505 research outputs found
Modeling lithium rich carbon stars in the Large Magellanic Cloud: an independent distance indicator ?
We present the first quantitative results explaining the presence in the
Large Magellanic Cloud of some asymptotic giant branch stars that share the
properties of lithium rich carbon stars. A self-consistent description of
time-dependent mixing, overshooting, and nuclear burning was required. We
identify a narrow range of masses and luminosities for this peculiar stars.
Comparison of these models with the luminosities of the few Li-rich C stars in
the Large Magellanic Cloud provides an independent distance indicator for the
LMCComment: 7 pages, 2 figure
Non-equilibrium Lifshitz theory as a steady state of a full dynamical quantum system
In this work we analyze the validity of Lifshitz's theory for the case of
non-equilibrium scenarios from a full quantum dynamical approach. We show that
Lifshitz's framework for the study of the Casimir pressure is the result of
considering the long-time regime (or steady state) of a well-defined fully
quantized problem, subjected to initial conditions for the electromagnetic
field interacting with real materials. For this, we implement the closed time
path formalism developed in previous works to study the case of two half spaces
(modeled as composite environments, consisting in quantum degrees of freedom
plus thermal baths) interacting with the electromagnetic field. Starting from
initial uncorrelated free subsystems, we solve the full time evolution,
obtaining general expressions for the different contributions to the pressure
that take part on the transient stage. Using the analytic properties of the
retarded Green functions, we obtain the long-time limit of these contributions
to the total Casimir pressure. We show that, in the steady state, only the
baths' contribute, in agreement with the results of previous works, where this
was assumed without justification. We also study in detail the physics of the
initial conditions' contribution and the concept of modified vacuum modes,
giving insights about in which situations one would expect a non vanishing
contribution at the steady state of a non-equilibrium scenario. This would be
the case when considering finite width slabs instead of half-spaces
Acceleration of heavy and light particles in turbulence: comparison between experiments and direct numerical simulations
We compare experimental data and numerical simulations for the dynamics of
inertial particles with finite density in turbulence. In the experiment,
bubbles and solid particles are optically tracked in a turbulent flow of water
using an Extended Laser Doppler Velocimetry technique. The probability density
functions (PDF) of particle accelerations and their auto-correlation in time
are computed. Numerical results are obtained from a direct numerical simulation
in which a suspension of passive pointwise particles is tracked, with the same
finite density and the same response time as in the experiment. We observe a
good agreement for both the variance of acceleration and the autocorrelation
timescale of the dynamics; small discrepancies on the shape of the acceleration
PDF are observed. We discuss the effects induced by the finite size of the
particles, not taken into account in the present numerical simulations.Comment: 7 pages, 4 figure
Conformal invariance and apparent universality of semiclassical gravity
In a recent work, it has been pointed out that certain observables of the
massless scalar field theory in a static spherically symmetric background
exhibit a universal behavior at large distances. More precisely, it was shown
that, unlike what happens in the case the coupling to the curvature \xi is
generic, for the special cases \xi=0 and \xi = 1/6 the large distance behavior
of the expectation value turns out to be independent of the
internal structure of the gravitational source. Here, we address a higher
dimensional generalization of this result: We first compute the difference
between a black hole and a static spherically symmetric star for the
observables and in the far field limit. Thus, we show
that the conformally invariant massless scalar field theory in a static
spherically symmetric background exhibits such universality phenomenon in D\geq
4 dimensions. Also, using the one-loop effective action, we compute
for a weakly gravitating object. These results lead to the
explicit expression of the expectation value for a
Schwarzschild-Tangherlini black hole in the far field limit. As an application,
we obtain quantum corrections to the gravitational potential in D dimensions,
which for D=4 are shown to agree with the one-loop correction to the graviton
propagator previously found in the literature.Comment: 11 page
Decoherence induced by a fluctuating Aharonov-Casher phase
Dipoles interference is studied when atomic systems are coupled to classical
electromagnetic fields. The interaction between the dipoles and the classical
fields induces a time-varying Aharonov-Casher phase. Averaging over the phase
generates a suppression of fringe visibility in the interference pattern. We
show that, for suitable experimental conditions, the loss of contrast for
dipoles can be observable and almost as large as the corresponding one for
coherent electrons. We analyze different trajectories in order to show the
dependence of the decoherence factor with the velocity of the particles.Comment: 13 pages, 3 figures. To appear in Phys. Rev.
A complete O(alpha_S^2) calculation of the signal-background interference for the Higgs diphoton decay channel
We present the full {\cal O}(\as^2) computation of the interference effects
between the Higgs diphoton signal and the continuum background at the LHC.
While the main contribution to the interference originates on the partonic
subprocess, we find that the corrections from the and channels
amount up to 35% of it. We discuss the effect of these new subprocesses in the
shift of the diphoton invariant mass peak recently reported by S. Martin in
Ref.\cite{Martin:2012xc}.Comment: 8 pages, 5 figure
Study of timing performance of Silicon Photomultiplier and application for a Cherenkov detector
Silicon photomultipliers are very versatile photo detectors due to their high
photon detection efficiency, fast response, single photon counting capability,
high amplification, and their insensitivity to magnetic fields. At our
institute we are studying the performance of these photo detectors at various
operating conditions. On the basis of the experience in the laboratory we built
a prototype of a timing Cherenkov detector consisting of a quartz radiator with
two mm MPPCs S10362-33-100C from Hamamatsu Photonics as
photodetectors. The MPPC sensors were operated with Peltier cooling to minimize
thermal noise and to avoid gain drifts. The test measurements at the DANE
Beam-Test Facility (BTF) at the Laboratori Nazionali di Frascati (LNF) with
pulsed 490 MeV electrons and the results on timing performance with Cherenkov
photons are presented.Comment: Conference proceedings of 12th Vienna Conference on Instrumentation
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