5,525 research outputs found

### Mass--dependence of the Lambda hypernuclear decay widths

Two different approaches have been employed for the evaluation of the decay
widths of Lambda-hypernuclei (ranging from $^5_{\Lambda}$He to
$^{208}_{\Lambda}$Pb) with the polarization propagator method. In ref.[1], the
two-nucleon stimulated non-mesonic decay, $\Lambda NN\to NNN$, has been
parameterized phenomenologically by means of data on the pion-nucleus optical
potential. The other approach [2] consisted in a fully microscopic description
of the non-mesonic decays through the first order approximation of the
so-called bosonic-loop-expansion. Both calculations reproduce, with
approximately the same accuracy, the experimental decay rates for the whole
range of mass numbers considered.Comment: 4 pages, 2 figures. Talk presented at HYP2000, Torino, 23-27 Oct.
2000. To appear in the proceedings (Nucl. Phys. A

### Microscopic Approach to Nucleon Spectra in Hypernuclear Non-Mesonic Weak Decay

A consistent microscopic diagrammatic approach is applied for the first time
to the calculation of the nucleon emission spectra in the non-mesonic weak
decay of Lambda-hypernuclei. We adopt a nuclear matter formalism extended to
finite nuclei via the local density approximation, a one--meson exchange weak
transition potential and a Bonn nucleon-nucleon strong potential. Ground state
correlations and final state interactions, at second order in the
nucleon--nucleon interaction, are introduced on the same footing for all the
isospin channels of one- and two-nucleon induced decays. Single and
double--coincidence nucleon spectra are predicted for 12_Lambda^C and compared
with recent KEK and FINUDA data. The key role played by quantum interference
terms allows us to improve the predictions obtained with intranuclear cascade
codes. Discrepancies with data remain for proton emission.Comment: 11 pages, 6 figures, 2 tables. To be published in Physics Letters

### Weak Decay of Hypernuclei

The focus of these Lectures is on the weak decay modes of hypernuclei, with
special attention to Lambda-hypernuclei. The subject involves many fields of
modern theoretical and experimental physics, from nuclear structure to the
fundamental constituents of matter and their interactions. The various weak
decay modes of Lambda-hypernuclei are described: the mesonic mode and the
non-mesonic ones. The latter are the dominant decay channels of medium--heavy
hypernuclei, where, on the contrary, the mesonic decay is disfavoured by Pauli
blocking effect on the outgoing nucleon. In particular, one can distinguish
between one-body and two-body induced decays. Theoretical models employed to
evaluate the (partial and total) decay widths of hypernuclei are illustrated,
and their results compared with existing experimental data. Open problems and
recent achievements are extensively discussed, in particular the determination
of the ratio Gamma_n/Gamma_p, possible tests of the Delta I=1/2 rule in
non-mesonic decays and the puzzling asymmetric non-mesonic decay of polarized
hypernuclei.Comment: 59 pages, 13 figures Lectures given at the International School of
Physics "Enrico Fermi" Course on: HADRON PHYSICS, Varenna (Lake Como), June
22nd - July 2nd, 200

### Compartmental analysis of dynamic nuclear medicine data: models and identifiability

Compartmental models based on tracer mass balance are extensively used in
clinical and pre-clinical nuclear medicine in order to obtain quantitative
information on tracer metabolism in the biological tissue. This paper is the
first of a series of two that deal with the problem of tracer coefficient
estimation via compartmental modelling in an inverse problem framework.
Specifically, here we discuss the identifiability problem for a general
n-dimension compartmental system and provide uniqueness results in the case of
two-compartment and three-compartment compartmental models. The second paper
will utilize this framework in order to show how non-linear regularization
schemes can be applied to obtain numerical estimates of the tracer coefficients
in the case of nuclear medicine data corresponding to brain, liver and kidney
physiology

### On the role of ground state correlations in hypernuclear non-mesonic weak decay

The contribution of ground state correlations (GSC) to the non--mesonic weak
decay of $^{12}_\Lambda$C and other medium to heavy hypernuclei is studied
within a nuclear matter formalism implemented in a local density approximation.
We adopt a weak transition potential including the exchange of the complete
octets of pseudoscalar and vector mesons as well as a residual strong
interaction modeled on the Bonn potential. Leading GSC contributions, at first
order in the residual strong interaction, are introduced on the same footing
for all isospin channels of one-- and two--nucleon induced decays. Together
with fermion antisymmetrization, GSC turn out to be important for an accurate
determination of the decay widths. Besides opening the two--nucleon stimulated
decay channels, for $^{12}_\Lambda$C GSC are responsible for 14% of the rate
$\Gamma_1$ while increasing the $\Gamma_{n}/\Gamma_{p}$ ratio by 4%. Our final
results for $^{12}_\Lambda$C are: $\Gamma_{\rm NM}=0.98$,
$\Gamma_{n}/\Gamma_{p}=0.34$ and $\Gamma_2/\Gamma_{\rm NM}=0.26$. The
saturation property of $\Gamma_{\rm NM}$ with increasing hypernuclear mass
number is clearly observed. The agreement with data of our predictions for
$\Gamma_{\rm NM}$, $\Gamma_n/\Gamma_p$ and $\Gamma_2$ is rather good.Comment: 32 pages, 9 figure

### Iterative algorithms for a non-linear inverse problem in atmospheric lidar

We consider the inverse problem of retrieving aerosol extinction coefficients
from Raman lidar measurements. In this problem the unknown and the data are
related through the exponential of a linear operator, the unknown is
non-negative and the data follow the Poisson distribution. Standard methods
work on the log-transformed data and solve the resulting linear inverse
problem, but neglect to take into account the noise statistics. In this study
we show that proper modelling of the noise distribution can improve
substantially the quality of the reconstructed extinction profiles. To achieve
this goal, we consider the non-linear inverse problem with non-negativity
constraint, and propose two iterative algorithms derived using the
Karush-Kuhn-Tucker conditions. We validate the algorithms with synthetic and
experimental data. As expected, the proposed algorithms outperform standard
methods in terms of sensitivity to noise and reliability of the estimated
profile.Comment: 19 pages, 6 figure

### Minimum Detection Efficiencies for a Loophole-Free Bell-type Test

We discuss the problem of finding the most favorable conditions for closing
the detection loophole in a test of local realism with a Bell inequality. For a
generic non-maximally entangled two-qubit state and two alternative measurement
bases we apply Hardy's proof of non-locality without inequality and derive an
Eberhard-like inequality. For an infinity of non-maximally entangled states we
find that it is possible to refute local realism by requiring perfect detection
efficiency for only one of the two measurements: the test is free from the
detection loophole for any value of the detection efficiency corresponding to
the other measurement. The maximum tolerable noise in a loophole-free test is
also evaluated.Comment: 4 pages, 2 figure

- â€¦