8,007 research outputs found
Deep Learning How to Fit an Intravoxel Incoherent Motion Model to Diffusion-Weighted MRI
Purpose: This prospective clinical study assesses the feasibility of training
a deep neural network (DNN) for intravoxel incoherent motion (IVIM) model
fitting to diffusion-weighted magnetic resonance imaging (DW-MRI) data and
evaluates its performance. Methods: In May 2011, ten male volunteers (age
range: 29 to 53 years, mean: 37 years) underwent DW-MRI of the upper abdomen on
1.5T and 3.0T magnetic resonance scanners. Regions of interest in the left and
right liver lobe, pancreas, spleen, renal cortex, and renal medulla were
delineated independently by two readers. DNNs were trained for IVIM model
fitting using these data; results were compared to least-squares and Bayesian
approaches to IVIM fitting. Intraclass Correlation Coefficients (ICC) were used
to assess consistency of measurements between readers. Intersubject variability
was evaluated using Coefficients of Variation (CV). The fitting error was
calculated based on simulated data and the average fitting time of each method
was recorded. Results: DNNs were trained successfully for IVIM parameter
estimation. This approach was associated with high consistency between the two
readers (ICCs between 50 and 97%), low intersubject variability of estimated
parameter values (CVs between 9.2 and 28.4), and the lowest error when compared
with least-squares and Bayesian approaches. Fitting by DNNs was several orders
of magnitude quicker than the other methods but the networks may need to be
re-trained for different acquisition protocols or imaged anatomical regions.
Conclusion: DNNs are recommended for accurate and robust IVIM model fitting to
DW-MRI data. Suitable software is available at (1)
U(2) and Maximal Mixing of nu_{mu}
A U(2) flavor symmetry can successfully describe the charged fermion masses
and mixings, and supress SUSY FCNC processes, making it a viable candidate for
a theory of flavor. We show that a direct application of this U(2) flavor
symmetry automatically predicts a mixing of 45 degrees for nu_mu to nu_s, where
nu_s is a light, right-handed state. The introduction of an additional flavor
symmetry acting on the right-handed neutrinos makes the model
phenomenologically viable, explaining the solar neutrino deficit as well as the
atmospheric neutrino anomaly, while giving a potential hot dark matter
candidate and retaining the theory's predictivity in the quark sector.Comment: 20 pages, 1 figur
Many-Body Theory of the Electroweak Nuclear Response
After a brief review of the theoretical description of nuclei based on
nonrelativistic many-body theory and realistic hamiltonians, these lectures
focus on its application to the analysis of the electroweak response. Special
emphasis is given to electron-nucleus scattering, whose experimental study has
provided a wealth of information on nuclear structure and dynamics, exposing
the limitations of the shell model. The extension of the formalism to the case
of neutrino-nucleus interactions, whose quantitative understanding is required
to reduce the systematic uncertainty of neutrino oscillation experiments, is
also discussed.Comment: Lectures delivered at the DAE-BRNS Workshop on Hadron Physics.
Aligarh Muslim University, Aligarh (India), February 18-23, 200
Knockout of proton-neutron pairs from O with electromagnetic probes
After recent improvements to the Pavia model of two-nucleon knockout from
O with electromagnetic probes the calculated cross sections are compared
to experimental data from such reactions. Comparison with data from a
measurement of the O(e,epn) reaction show much better agreement
between experiment and theory than was previously observed. In a comparison
with recent data from a measurement of the O(,pn) reaction the
model over-predicts the measured cross section at low missing momentum.Comment: 6 pages, 5 figure
Less Minimal Flavour Violation
We consider the approximate U(2)^3 flavour symmetry exhibited by the quark
sector of the Standard Model and all its possible breaking terms appearing in
the quark Yukawa couplings. Taking an Effective Field Theory point of view, we
determine the current bounds on these parameters, assumed to control the
breaking of flavour in a generic extension of the Standard Model at a reference
scale Lambda. In particular, a significant bound from epsilon'/epsilon is
derived, which is relevant to Minimal Flavour Violation as well. In the
up-quark sector, the recently observed CP violation in D -> pi+ pi-, K+ K-
decays might be accounted for in this generic framework, consistently with any
other constraint.Comment: 15 pages, 1 figur
Radii and binding energies in oxygen isotopes: a puzzle for nuclear forces
We present a systematic study of both nuclear radii and binding energies in
(even) oxygen isotopes from the valley of stability to the neutron drip line.
Both charge and matter radii are compared to state-of-the-art {\it ab initio}
calculations along with binding energy systematics. Experimental matter radii
are obtained through a complete evaluation of the available elastic proton
scattering data of oxygen isotopes. We show that, in spite of a good
reproduction of binding energies, {\it ab initio} calculations with
conventional nuclear interactions derived within chiral effective field theory
fail to provide a realistic description of charge and matter radii. A novel
version of two- and three-nucleon forces leads to considerable improvement of
the simultaneous description of the three observables for stable isotopes, but
shows deficiencies for the most neutron-rich systems. Thus, crucial challenges
related to the development of nuclear interactions remain.Comment: 6 pages, 5 figures, Submitted to Nature Physics, April 12th 2016;
first version (v1 Arxiv) Internal Report Preprint Irfu-18 December 2015. 6
p., 5 fig., Submitted to Physical Review Letters, April 29, May 3rd 2016; 2nd
version. Int. Rep. Irfu-24 May 2016. Published in PRL, 27 July 2016 with the
modified title (Radii and binding energies in oxygen isotopes: a challenge
for nuclear forces
New Physics and CP Violation in Singly Cabibbo Suppressed D Decays
We analyze various theoretical aspects of CP violation in singly Cabibbo
suppressed (SCS) D-meson decays, such as . In particular, we
explore the possibility that CP asymmetries will be measured close to the
present level of experimental sensitivity of . Such measurements
would signal new physics. We make the following points: (i) The mechanism at
work in neutral D decays could be indirect or direct CP violation (or both).
(ii) One can experimentally distinguish between these possibilities. (iii) If
the dominant CP violation is indirect, then there are clear predictions for
other modes. (iv) Tree-level direct CP violation in various known models is
constrained to be much smaller than . (v) SCS decays, unlike Cabibbo
favored or doubly Cabibbo suppressed decays, are sensitive to new contributions
from QCD penguin operators and especially from chromomagnetic dipole operators.
This point is illustrated with supersymmetric gluino-squark loops, which can
yield direct CP violating effects of .Comment: 36 pages, 5 figure
Many-body approach to proton emission and the role of spectroscopic factors
The process of proton emission from nuclei is studied by utilizing the
two-potential approach of Gurvitz and Kalbermann in the context of the full
many-body problem. A time-dependent approach is used for calculating the decay
width. Starting from an initial many-body quasi-stationary state, we employ the
Feshbach projection operator approach and reduce the formalism to an effective
one-body problem. We show that the decay width can be expressed in terms of a
one-body matrix element multiplied by a normalization factor. We demonstrate
that the traditional interpretation of this normalization as the square root of
a spectroscopic factor is only valid for one particular choice of projection
operator. This causes no problem for the calculation of the decay width in a
consistent microscopic approach, but it leads to ambiguities in the
interpretation of experimental results. In particular, spectroscopic factors
extracted from a comparison of the measured decay width with a calculated
single-particle width may be affected.Comment: 17 pages, Revte
"All-versus-nothing" nonlocality test of quantum mechanics by two-photon hyperentanglement
We report the experimental realization and the characterization of
polarization and momentum hyperentangled two photon states, generated by a new
parametric source of correlated photon pairs. By adoption of these states an
"all versus nothing" test of quantum mechanics was performed. The two photon
hyperentangled states are expected to find at an increasing rate a widespread
application in state engineering and quantum information. PACS: 03.65.Ud,
03.67.Mn, 42.65. LmComment: Replaced with published versio
A tradeoff in simultaneous quantum-limited phase and loss estimation in interferometry
Interferometry with quantum light is known to provide enhanced precision for
estimating a single phase. However, depending on the parameters involved, the
quantum limit for the simultaneous estimation of multiple parameters may not
attainable, leading to trade-offs in the attainable precisions. Here we study
the simultaneous estimation of two parameters related to optical
interferometry: phase and loss, using a fixed number of photons. We derive a
trade-off in the estimation of these two parameters which shows that, in
contrast to single-parameter estimation, it is impossible to design a strategy
saturating the quantum Cramer-Rao bound for loss and phase estimation in a
single setup simultaneously. We design optimal quantum states with a fixed
number of photons achieving the best possible simultaneous precisions. Our
results reveal general features about concurrently estimating Hamiltonian and
dissipative parameters, and has implications for sophisticated sensing
scenarios such as quantum imaging.Comment: 9 pages, 6 figure
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