter regions, and they can be characterized by the
static spin and pseudospin susceptibilities calculated in SBMFT scheme. The
result reveals that the spin-orbit coupling of type serves
as both the spin-Peierls and orbital-Peierles mechanisms that responsible for
the spin-singlet and orbital-singlet formations respectively.Comment: 6 pages, 3 figure
A Minimal Type II Seesaw Model
We propose a minimal type II seesaw model by introducing only one
right-handed neutrino besides the triplet Higgs to the standard
model. In the usual type II seesaw models with several right-handed neutrinos,
the contributions of the right-handed neutrinos and the triplet Higgs to the CP
asymmetry, which stems from the decay of the lightest right-handed neutrino,
are proportional to their respective contributions to the light neutrino mass
matrix. However, in our minimal type II seesaw model, this CP asymmetry is just
given by the one-loop vertex correction involving the triplet Higgs, even
though the contribution of the triplet Higgs does not dominate the light
neutrino masses. For illustration, the Fritzsch-type lepton mass matrices are
considered.Comment: 5 pages, 4 figures, some points clarified, useful references added,
to appear in Phys. Rev.
Possible Deviation from the Tri-bimaximal Neutrino Mixing in a Seesaw Model
We propose a simple but suggestive seesaw model with two phenomenological
conjectures: three heavy (right-handed) Majorana neutrinos are degenerate in
mass in the symmetry limit and three light Majorana neutrinos have the
tri-bimaximal mixing pattern . We show that a small mass splitting
between the first generation and the other two generations of heavy Majorana
neutrinos is responsible for the deviation of the solar neutrino mixing angle
from its initial value given by , and the
slight breaking of the mass degeneracy between the second and third generations
of heavy Majorana neutrinos results in a small mixing angle
and a tiny departure of the atmospheric neutrino mixing angle
from . It turns out that a normal hierarchy of the light neutrino
mass spectrum is favored in this seesaw scenario.Comment: RevTex 12 pages (2 EPS figures included). More discussions and
references adde
Implicit Decomposition for Write-Efficient Connectivity Algorithms
The future of main memory appears to lie in the direction of new technologies
that provide strong capacity-to-performance ratios, but have write operations
that are much more expensive than reads in terms of latency, bandwidth, and
energy. Motivated by this trend, we propose sequential and parallel algorithms
to solve graph connectivity problems using significantly fewer writes than
conventional algorithms. Our primary algorithmic tool is the construction of an
-sized "implicit decomposition" of a bounded-degree graph on
nodes, which combined with read-only access to enables fast answers to
connectivity and biconnectivity queries on . The construction breaks the
linear-write "barrier", resulting in costs that are asymptotically lower than
conventional algorithms while adding only a modest cost to querying time. For
general non-sparse graphs on edges, we also provide the first writes
and operations parallel algorithms for connectivity and biconnectivity.
These algorithms provide insight into how applications can efficiently process
computations on large graphs in systems with read-write asymmetry
Updated Values of Running Quark and Lepton Masses
Reliable values of quark and lepton masses are important for model building
at a fundamental energy scale, such as the Fermi scale M_Z \approx 91.2 GeV and
the would-be GUT scale \Lambda_GUT \sim 2 \times 10^16 GeV. Using the latest
data given by the Particle Data Group, we update the running quark and
charged-lepton masses at a number of interesting energy scales below and above
M_Z. In particular, we take into account the possible new physics scale (\mu
\sim 1 TeV) to be explored by the LHC and the typical seesaw scales (\mu \sim
10^9 GeV and \mu \sim 10^12 GeV) which might be relevant to the generation of
neutrino masses. For illustration, the running masses of three light Majorana
neutrinos are also calculated. Our up-to-date table of running fermion masses
are expected to be very useful for the study of flavor dynamics at various
energy scales.Comment: 23 pages, 6 tables, 2 figures; version published in PR
Temperature Dependence of Electrical and Optical Modulation Responses of Quantum-Well Lasers
We present theory and experiment for high-speed optical injection in the absorption region of a quantum-well laser and compare the results with those of electrical injection including the carrier transport effect. We show that the main difference between the two responses is the low-frequency roll-off. By using both injection methods, we obtain more accurate and consistent measurements of many important dynamic laser parameters, including the differential gain, carrier lifetime, K factor, and gain compression factor. Temperature-dependent data of the test laser are presented which show that the most dominant effect is the linear degradation of differential gain and injection efficiency with increasing temperature. While the K-factor is insensitive to temperature variation for multiple-quantum-well lasers, we find that the carrier capture time and nonlinear gain suppression coefficient decreases as temperature increases
Non-linear Evolution of Baryon Acoustic Oscillations from Improved Perturbation Theory in Real and Redshift Spaces
We study the non-linear evolution of baryon acoustic oscillations in the
matter power spectrum and correlation function from the improved perturbation
theory (PT). Based on the framework of renormalized PT, we apply the {\it
closure approximation} that truncates the infinite series of loop contributions
at one-loop order, and obtain a closed set of integral equations for power
spectrum and non-linear propagator. The resultant integral expressions keep
important non-perturbative properties which can dramatically improve the
prediction of non-linear power spectrum. Employing the Born approximation, we
then derive the analytic expressions for non-linear power spectrum and the
predictions are made for non-linear evolution of baryon acoustic oscillations
in power spectrum and correlation function. A detailed comparison between
improved PT results and N-body simulations shows that a percent-level agreement
is achieved in a certain range in power spectrum and in a rather wider range in
correlation function. Combining a model of non-linear redshift-space
distortion, we also evaluate the power spectrum and correlation function in
correlation function. In contrast to the results in real space, the agreement
between N-body simulations and improved PT predictions tends to be worse, and a
more elaborate modeling for redshift-space distortion needs to be developed.
Nevertheless, with currently existing model, we find that the prediction of
correlation function has a sufficient accuracy compared with the
cosmic-variance errors for future galaxy surveys with volume of a few (Gpc/h)^3
at z>=0.5.Comment: 25 pages, 15 figures, accepted for publication in Phys.Rev.
- …