266 research outputs found
Light scalar field constraints from gravitational-wave observations of compact binaries
Scalar-tensor theories are among the simplest extensions of general
relativity. In theories with light scalars, deviations from Einstein's theory
of gravity are determined by the scalar mass m_s and by a Brans-Dicke-like
coupling parameter \omega_{BD}. We show that gravitational-wave observations of
nonspinning neutron star-black hole binary inspirals can be used to set lower
bounds on \omega_{BD} and upper bounds on the combination
m_s/\sqrt{\omega_{BD}}$. We estimate via a Fisher matrix analysis that
individual observations with signal-to-noise ratio \rho would yield
(m_s/\sqrt{\omega_{BD}})(\rho/10)<10^{-15}, 10^{-16} and 10^{-19} eV for
Advanced LIGO, ET and eLISA, respectively. A statistical combination of
multiple observations may further improve these bounds.Comment: 9 pages, 4 figures. Matches version accepted in Physical Review
Gravitational radiation from compact binary systems in the massive Brans-Dicke theory of gravity
We derive the equations of motion, the periastron shift, and the
gravitational radiation damping for quasicircular compact binaries in a massive
variant of the Brans-Dicke theory of gravity. We also study the Shapiro time
delay and the Nordtvedt effect in this theory. By comparing with recent
observational data, we put bounds on the two parameters of the theory: the
Brans-Dicke coupling parameter \omega_{BD} and the scalar mass m_s. We find
that the most stringent bounds come from Cassini measurements of the Shapiro
time delay in the Solar System, that yield a lower bound \omega_{BD}>40000 for
scalar masses m_s<2.5x10^{-20} eV, to 95% confidence. In comparison,
observations of the Nordtvedt effect using Lunar Laser Ranging (LLR)
experiments yield \omega_{BD}>1000 for m_s<2.5x10^{-20} eV. Observations of the
orbital period derivative of the quasicircular white dwarf-neutron star binary
PSR J1012+5307 yield \omega_{BD}>1250 for m_s<10^{-20} eV. A first estimate
suggests that bounds comparable to the Shapiro time delay may come from
observations of radiation damping in the eccentric white dwarf-neutron star
binary PSR J1141-6545, but a quantitative prediction requires the extension of
our work to eccentric orbits.Comment: 19 pages, 2 figures, 2 tables. Added new Appendix and slightly
rephrased section on Shapiro time delay. Matches version in press in PR
Effect of symmetry breaking perturbations in the one-dimensional SU(4) spin-orbital model
We study the effect of symmetry breaking perturbations in the one-dimensional
SU(4) spin-orbital model. We allow the exchange in spin () and orbital
() channel to be different and thus reduce the symmetry to SU(2)
SU(2). A magnetic field along the direction is also applied. Using
the formalism developped by Azaria et al we extend their analysis of the
isotropic , h=0 case and obtain the low-energy effective theory near
the SU(4) point in the asymmetric case. An accurate analysis of the
renormalization group flow is presented with a particular emphasis on the
effect of the anisotropy. In zero magnetic field, we retrieve the same
qualitative low-energy physics than in the isotropic case. In particular, the
massless behavior found on the line extends in a large
anisotropic region. We discover though that the anisotropy plays its trick in
allowing non trivial scaling behaviors of the physical quantities. When a
magnetic field is present the effect of the anisotropy is striking. In addition
to the usual commensurate-incommensurate phase transition that occurs in the
spin sector of the theory, we find that the field may induce a second
transition of the KT type in the remaining degrees of freedom to which it does
not couple directly. In this sector, we find that the effective theory is that
of an SO(4) Gross-Neveu model with an h-dependent coupling that may change its
sign as h varies.Comment: 14 pages, 5 Figs, added referenc
Theory of a spherical quantum rotors model: low--temperature regime and finite-size scaling
The quantum rotors model can be regarded as an effective model for the
low-temperature behavior of the quantum Heisenberg antiferromagnets. Here, we
consider a -dimensional model in the spherical approximation confined to a
general geometry of the form (
-linear space size and -temporal size) and subjected to periodic
boundary conditions. Due to the remarkable opportunity it offers for rigorous
study of finite-size effects at arbitrary dimensionality this model may play
the same role in quantum critical phenomena as the popular Berlin-Kac spherical
model in classical critical phenomena. Close to the zero-temperature quantum
critical point, the ideas of finite-size scaling are utilized to the fullest
extent for studying the critical behavior of the model. For different
dimensions and a detailed analysis, in terms of the
special functions of classical mathematics, for the susceptibility and the
equation of state is given. Particular attention is paid to the two-dimensional
case.Comment: 33pages, revtex+epsf, 3ps figures included submitted to PR
Critical exponents from two-particle irreducible 1/N expansion
We calculate the critical exponent in the 1/N expansion of the
two-particle-irreducible (2PI) effective action for the O(N) symmetric model in three spatial dimensions. The exponent controls the behavior
of a two-point function {\it near} the critical point , but can be evaluated on the critical point by the use of the
vertex function . We derive a self-consistent equation for
within the 2PI effective action, and solve it by iteration in
the 1/N expansion. At the next-to-leading order in the 1/N expansion, our
result turns out to improve those obtained in the standard
one-particle-irreducible calculation.Comment: 18 page
Mesobot : An Autonomous Underwater Vehicle for Tracking and Sampling Midwater Targets
Mesobot, a new class of autonomous underwater vehicle, will address specific unmet needs for observing slow-moving targets in the midwater ocean. Mesobot will track targets such as zooplankton, fish, and descending particle aggregates using a control system based on stereo cameras and a combination of thrusters and a variable buoyancy system. The vehicle will also be able to collect biogeochemical and environmental DNA (eDNA) samples using a pumped filter sampler
Chronic Alcohol Exposure Alters Behavioral and Synaptic Plasticity of the Rodent Prefrontal Cortex
In the present study, we used a mouse model of chronic intermittent ethanol (CIE) exposure to examine how CIE alters the plasticity of the medial prefrontal cortex (mPFC). In acute slices obtained either immediately or 1-week after the last episode of alcohol exposure, voltage-clamp recording of excitatory post-synaptic currents (EPSCs) in mPFC layer V pyramidal neurons revealed that CIE exposure resulted in an increase in the NMDA/AMPA current ratio. This increase appeared to result from a selective increase in the NMDA component of the EPSC. Consistent with this, Western blot analysis of the postsynaptic density fraction showed that while there was no change in expression of the AMPA GluR1 subunit, NMDA NR1 and NRB subunits were significantly increased in CIE exposed mice when examined immediately after the last episode of alcohol exposure. Unexpectedly, this increase in NR1 and NR2B was no longer observed after 1-week of withdrawal in spite of a persistent increase in synaptic NMDA currents. Analysis of spines on the basal dendrites of layer V neurons revealed that while the total density of spines was not altered, there was a selective increase in the density of mushroom-type spines following CIE exposure. Examination of NMDA-receptor mediated spike-timing-dependent plasticity (STDP) showed that CIE exposure was associated with altered expression of long-term potentiation (LTP). Lastly, behavioral studies using an attentional set-shifting task that depends upon the mPFC for optimal performance revealed deficits in cognitive flexibility in CIE exposed mice when tested up to 1-week after the last episode of alcohol exposure. Taken together, these observations are consistent with those in human alcoholics showing protracted deficits in executive function, and suggest these deficits may be associated with alterations in synaptic plasticity in the mPFC
Mechanical design of the optical modules intended for IceCube-Gen2
IceCube-Gen2 is an expansion of the IceCube neutrino observatory at the South Pole that aims to increase the sensitivity to high-energy neutrinos by an order of magnitude. To this end, about 10,000 new optical modules will be installed, instrumenting a fiducial volume of about 8 km3. Two newly developed optical module types increase IceCube’s current sensitivity per module by a factor of three by integrating 16 and 18 newly developed four-inch PMTs in specially designed 12.5-inch diameter pressure vessels. Both designs use conical silicone gel pads to optically couple the PMTs to the pressure vessel to increase photon collection efficiency. The outside portion of gel pads are pre-cast onto each PMT prior to integration, while the interiors are filled and cast after the PMT assemblies are installed in the pressure vessel via a pushing mechanism. This paper presents both the mechanical design, as well as the performance of prototype modules at high pressure (70 MPa) and low temperature (−40∘C), characteristic of the environment inside the South Pole ice
The next generation neutrino telescope: IceCube-Gen2
The IceCube Neutrino Observatory, a cubic-kilometer-scale neutrino detector at the geographic South Pole, has reached a number of milestones in the field of neutrino astrophysics: the discovery of a high-energy astrophysical neutrino flux, the temporal and directional correlation of neutrinos with a flaring blazar, and a steady emission of neutrinos from the direction of an active galaxy of a Seyfert II type and the Milky Way. The next generation neutrino telescope, IceCube-Gen2, currently under development, will consist of three essential components: an array of about 10,000 optical sensors, embedded within approximately 8 cubic kilometers of ice, for detecting neutrinos with energies of TeV and above, with a sensitivity five times greater than that of IceCube; a surface array with scintillation panels and radio antennas targeting air showers; and buried radio antennas distributed over an area of more than 400 square kilometers to significantly enhance the sensitivity of detecting neutrino sources beyond EeV. This contribution describes the design and status of IceCube-Gen2 and discusses the expected sensitivity from the simulations of the optical, surface, and radio components
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