2,725 research outputs found
On the maximum mass of hyperonic neutron stars
Chiral Lagrangian and quark-meson coupling models of hyperon matter are used
to estimate the maximum mass of neutron stars. Our relativistic calculations
include, for the first time, both Hartree and Fock contributions in a
consistent manner. Being related to the underlying quark structure of baryons,
these models are considered to be good candidates for describing the dense core
of neutron stars. Taking account of the known experimental constraints at
saturation density, the equations of state deduced from these relativistic
approaches cannot sustain a neutron star with a mass larger than 1.6-1.66
Minimizing the Dick Effect in an Optical Lattice Clock
We discuss the minimization of the Dick effect in an optical lattice clock.
We show that optimizing the time sequence of operation of the clock can lead to
a significant reduction of the clock stability degradation by the frequency
noise of the interrogation laser. By using a non-destructive detection of the
atoms, we are able to recycle most of the atoms between cycles and consequently
to strongly reduce the time spent capturing the atoms in each cycle. With
optimized parameters, we expect a fractional Allan deviation better than
2E-16 for the lattice clock.Comment: 6 pages, 10 figures. Submitted to IEEE Transactions on Ultrasonics,
Ferroelectrics, and Frequency Contro
A Euclidean Geometric Invariant of Framed (Un)Knots in Manifolds
We present an invariant of a three-dimensional manifold with a framed knot in
it based on the Reidemeister torsion of an acyclic complex of Euclidean
geometric origin. To show its nontriviality, we calculate the invariant for
some framed (un)knots in lens spaces. Our invariant is related to a
finite-dimensional fermionic topological quantum field theory
Vortical control of forced two-dimensional turbulence
A new numerical technique for the simulation of forced two-dimensional turbulence (Dritschel and Fontane, 2010) is used to examine the validity of Kraichnan-Batchelor scaling laws at higher Reynolds number than previously accessible with classical pseudo-spectral methods,making use of large simulation ensembles to allow a detailed consideration of the inverse cascade in a quasi-steady state. Our results support the recent finding of Scott (2007), namely that when a direct enstrophy cascading range is well-represented numerically, a steeper energy spectrum proportional to k^(−2) is obtained in place of the classical k^(−5/3) prediction. It is further shown that this steep spectrum is associated with a faster growth of energy at large scales, scaling like t^(−1) rather than Kraichnan’s prediction of t^(−3/2). The deviation from Kraichnan’s theory is related to the emergence of a population of vortices that dominate the distribution of energy across scales, and whose number density and vorticity distribution with respect to vortex area are related to the shape of the enstrophy spectrum. An analytical model is proposed which closely matches the numerical spectra between the large scales and the forcing scale
Direct Detection is testing Freeze-in
Dark Matter (DM) may belong to a hidden sector that is only feebly
interacting with the Standard Model (SM) and may have never been in thermal
equilibrium in the Early Universe. In this case, the observed abundance of dark
matter particles could have built up through a process known as Freeze-in. We
show that, for the first time, direct detection experiments are testing this DM
production mechanism. This applies to scenarios where the SM and hidden sectors
communicate through a light mediator particle of mass less than a few MeV.
Through the exchange of such light mediator, the very same FIMP candidates can
have self-interactions that are in the range required to address the small
scale structure issues of collisionless cold dark matter.Comment: 7 pages, 4 figures. References added. Discussion of further
constraints on parameters. Figures updated. Conclusions unchanged. Matches
published versio
Low-rate coding using incremental redundancy for GLDPC codes
In this paper we propose a low-rate coding method, suited for application-layer forward error correction. Depending on channel conditions, the coding scheme we propose can switch from a fixed-rate LDPC code to various low-rate GLDPC codes. The source symbols are first encoded by using a staircase or triangular LDPC code. If additional symbols are needed, the encoder is then switched to the GLDPC mode and extra-repair symbols are produced, on demand. In order to ensure small overheads, we consider irregular distributions of extra-repair symbols optimized by density evolution techniques. We also show that increasing the number of extra-repair symbols improves the successful decoding probability, which becomes very close to 1 for sufficiently many extra-repair symbols
Non-destructive measurement of the transition probability in a Sr optical lattice clock
We present the experimental demonstration of non-destructive probing of the
1S0-3P0 clock transition probability in an optical lattice clock with 87Sr
atoms. It is based on the phase shift induced by the atoms on a weak
off-resonant laser beam. The method we propose is a differential measurement of
this phase shift on two modulation sidebands with opposite detuning with
respect to the 1S0-1P1 transition, allowing a detection limited by the photon
shot noise. We have measured an atomic population of 10^4 atoms with a signal
to noise ratio of 100 per cycle, while keeping more than 95% of the atoms in
the optical lattice with a depth of 0.1 mK. The method proves simple and robust
enough to be operated as part of the whole clock setup. This detection scheme
enables us to reuse atoms for subsequent clock state interrogations,
dramatically reducing the loading time and thereby improving the clock
frequency stability.Comment: 4 pages, 5 figure
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