165 research outputs found
Neutrino Induced Coherent Pion Production off Nuclei and PCAC
We review the Rein--Sehgal model and criticize its use for low energy
neutrino induced coherent pion production. We have studied the validity of the
main approximations implicit in that model, trying to compare with physical
observables when that is possible and with microscopical calculations. Next, we
have tried to elaborate a new improved model by removing the more problematic
approximations, while keeping the model still reasonably simple. Last, we have
discussed the limitations intrinsic to any approach based on the partial
conservation of the axial current hypothesis. In particular, we have shown the
inability of such models to determine the angular distribution of the outgoing
pion with respect to the direction of the incoming neutrino, except for the
kinematical point.Comment: 19 latex pages, 7 figures, 1 table. Version accepted for publication
in Physical Review
Theoretical study of neutrino-induced coherent pion production off nuclei at T2K and MiniBooNE energies
We have developed a model for neutrino-induced coherent pion production off
nuclei in the energy regime of interest for present and forthcoming neutrino
oscillation experiments. It is based on a microscopic model for pion production
off the nucleon that, besides the dominant Delta pole contribution, takes into
account the effect of background terms required by chiral symmetry. Moreover,
the model uses a reduced nucleon-to-Delta resonance axial coupling, which leads
to coherent pion production cross sections around a factor two smaller than
most of the previous theoretical estimates. In the coherent production, the
main nuclear effects, namely medium corrections on the Delta propagator and the
final pion distortion, are included. We have improved on previous similar
models by taking into account the nucleon motion and employing a more
sophisticated optical potential. As found in previous calculations the
modification of the Delta self-energy inside the nuclear medium strongly
reduces the cross section, while the final pion distortion mainly shifts the
peak position to lower pion energies. The angular distribution profiles are not
much affected by nuclear effects. Nucleon motion increases the cross section by
15% at neutrino energies of 650 MeV, while Coulomb effects on charged pions are
estimated to be small. Finally, we discuss at length the deficiencies of the
Rein-Sehgal pion coherent production model for neutrino energies below 2 GeV,
and in particular for the MiniBooNE and T2K experiments. We also predict flux
averaged cross sections for these two latter experiments and K2K.Comment: 19 latex pages, 10 figures, 2 tables. Minor changes. Version accepted
for publication in Physical Review
Neutral current coherent pion production
We investigate the neutrino induced coherent pion production reaction at low
and intermediate energies. The model includes pion, nucleon and Delta(1232)
resonance as the relevant hadronic degrees of freedom. Nuclear medium effects
on the production mechanisms and pion distortion are taken into account. We
obtain that the dominance of the Delta excitation holds due to large
cancellations among the background contributions. We consider two sets of
vector and axial-vector N-Delta transition form-factors, evidencing the strong
sensitivity of the results to the axial coupling C5A(0). The differences
between neutrino and antineutrino cross sections, emerging from interference
terms, are also discussed.Comment: 4 pages, 3 figures, minor corrections, a few references adde
Nuclear Effects in Neutrino Induced Coherent Pion Production at K2K and MiniBooNE Neutrino Energies
The coherent pion production induced by neutrinos in nuclei is studied using
a delta hole model in local density approximation taking into account the
renormalization of properties in a nuclear medium. The pion absorption
effects have been included in an eikonal approximation. These effects give a
large reduction in the total cross section. The numerical results for the total
cross section are found to be consistent with recent experimental results from
K2K and MiniBooNE collaborations and other older experiments in the
intermediate energy region.Comment: 4pages, 5figure
Components of the concept «pedagogical scenario»
This paper presents an analysis of the concept of "pedagogical scenario". Various aspects of the concept are presented. The result is a list of the main components of the concept in terms of its application in electronic courses.В работе представлен анализ понятия «педагогический сценарий». Рассмотрены разные аспекты этого понятия. В результате представлен список основных компонент понятия с точки зрения применения его в электронных курсах
Coherent pion production by neutrinos on nuclei
The main part of coherent pion production by neutrinos on nuclei is
essentially determined by PCAC, provided that the leptonic momentum transferred
square Q^2 remains sufficiently small. We give the formulas for the charged and
neutral current cross sections, including also the small non-PCAC transverse
current contributions and taking into account the effect of the \mu^- mass. Our
results are compared with the experimental ones and other theoretical
treatments.Comment: 18 pages, 4 figure
Breakdown of PCAC in diffractive neutrino interactions
We test the hypothesis of partially conserved axial current (PCAC) in high
energy diffractive neutrino production of pions. Since the pion pole
contribution to the Adler relation (AR) is forbidden by conservation of the
lepton current, the heavier states, like the a_1 pole, \rho-\pi-cut, etc.,
control the lifetime of the hadronic fluctuations of the neutrino. We evaluate
the deviation from the AR in diffractive neutrino-production of pions on proton
and nuclear targets. At high energies, when all the relevant time scales
considerably exceed the size of the target, the AR explicitly breaks down on an
absorptive target, such as a heavy nucleus. In this regime, close to the black
disc limit, the off-diagonal diffractive amplitudes vanish, while the diagonal
one, \pi->\pi, which enters the AR, maximizes and saturates the unitarity
bound. At lower energies, in the regime of short lifetime of heavy hadronic
fluctuations the AR is restored, i.e. it is not altered by the nuclear effects.Comment: 10 pages, 5 figure
Masses and widths of scalar-isoscalar multi-channel resonances from data analysis
Peculiarities of obtaining parameters for broad multi-channel resonances from
data are discussed analyzing the experimental data on processes
in the channel in a
model-independent approach based on analyticity and unitarity and using an
uniformization procedure. We show that it is possible to obtain a good
description of the scattering data from the threshold to 1.89 GeV with
parameters of resonances cited in the PDG tables as preferred. However, in this
case, first, representation of the background is unsatisfactory;
second, the data on the coupled process are not well
described even qualitatively above 1.15 GeV when using the resonance parameters
from the only scattering analysis. The combined analysis of these
coupled processes is needed, which is carried out satisfactorily. Then both
above-indicated flaws, related to the analysis of solely the
-scattering, are cured. The most remarkable change of parameters with
respect to the values of only scattering analysis appears for the mass
of the which is now in some accordance with the Weinberg prediction
on the basis of mended symmetry and with an analysis using the large-
consistency conditions between the unitarization and resonance saturation. The
obtained -scattering length in case when we restrict to the
analysis of the scattering or consider so-called A-solution (with a
lower mass and width of meson) agrees well with prediction of chiral
perturbation theory (ChPT) and with data extracted at CERN by the NA48/2
Collaboration from the analysis of the decay and by the DIRAC
Collaboration from the measurement of the lifetime.Comment: 21 pages, 3 figures, 6 table
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