3,865 research outputs found
Response of VIRGO detectors to pre-big-bang gravitons
The sensitivity achievable by a pair of VIRGO detectors to stochastic and
isotropic gravitational wave backgrounds produced in pre-big-bang models is
discussed in view of the development of a second VIRGO interferometer. We
describe a semi-analytical technique allowing to compute the signal-to-noise
ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic
gravitons of arbitrary slope. We apply our results to the case of two
correlated and coaligned VIRGO detectors and we compute their achievable
sensitivities. We perform our calculations both for the usual case of minimal
string cosmological scenario and in the case of a non-minimal scenario where a
long dilaton dominated phase is present prior to the onset of the ordinary
radiation dominated phase. In this framework, we investigate possible
improvements of the achievable sensitivities by selective reduction of the
thermal contributions (pendulum and pendulum's internal modes) to the noise
power spectra of the detectors. Since a reduction of the shot noise does not
increase significantly the expected sensitivity of a VIRGO pair (in spite of
the relative spatial location of the two detectors) our findings support the
experimental efforts directed towards a substantial reduction of thermal noise.Comment: 23 pages in Latex styl
Description of the Scenario Machine
We present here an updated description of the "Scenario Machine" code. This
tool is used to carry out a population synthesis of binary stars. Previous
version of the description can be found at
http://xray.sai.msu.ru/~mystery//articles/review/contents.htmlComment: 32 pages, 3 figures. Corrected typo
Insecurity for compact surfaces of positive genus
A pair of points in a riemannian manifold is secure if the geodesics
between the points can be blocked by a finite number of point obstacles;
otherwise the pair of points is insecure. A manifold is secure if all pairs of
points in are secure. A manifold is insecure if there exists an insecure
point pair, and totally insecure if all point pairs are insecure.
Compact, flat manifolds are secure. A standing conjecture says that these are
the only secure, compact riemannian manifolds. We prove this for surfaces of
genus greater than zero. We also prove that a closed surface of genus greater
than one with any riemannian metric and a closed surface of genus one with
generic metric are totally insecure.Comment: 37 pages, 11 figure
Sensitivity of a VIRGO pair to stochastic GW backgrounds
The sensitivity of a pair of VIRGO interferometers to gravitational waves
backgrounds (GW) of cosmological origin is analyzed for the cases of maximal
and minimal overlap of the two detectors. The improvements in the detectability
prospects of scale-invariant and non-scale-invariant logarithmic energy spectra
of relic GW are discussed.Comment: 25 pages in RevTex style with 6 figure
Sensitivity of wide band detectors to quintessential gravitons
There are no reasons why the energy spectra of the relic gravitons, amplified
by the pumping action of the background geometry, should not increase at high
frequencies. A typical example of this behavior are quintessential inflationary
models where the slopes of the energy spectra can be either blue or mildly
violet. In comparing the predictions of scenarios leading to blue and violet
graviton spectra we face the problem of correctly deriving the sensitivities of
the interferometric detectors. Indeed, the expression of the signal-to-noise
ratio not only depends upon the noise power spectra of the detectors but also
upon the spectral form of the signal and, therefore, one can reasonably expect
that models with different spectral behaviors will produce different
signal-to-noise ratios. By assuming monotonic (blue) spectra of relic gravitons
we will give general expressions for the signal-to-noise ratio in this class of
models. As an example we studied the case of quintessential gravitons. The
minimum achievable sensitivity to of different pairs of
detectors is computed, and compared with the theoretical expectations.Comment: 10 pages in Revtex style, 3 figure
Ergodic Jacobi matrices and conformal maps
We study structural properties of the Lyapunov exponent and the
density of states for ergodic (or just invariant) Jacobi matrices in a
general framework. In this analysis, a central role is played by the function
as a conformal map between certain domains. This idea goes
back to Marchenko and Ostrovskii, who used this device in their analysis of the
periodic problem
Joint profiling of DNA methylation and chromatin architecture in single cells.
We report a molecular assay, Methyl-HiC, that can simultaneously capture the chromosome conformation and DNA methylome in a cell. Methyl-HiC reveals coordinated DNA methylation status between distal genomic segments that are in spatial proximity in the nucleus, and delineates heterogeneity of both the chromatin architecture and DNA methylome in a mixed population. It enables simultaneous characterization of cell-type-specific chromatin organization and epigenome in complex tissues
Magnetic Knots as The origin of Spikes in the Gravitational Waves Backgrounds
The dynamical symmetries of hot and electrically neutral plasmas in a highly
conducting medium suggest that, after the epoch of the electron-positron
annihilation, magnetohydrodynamical configurations carrying a net magnetic
helicity can be present. The simultaneous conservation of the magnetic flux and
helicity implies that the (divergence free) field lines will possess
inhomogeneous knot structures acting as source seeds in the evolution equations
of the scalar, vector and tensor fluctuations of the background geometry. We
give explicit examples of magnetic knot configurations with finite energy and
we compute the induced metric fluctuations. Since magnetic knots are
(conformally) coupled to gravity via the vertex dictated by the equivalence
principle, they can imprint spikes in the gravitational wave spectrum for
frequencies compatible with the typical scale of the knot corresponding, in our
examples, to a present frequency range of -- Hertz. At
lower frequencies the spectrum is power-suppressed and well below the COBE
limit. For smaller length scales (i.e. for larger frequencies) the spectrum is
exponentially suppressed and then irrelevant for the pulsar bounds. Depending
upon the number of knots of the configuration, the typical amplitude of the
gravitational wave logarithmic energy spectrum (in critical units) can be even
four orders of magnitude larger than the usual flat (inflationary) energy
spectrum generated thanks to the parametric amplification of the vacuum
fluctuations.Comment: Accepted for publication in Physical Review D, 20 pages in RevTex
style, 4 Encapsulated figure
Production and detection of relic gravitons in quintessential inflationary models
A large class of quintessential inflationary models, recently proposed by
Peebles and Vilenkin, leads to post-inflationary phases whose effective
equation of state is stiffer than radiation. The expected gravitational waves
logarithmic energy spectra are tilted towards high frequencies and
characterized by two parameters: the inflationary curvature scale at which the
transition to the stiff phase occurs and the number of (non conformally
coupled) scalar degrees of freedom whose decay into fermions triggers the onset
of a gravitational reheating of the Universe. Depending upon the parameters of
the model and upon the different inflationary dynamics (prior to the onset of
the stiff evolution) the relic gravitons energy density can be much more
sizeable than in standard inflationary models, for frequencies larger than 1
Hz. We estimate the required sensitivity for detection of the predicted
spectral amplitude and show that the allowed region of our parameter space
leads to a signal smaller (by one 1.5 orders of magnitude) than the advanced
LIGO sensitivity at a frequency of 0.1 KHz. The maximal signal, in our context,
is expected in the GHz region where the energy density of relic gravitons in
critical units (i.e. ) is of the order of , roughly
eight orders of magnitude larger than in ordinary inflationary models. Smaller
detectors (not necessarily interferometers) can be relevant for detection
purposes in the GHz frequency window. We suggest/speculate that future
measurements through microwave cavities can offer interesting perspectives.Comment: 24 pages in Revtex style, 7 figure
Spin Correlation in tt-bar Production from pp-bar Collisions at sqrt(s)=1.8 TeV
The D0 collaboration has performed a study of spin correlation in tt-bar
production for the process tt-bar to bb-bar W^+W^-, where the W bosons decay to
e-nu or mu-nu. A sample of six events was collected during an exposure of the
D0 detector to an integrated luminosity of approximately 125 pb^-1 of
sqrt{s}=1.8 TeV pp-bar collisions. The standard model (SM) predicts that the
short lifetime of the top quark ensures the transmission of any spin
information at production to the tt-bar decay products.
The degree of spin correlation is characterized by a correlation coefficient
k. We find that k>-0.25 at the 68% confidence level, in agreement with the SM
prediction of k=0.88.Comment: Submitted to PRL, Added references, minor changes to tex
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