523 research outputs found
Global superscaling analysis of quasielastic electron scattering with relativistic effective mass
We present a global analysis of the inclusive quasielastic electron
scattering data with a superscaling approach with relativistic effective mass.
The SuSAM* model exploits the approximation of factorization of the scaling
function out of the cross section under quasifree conditions. Our
approach is based on the relativistic mean field theory of nuclear matter where
a relativistic effective mass for the nucleon encodes the dynamics of nucleons
moving in presence of scalar and vector potentials. Both the scaling variable
and the single nucleon cross sections include the effective mass as a
parameter to be fitted to the data alongside the Fermi momentum . Several
methods to extract the scaling function and its uncertainty from the data are
proposed and compared. The model predictions for the quasielastic cross section
and the theoretical error bands are presented and discussed for nuclei along
the periodic table from to : H, H, He, He,
C, Li, Be, Mg, Ni,
Y, Sn, Ta, W, Au, O, Al,
Ca, Ca, Fe, Pb, and U.
We find that more than 9000 of the total data fall within the
quasielastic theoretical bands. Predictions for Ti and Ar are
also provided for the kinematics of interest to neutrino experiments.Comment: 26 pages, 20 figures and 4 table
Relativistic effects in two-particle emission for electron and neutrino reactions
Two-particle two-hole contributions to electroweak response functions are
computed in a fully relativistic Fermi gas, assuming that the electroweak
current matrix elements are independent of the kinematics. We analyze the
genuine kinematical and relativistic effects before including a realistic
meson-exchange current (MEC) operator. This allows one to study the
mathematical properties of the non-trivial seven-dimensional integrals
appearing in the calculation and to design an optimal numerical procedure to
reduce the computation time. This is required for practical applications to CC
neutrino scattering experiments, where an additional integral over the neutrino
flux is performed. Finally we examine the viability of this model to compute
the electroweak 2p-2h response functions.Comment: Major revision (shortened). 22 pages, 18 figure
2p-2h excitations in neutrino scattering: angular distribution and frozen approximation
We study the phase-space dependence of 2p-2h excitations in neutrino
scattering using the relativistic Fermi gas model. We follow a similar approach
to other authors, but focusing in the phase-space properties, comparing with
the non-relativistic model. A careful mathematical analysis of the angular
distribution function for the outgoing nucleons is performed. Our goals are to
optimize the CPU time of the 7D integral to compute the hadron tensor in
neutrino scattering, and to conciliate the different relativistic and non
relativistic models by describing general properties independently of the
two-body current. For some emission angles the angular distribution becomes
infinite in the Lab system, and we derive a method to integrate analytically
around the divergence. Our results show that the frozen approximation, obtained
by neglecting the momenta of the two initial nucleons inside the integral of
the hadron tensor, reproduces fairly the exact response functions for constant
current matrix elements.Comment: 8 pages, 4 figures. Contribution to 16th International Workshop on
Neutrino Factories and Future Neutrino Beam Facilities, 25-30 August, 2014.
Held at University of Glasgow, United Kingdo
The frozen nucleon approximation in two-particle two-hole response functions
We present a fast and efficient method to compute the inclusive two-particle
two-hole (2p-2h) electroweak responses in the neutrino and electron
quasielastic inclusive cross sections. The method is based on two
approximations. The first neglects the motion of the two initial nucleons below
the Fermi momentum, which are considered to be at rest. This approximation,
which is reasonable for high values of the momentum transfer, turns out also to
be quite good for moderate values of the momentum transfer . The
second approximation involves using in the "frozen" meson-exchange currents
(MEC) an effective -propagator averaged over the Fermi sea. Within the
resulting "frozen nucleon approximation", the inclusive 2p-2h responses are
accurately calculated with only a one-dimensional integral over the emission
angle of one of the final nucleons, thus drastically simplifying the
calculation and reducing the computational time. The latter makes this method
especially well-suited for implementation in Monte Carlo neutrino event
generators.Comment: 8 pages, 5 figures and 1 tabl
Two-nucleon emission in neutrino and electron scattering from nuclei: the modified convolution approximation
The theoretical formalism of inclusive lepton-nucleus scattering in the
two-nucleon emission channel is discussed in the context of a simplified
approach, the modified convolution approximation. This allows one to write the
2p2h responses of the relativistic Fermi gas as a folding integral of two 1p1h
responses with the energies and momenta transferred to each nucleon. The idea
behind this method is to introduce different average momenta for the two
initial nucleons in the matrix elements of the two-body current, with the
innovation that they depend on the transferred energies and momenta. This
method treats exactly the two-body phase space kinematics, and reduces the
formulae of the response functions from seven-dimensional integrals over
momenta to much simpler three-dimensional ones. The applicability of the method
is checked by comparing with the full results within a model of electroweak
meson-exchange currents. The predictions are accurate enough, especially in the
low-energy threshold region where the average momentum approximation works the
best.Comment: 35 pages, 13 figure
Center of mass momentum dependence of short-range correlations with the coarse-grained Granada potential
The effect of the center of mass motion on the high-momentum distributions of
correlated nucleon pairs is studied by solving the Bethe-Goldstone equation in
nuclear matter with the Granada nucleon-nucleon potential. We show that this
coarse-grained potential reduces the problem to an algebraic linear system of
five (ten) equations for uncoupled (coupled) partial waves that can be easily
solved. The corresponding relative wave functions of correlated pn, pp and nn
pairs are computed for different values of their CM momentum. We find that the
pn pairs dominate the high-momentum tail of the relative momentum distribution,
and that this only depends marginally on center of mass momentum. Our results
provide further justification and agreement for the factorization approximation
commonly used in the literature. This approximation assumes that the momentum
distribution of nucleon pairs can be factorized as the product of the center of
mass momentum distribution and the relative momentum distribution.Comment: 27 pages, 12 figures, corrected the treatment of the coupled
nucleon-nucleon partial waves, new authors adde
Nuclear dependence of the 2p2h electroweak response in the Relativistic Fermi Gas model
We present the results of a recent study of meson-exchange two-body currents
in lepton-nucleus inclusive scattering at various kinematics and for different
nuclei within the Relativistic Fermi Gas model. We show that the associated
nuclear response functions at their peaks scale as , for Fermi
momentum going from 200 to 300 MeV/c and momentum transfer from
to 2 GeV/c. This behavior is different from what is found for the
quasielastic response, which scales as . This result can be valuable in
the analyses of long-baseline neutrino oscillation experiments, which need to
implement these nuclear effects in Monte Carlo simulations for different
kinematics and nuclear targets.Comment: 11 pages, 6 figures, Proccedings of the Workshop "Advanced Aspects in
Nuclear Structure and Reactions at Different Energy Scales", 25-28 April
2017, Arbanasi, Bulgari
Emission of neutron-proton and proton-proton pairs in electron scattering induced by meson-exchange currents
We use a relativistic model of meson-exchange currents to compute the
proton-neutron and proton-proton yields in scattering from C in
the 2p-2h channel. We compute the response functions and cross section with the
relativistic Fermi gas model for a range of kinematics from intermediate to
high momentum transfers. We find a large contribution of neutron-proton
configurations in the initial state, as compared to proton-proton pairs. The
different emission probabilities of distinct species of nucleon pairs are
produced in our model only by meson-exchange currents, mainly by the
isobar current. We also analyze the effect of the exchange contribution and
show that the direct/exchange interference strongly affects the determination
of the np/pp ratio.Comment: 5 pages, 6 figure
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