13,919 research outputs found
Photon-tagged correlations in heavy-ion collisions: kinematic requirements and a case study
Photon-tagged correlations may be useful to determine how the dense partonic
medium produced in heavy-ion collisions affects the fragmentation of
high-energy quarks and gluons into a leading hadron. In these proceedings, I
discuss the kinematic requirements for the hadron and the prompt photon
transverse momentum cuts. A case study at LHC energy, tagging on p_T > 20 GeV
and p_T > 50 GeV photons, is then briefly examined.Comment: 4 pages, 2 figures. To appear in the proceedings of the 19th
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(QM2006), Shanghai, China, 14-20 November 200
Theory of the spin-galvanic effect and the anomalous phase-shift in superconductors and Josephson junctions with intrinsic spin-orbit coupling
Due to the spin-orbit coupling (SOC) an electric current flowing in a normal
metal or semiconductor can induce a bulk magnetic moment. This effect is known
as the Edelstein (EE) or magneto-electric effect. Similarly, in a bulk
superconductor a phase gradient may create a finite spin density. The inverse
effect, also known as the spin-galvanic effect, corresponds to the creation of
a supercurrent by an equilibrium spin polarization. Here, by exploiting the
analogy between a linear-in-momentum SOC and a background SU(2) gauge field, we
develop a quasiclassical transport theory to deal with magneto-electric effects
in superconducting structures. For bulk superconductors this approach allows us
to easily reproduce and generalize a number of previously known results. For
Josephson junctions we establish a direct connection between the inverse EE and
the appearance of an anomalous phase-shift in the current-phase
relation. In particular we show that is proportional to the
equilibrium spin-current in the weak link. We also argue that our results are
valid generically, beyond the particular case of linear-in-momentum SOC. The
magneto-electric effects discussed in this study may find applications in the
emerging field of coherent spintronics with superconductors.Comment: v1: article version of the preprints arXiv:1408.4533 and
arXiv:1409.4563 in letter format, with far more results and details. v2: some
typos and mistakes corrected, new presentation of the derivation at all
temperature in the ballistic regime (section VI), including a new fig.2 to
illustrate this section. v3: accepted version, with extra reference
Ballistic Josephson junctions in the presence of generic spin dependent fields
Ballistic Josephson junctions are studied in the presence of a spin-splitting
field and spin-orbit coupling. A generic expression for the quasi-classical
Green's function is obtained and with its help we analyze several aspects of
the proximity effect between a spin-textured normal metal (N) and singlet
superconductors (S). In particular, we show that the density of states may show
a zero-energy peak which is a generic consequence of the spin-dependent
couplings in heterostructures. In addition we also obtain the spin current and
the induced magnetic moment in a SNS structure and discuss possible coherent
manipulation of the magnetization which results from the coupling between the
superconducting phase and the spin degree of freedom. Our theory predicts a
spin accumulation at the S/N interfaces, and transverse spin currents flowing
perpendicular to the junction interfaces. Some of these findings can be
understood in the light of a non-Abelian electrostatics.Comment: published versio
Multi-spectral piston sensor for co-phasing giant segmented mirrors and multi-aperture interferometric arrays
This paper presents the optical design of a multi-spectral piston sensor
suitable to co-phasing giant segmented mirrors equipping the Future Extremely
Large Telescopes (ELTs). The general theory of the sensor is described in
detail and numerical simulations have been carried out, demonstrating that
direct piston and tip-tilt measurements are feasible within accuracies
respectively close to 20 nm and 10 nano-radians. Those values are compatible
with the co-phasing requirements, although the method seems to be perturbed by
uncorrected atmospheric seein
Comment on "Superinstantons and the Reliability of Perturbation Theory in Non-Abelian Models"
In a recent letter (hep-lat/9311019) A. Patrascioiu and E. Seiler argued that
when taking into account "superinstantons configurations" the perturbative
expansion and the beta-function of the two-dimensional non-linear sigma-model
are modified at two loops order. I point out that: (1) perturbation theory in a
superinstanton background is infra-red singular beyond three loops; (2) the new
infra-red singular terms, which change the two loop terms, come from singular
operators - describing superinstanton insertions - in the OPE; (3) taking into
account these operators, the beta-function is not modified. Therefore the
results of Patrascioiu and Seiler do not contradict perturbation theory.Comment: 1 page, REVTeX, no figure
Advances in Feature Selection with Mutual Information
The selection of features that are relevant for a prediction or
classification problem is an important problem in many domains involving
high-dimensional data. Selecting features helps fighting the curse of
dimensionality, improving the performances of prediction or classification
methods, and interpreting the application. In a nonlinear context, the mutual
information is widely used as relevance criterion for features and sets of
features. Nevertheless, it suffers from at least three major limitations:
mutual information estimators depend on smoothing parameters, there is no
theoretically justified stopping criterion in the feature selection greedy
procedure, and the estimation itself suffers from the curse of dimensionality.
This chapter shows how to deal with these problems. The two first ones are
addressed by using resampling techniques that provide a statistical basis to
select the estimator parameters and to stop the search procedure. The third one
is addressed by modifying the mutual information criterion into a measure of
how features are complementary (and not only informative) for the problem at
hand
The low Sr/Ba ratio on some extremely metal-poor stars
It has been noted that, in classical extremely metal-poor (EMP) stars, the
abundance ratio of Sr and Ba, is always higher than [Sr/Ba] = -0.5, the value
of the solar r-only process; however, a handful of EMP stars have recently been
found with a very low Sr/Ba ratio. We try to understand the origin of this
anomaly by comparing the abundance pattern of the elements in these stars and
in the classical EMP stars. Four stars with very low Sr/Ba ratios were observed
and analyzed within LTE approximation through 1D (hydrostatic) model
atmosphere, providing homogeneous abundances of nine neutron-capture elements.
In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in
fact, found to be higher than the r-only solar ratio, so the star is discarded.
The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved
giants. They do not present a clear carbon enrichment. The abundance patterns
of the neutron-capture elements in the three stars are strikingly similar to a
theoretical s-process pattern. This pattern could at first be attributed to
pollution by a nearby AGB, but none of the stars presents a clear variation in
the radial velocity indicating the presence of a companion. The stellar
parameters seem to exclude any internal pollution in a TP-AGB phase for at
least two of these stars. The possibility that the stars are early-AGB stars
polluted during the core He flash does not seem compatible with the theory.Comment: Accepted for publication in Astronomy and Astrophysic
Chemical evolution of the Milky Way: the origin of phosphorus
Context. Recently, for the first time the abundance of P has been measured in
disk stars. This provides the opportunity of comparing the observed abundances
with predictions from theoretical models. Aims. We aim at predicting the
chemical evolution of P in the Milky Way and compare our results with the
observed P abundances in disk stars in order to put constraints on the P
nucleosynthesis. Methods. To do that we adopt the two-infall model of galactic
chemical evolution, which is a good model for the Milky Way, and compute the
evolution of the abundances of P and Fe. We adopt stellar yields for these
elements from different sources. The element P should have been formed mainly
in Type II supernovae. Finally, Fe is mainly produced by Type Ia supernovae.
Results. Our results confirm that to reproduce the observed trend of [P/Fe] vs.
[Fe/H] in disk stars, P is formed mainly in massive stars. However, none of the
available yields for P can reproduce the solar abundance of this element. In
other words, to reproduce the data one should assume that massive stars produce
more P than predicted by a factor of ~ 3. Conclusions. We conclude that all the
available yields of P from massive stars are largely underestimated and that
nucleosynthesis calculations should be revised. We also predict the [P/Fe]
expected in halo stars.Comment: Accepted for publication in A&A (minor changes with respect to the
submitted version
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