24,027 research outputs found
Speedy motions of a body immersed in an infinitely extended medium
We study the motion of a classical point body of mass M, moving under the
action of a constant force of intensity E and immersed in a Vlasov fluid of
free particles, interacting with the body via a bounded short range potential
Psi. We prove that if its initial velocity is large enough then the body
escapes to infinity increasing its speed without any bound "runaway effect".
Moreover, the body asymptotically reaches a uniformly accelerated motion with
acceleration E/M. We then discuss at a heuristic level the case in which Psi(r)
diverges at short distances like g r^{-a}, g,a>0, by showing that the runaway
effect still occurs if a<2.Comment: 15 page
A compact high-flux source of cold sodium atoms
We present a compact source of cold sodium atoms suitable for the production
of quantum degenerate gases and versatile for a multi-species experiment. The
magnetic field produced by permanent magnets allows to simultaneously realize a
Zeeman slower and a two-dimensional MOT within an order of magnitude smaller
length than standard sodium sources. We achieve an atomic flux exceeding 4x10^9
atoms/s loaded in a MOT, with a most probable longitudinal velocity of 20 m/s,
and a brightness larger than 2.5x10^(12) atoms/s/sr. This atomic source allowed
us to produce a pure BEC with more than 10^7 atoms and a background pressure
limited lifetime of 5 minutes.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
Renormalization Group Improvement and Dynamical Breaking of Symmetry in a Supersymmetric Chern-Simons-matter Model
In this work, we investigate the consequences of the Renormalization Group
Equation (RGE) in the determination of the effective superpotential and the
study of Dynamical Symmetry Breaking (DSB) in an N = 1 supersymmetric theory
including an Abelian Chern-Simons superfield coupled to N scalar superfields in
(2+1) dimensional spacetime. The classical Lagrangian presents scale
invariance, which is broken by radiative corrections to the effective
superpotential. We calculate the effective superpotential up to two-loops by
using the RGE and the beta functions and anomalous dimensions known in the
literature. We then show how the RGE can be used to improve this calculation,
by summing up properly defined series of leading logs (LL), next-to-leading
logs (NLL) contributions, and so on... We conclude that even if the RGE
improvement procedure can indeed be applied in a supersymmetric model, the
effects of the consideration of the RGE are not so dramatic as it happens in
the non-supersymmetric case.Comment: v4: 11 pages, 1 figure. Version accepted for publication in NP
The SU(2) X U(1) Electroweak Model based on the Nonlinearly Realized Gauge Group
The electroweak model is formulated on the nonlinearly realized gauge group
SU(2) X U(1). This implies that in perturbation theory no Higgs field is
present. The paper provides the effective action at the tree level, the Slavnov
Taylor identity (necessary for the proof of unitarity), the local functional
equation (used for the control of the amplitudes involving the Goldstone
bosons) and the subtraction procedure (nonstandard, since the theory is not
power-counting renormalizable). Particular attention is devoted to the number
of independent parameters relevant for the vector mesons; in fact there is the
possibility of introducing two mass parameters. With this choice the relation
between the ratio of the intermediate vector meson masses and the Weinberg
angle depends on an extra free parameter. We briefly outline a method for
dealing with \gamma_5 in dimensional regularization. The model is formulated in
the Landau gauge for sake of simplicity and conciseness: the QED Ward identity
has a simple and intriguing form.Comment: 19 pages, final version published by Int. J. Mod. Phys. A, some typos
corrected in eqs.(1) and (41). The errors have a pure editing origin.
Therefore they do not affect the content of the pape
On the asymmetric zero-range in the rarefaction fan
We consider the one-dimensional asymmetric zero-range process starting from a
step decreasing profile. In the hydrodynamic limit this initial condition leads
to the rarefaction fan of the associated hydrodynamic equation. Under this
initial condition and for totally asymmetric jumps, we show that the weighted
sum of joint probabilities for second class particles sharing the same site is
convergent and we compute its limit. For partially asymmetric jumps we derive
the Law of Large Numbers for the position of a second class particle under the
initial configuration in which all the positive sites are empty, all the
negative sites are occupied with infinitely many first class particles and with
a single second class particle at the origin. Moreover, we prove that among the
infinite characteristics emanating from the position of the second class
particle, this particle chooses randomly one of them. The randomness is given
in terms of the weak solution of the hydrodynamic equation through some sort of
renormalization function. By coupling the zero-range with the exclusion process
we derive some limiting laws for more general initial conditions.Comment: 22 pages, to appear in Journal of Statistical Physic
Atmospheric neutrinos in a Large Liquid Argon detector
In view of the evaluation of the physics goals of a large Liquid Argon TPC,
evolving from the ICARUS technology, we have studied the possibility of
performing precision measurements on atmospheric neutrinos. For this purpose we
have improved existing Monte Carlo neutrino event generators based on FLUKA and
NUX by including the 3-flavor oscillation formalism and the numerical treatment
of Earth matter effects. By means of these tools we have studied the
sensitivity in the measurement of Theta(23) through the accurate measurement of
electron neutrinos. The updated values for Delta m^2(23) from Super-Kamiokande
and the mixing parameters as obtained by solar and KamLand experiments have
been used as reference input, while different values of Theta(13) have been
considered. An exposure larger than 500 kton yr seems necessary in order to
achieve a significant result, provided that the present knowledge of systematic
uncertainties is largely improved.Comment: Talk given at the worksgop "Cryogenic Liquid Detectors for Future
Particle Physics", LNGS (Italy) March 13th-14th, 200
Modelization of Thermal Fluctuations in G Protein-Coupled Receptors
We simulate the electrical properties of a device realized by a G protein
coupled receptor (GPCR), embedded in its membrane and in contact with two
metallic electrodes through which an external voltage is applied. To this
purpose, recently, we have proposed a model based on a coarse graining
description, which describes the protein as a network of elementary impedances.
The network is built from the knowledge of the positions of the C-alpha atoms
of the amino acids, which represent the nodes of the network. Since the
elementary impedances are taken depending of the inter-nodes distance, the
conformational change of the receptor induced by the capture of the ligand
results in a variation of the network impedance. On the other hand, the
fluctuations of the atomic positions due to thermal motion imply an impedance
noise, whose level is crucial to the purpose of an electrical detection of the
ligand capture by the GPCR. Here, in particular, we address this issue by
presenting a computational study of the impedance noise due to thermal
fluctuations of the atomic positions within a rhodopsin molecule. In our model,
the C-alpha atoms are treated as independent, isotropic, harmonic oscillators,
with amplitude depending on the temperature and on the position within the
protein (alpha-helix or loop). The relative fluctuation of the impedance is
then calculated for different temperatures.Comment: 5 pages, 2 figures, Proceeding of the 18-th International Conference
on Fluctuations and Noise, 19-23 September 2005, Salamanca, Spain -minor
proofreadings
Kohn Anomalies and Electron-Phonon Interaction in Graphite
We demonstrate that graphite phonon dispersions have two Kohn anomalies at
the Gamma-E_2g and K-A'1 modes. The anomalies are revealed by two sharp kinks.
By an exact analytic derivation, we show that the slope of these kinks is
proportional to the square of the electron-phonon coupling (EPC). Thus, we can
directly measure the EPC from the experimental dispersions. The Gamma-E_2g and
K-A'1 EPCs are particularly large, whilst they are negligible for all the other
modes at Gamma and K.Comment: 4 pages, 2 figure
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