1,965 research outputs found
Electroweak structure of light nuclei within chiral effective field theory
We review the results of the most recent calculations for the electromagnetic
structure of light nuclei, the weak muon capture on deuteron and 3He and the
weak proton-proton capture reaction at energies of astrophysical interest,
performed within the chiral effective field theory framework.Comment: 6 pages, 3 figures, submitted to Few-Body System
The hep reaction and the solar neutrino problem
The results of a new calculation of the astrophysical S-factor for the proton
weak capture on 3He are here reviewed. The methods used to obtain very accurate
initial and final state wave functions and to construct the nuclear weak
current operator are described. Finally the implications of these results for
the Super-Kamiokande solar neutrino data are discussed.Comment: 10 pages, 1 figure, XVII European Few-Body plenary talk, Evora,
September 200
Muon capture on deuteron and the neutron-neutron scattering length
We study the capture rate in the doublet hyperfine initial state for the muon
capture reaction \mu^- + \,^2{\rm H} \rightarrow \nu_\mu + n + n ()
and the total capture rate for the reaction \mu^- + \,^3{\rm He} \rightarrow
\nu_\mu + \,^3{\rm H} (). We investigate whether and
could be sensitive to the -wave scattering length
(). To this aim, we consider nuclear potentials and weak currents
derived within EFT. We employ the N3LO chiral potential with cutoff
=500 MeV, but the low-energy constant (LEC) determining is
varied so as to obtain =-18.95 (the present empirical value), -16.0,
-22.0, and +18.22 fm. The last value leads to a bound state with a binding
energy of 139 keV. The LECs and , present in the three-nucleon
potential and axial-vector current, are fitted to reproduce the binding
energies and the triton Gamow-Teller matrix element. The capture rate
is found to be 399(3) s for =-18.95 and -16.0 fm; and
400(3) s for =-22.0 fm. For =+18.22 fm, we obtain 275(3)
s (135(3) s), when the final system is unbound (bound). The
rate is found to be 1494(15), 1491(16), 1488(18), and 1475(16)
s for =-18.95, -16.0, -22.0, and +18.22 fm, respectively. The
theoretical uncertainties are due to the fitting procedure and radiative
corrections. Our results seem to exclude the possibility of constraining a
negative with an uncertainty of less than 3 fm through an
accurate determination of the muon capture rates, but the uncertainty on the
present empirical value will not complicate the interpretation of the
(forth-coming) experimental results for . Finally, a comparison with
the already available experimental data discourages the possibility of a bound
state.Comment: 5 pages, 2 figures; revisited version accepted for publication on
Phys. Rev.
Muon capture on light nuclei
This work investigates the muon capture reactions 2H(\mu^-,\nu_\mu)nn and
3He(\mu^-,\nu_\mu)3H and the contribution to their total capture rates arising
from the axial two-body currents obtained imposing the
partially-conserved-axial-current (PCAC) hypothesis. The initial and final A=2
and 3 nuclear wave functions are obtained from the Argonne v_{18} two-nucleon
potential, in combination with the Urbana IX three-nucleon potential in the
case of A=3. The weak current consists of vector and axial components derived
in chiral effective field theory. The low-energy constant entering the vector
(axial) component is determined by reproducting the isovector combination of
the trinucleon magnetic moment (Gamow-Teller matrix element of tritium
beta-decay). The total capture rates are 393.1(8) s^{-1} for A=2 and 1488(9)
s^{-1} for A=3, where the uncertainties arise from the adopted fitting
procedure.Comment: 6 pages, submitted to Few-Body Sys
Second fundamental form of the Prym map in the ramified case
In this paper we study the second fundamental form of the Prym map in the ramified case .
We give an expression of it in terms of the second fundamental form of the
Torelli map of the covering curves. We use this expression to give an upper
bound for the dimension of a germ of a totally geodesic submanifold, and hence
of a Shimura subvariety of , contained in the
Prym locus.Comment: To appear in Galois Covers, Grothendieck-Teichmueller Theory and
Dessins d'Enfants - Interactions between Geometry, Topology, Number Theory
and Algebra. Springer Proceedings in Mathematics & Statistics. arXiv admin
note: text overlap with arXiv:1711.0342
The proton-proton weak capture in chiral effective field theory
The astrophysical S-factor for proton-proton weak capture is calculated in
chiral effective field theory over the center-of-mass relative-energy range
0--100 keV. The chiral two-nucleon potential derived up to
next-to-next-to-next-to leading order is augmented by the full electromagnetic
interaction including, beyond Coulomb, two-photon and vacuum-polarization
corrections. The low-energy constants (LEC's) entering the weak current
operators are fixed so as to reproduce the A=3 binding energies and magnetic
moments, and the Gamow-Teller matrix element in tritium beta decay.
Contributions from S and P partial waves in the incoming two-proton channel are
retained. The S-factor at zero energy is found to be S(0)=(4.030 +/- 0.006) x
10^{-23} MeV fm^2, with a P-wave contribution of 0.020 x 10^{-23} MeV fm^2. The
theoretical uncertainty is due to the fitting procedure of the LEC's and to the
cutoff dependence.Comment: 4 pages, 3 figures; revisited version accepted for publication on
Phys. Rev. Lett. A misprint in Table II has been correcte
DESIGN OF A STATED RANKING EXPERIMENT TO STUDY INTERACTIVE FREIGHT BEHAVIOUR: AN APPLICATION TO ROME'S LTZ
City logistics policies require an understanding of several issues (e.g. freight distribution context, preferences and relationship among agents) seldom accounted for in current research. Policies run the risk of producing unsatisfactory results because behavioural and contextual aspects are not considered. The acquisition of relevant data is crucial to test hypothesis and forecast agents' reactions to policy changes. Despite recent methodological advances in modelling interactive behaviour the development of apt survey instruments is still lacking to test innovative policies acceptability. This paper expands and innovate the methodological literature by describing a stated ranking experiment to study freight agent interactive behaviour and discusses the experimental design implemented to incorporate agent-specific priors when efficient design techniques are employed.urban freight distribution, group decision making, agent-specific interaction, stated preference, stated ranking experiments
Electromagnetic Structure of the Trinucleons
The electromagnetic form factors of the trinucleons 3H and 3He are calculated
with wave functions obtained with the Argonne AV18 two-nucleon and Urbana IX
three-nucleon interactions. Full account is taken of the two-body currents
required by current conservation with the AV18 interaction as well as those
associated with N-Delta transition currents and the currents of Delta resonance
components in the wave functions. Explicit three-nucleon current operators
associated with the two-pion exchange three-nucleon interaction arising from
irreducible S-wave pion-nucleon scattering is constructed and shown to have
very little effect on the calculated magnetic form factors. The calculated
magnetic form factor of 3H, and charge form factors of both 3H and 3He are in
satisfactory agreement with the experimental data. However, the position of the
zero in the magnetic form factor of 3He is slightly underpredicted.Comment: 27 pages RevTex file, 19 PostScript figures, submitted to Phys. Rev.
Astrophysical implications of the proton-proton cross section updates
The p(p,e^+ \nu_e)^2H reaction rate is an essential ingredient for
theoretical computations of stellar models. In the past several values of the
corresponding S-factor have been made available by different authors. Prompted
by a recent evaluation of S(E), we analysed the effect of the adoption of
different proton-proton reaction rates on stellar models, focusing, in
particular, on the age of mid and old stellar clusters (1-12 Gyr) and on
standard solar model predictions. By comparing different widely adopted p(p,e^+
\nu_e)^2H reaction rates, we found a maximum difference in the temperature
regimes typical of main sequence hydrogen-burning stars (5x10^6 - 3x10^7 K) of
about 3%. Such a variation translates into a change of cluster age
determination lower than 1%. A slightly larger effect is observed in the
predicted solar neutrino fluxes with a maximum difference, in the worst case,
of about 8%. Finally we also notice that the uncertainty evaluation of the
present proton-proton rate is at the level of few \permil, thus the p(p,e^+
\nu_e)^2H reaction rate does not constitute anymore a significant uncertainty
source in stellar models.Comment: accepte
Comparative study of three-nucleon force models in systems
Using modern nucleon-nucleon interactions in the description of the
nuclei, it is not possible to reproduce both the three- and four-nucleon
binding energies simultaneously. This is one manifestation of the necessity of
including a three-nucleon force in the nuclear Hamiltonian. In this paper we
will perform a comparative study of some, widely used, three-nucleon force
models. We will analyze their capability to describe the aforementioned binding
energies as well as the doublet scattering length. A correct description
of these quantities can be considered a stringent requirement for a nuclear
Hamiltonian containing two- and three-nucleon interaction terms. As we will
show, this requirement is not fulfilled by several of the models available in
the literature. To satisfy it, we propose modifications in the parametrization
of the three-nucleon forces and we study their effects on few selected
low energy scattering observables.Comment: 30 pages, 10 figure
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