164 research outputs found
Predicting the distributions of under-recorded Odonata using species distribution models
1. Absences in distributional data may result either from the true absence of a species or from a false absence due to lack of recording effort. I use general linear models (GLMs) and species distribution models (SDMs) to investigate this problem in North American Odonata and present a potential solution. 2. I use multi-model selection methods based on Akaike's information criterion to evaluate the ability of water-energy variables, human population density, and recording effort to explain patterns of odonate diversity in the USA and Canada using GLMs. Water-energy variables explain a large proportion of the variance in odonate diversity, but the residuals of these models are significantly related to recorder effort. 3. I then create SDMs for 176species that are found solely in the USA and Canada using model averaging of eight different methods. These give predictions of hypothetical true distributions of each of the 176species based on climate variables, which I compare with observed distributions to identify areas where potential under-recording may occur. 4. Under-recording appears to be highest in northern Canada, Alaska, and Quebec, as well as the interior of the USA. The proportion of predicted species that have been observed is related to recorder effort and population density. Maps for individual species have been made available online () to facilitate recording in the future. 5. This analysis has illustrated a problem with current odonate recording in the form of unbalanced recorder effort. However, the SDM approach also provides the solution, targeting recorder effort in such a way as to maximise returns from limited resources
Parity-violating longitudinal response
The longitudinal quasielastic parity-violating electron scattering response
is explored within the context of a model that builds antisymmetrized RPA-HF
correlations on a relativistic Fermi gas basis. The large sensitivity to
nuclear dynamics of this observable, found in previous studies where only
pionic correlations were included, is shown to survive in the present model
where the effects from pion, rho, sigma and omega exchange in a version of the
Bonn potential are incorporated. Through an intricate diagrammatic
cancellation/filtration mechanism the longitudinal parity-violating response
turns out to be close to the one obtained in first-order perturbation theory
with the pion alone. Finally, in accord with our previous work, the
parity-violating response is seen to display appreciable sensitivity to the
electric strangeness content of the nucleon, especially at high momentum
transfer.Comment: 13 pages, uses REVTeX and epsfig, 10 postscript figures; a postscript
version of the paper is available by anonymous ftp at
ftp://carmen.to.infn.it/pub/barbaro/papers/951
Parity-Violating Electron Scattering from the Pion-Correlated Relativistic Fermi Gas
Parity-violating quasielastic electron scattering is studied within the
context of the relativistic Fermi gas and its extensions to include the effects
of pionic correlations and meson-exchange currents. The work builds on previous
studies using the same model; here the part of the parity-violating asymmetry
that contains axial-vector hadronic currents is developed in detail using those
previous studies and a link is provided to the transverse vector-isovector
response. Various integrated observables are constructed from the differential
asymmetry. These include an asymmetry averaged over the quasielastic peak, as
well as the difference of the asymmetry integrated to the left and right of the
peak -- the latter is shown to be optimal for bringing out the nature of the
pionic correlations. Special weighted integrals involving the differential
asymmetry and electromagnetic cross section, based on the concepts of y-scaling
and sum rules, are constructed and shown to be suited to studies of the
single-nucleon form factor content in the problem, in particular, to
determinations of the isovector/axial-vector and electric strangeness form
factors. Comparisons are also made with recent predictions made on the basis of
relativistic mean-field theory.Comment: 28 pages, LATeX, 13 figures (tar-compressed postscript files,
available from the authors), MIT preprint CTP#222
Influence of nucleonic motion in Relativistic Fermi Gas inclusive responses
Impulsive hadronic descriptions of electroweak processes in nuclei involve
two distinctly different elements: one stems from the nuclear many-body physics
--- the medium --- which is rather similar for the various inclusive response
functions, and the other embodies the responses of the hadrons themselves to
the electroweak probe and varies with the channel selected. In this letter we
investigate within the context of the relativistic Fermi gas in both the
quasi-elastic and regimes the interplay between these two
elements. Specifically, we focus on expansions in the one small parameter in
the problem, namely, the momentum of a nucleon in the initial wave function
compared with the hadronic scale, the nucleon mass. Both parity-conserving and
-violating inclusive responses are studied and the interplay between
longitudinal () and transverse ( and ) contributions is highlighted.Comment: 11 pages, 1 figur
Spin Observables in Coincidence Electron Scattering from Nuclei I: Reduced Response Functions
A theoretical description of nucleon knockout reactions initiated by
polarized electron scattering from polarized nuclei is presented. Explicit
expressions for the complete set of reduced response functions (independent of
the polarization angle) that can be experimentally obtained assuming plane
waves for the electron are given in a general multipole expansion. The
formalism is applied to the particular case of closed-shell-minus-one nuclei
using two models for the ejected nucleon, including the final-state interaction
phenomenologically with a complex optical potential and in the factorized
plane-wave impulse approximation. Relativistic effects in the kinematics and in
the electromagnetic current are incorporated throughout --- specifically a new
expansion of the electromagnetic current in powers only of the struck nucleon
momentum is employed. Results are presented for the nucleus 39K.Comment: 51 pages (LaTeX), 17 figures (postScript
Superscaling and Charge-changing Neutrino Cross Sections
The superscaling function extracted from inclusive electron scattering data
is used to predict high energy charge-changing neutrino cross sections in the
quasi-elastic and regions.Comment: 3 pages, 2 figures, to appear in the Proceedings of the 7th
International Workshop on Neutrino Factories and Superbeams, Laboratori
Nazionali di Frascati, Frascati (Rome), June 21 - 26, 200
Nuclear response functions for the N-N*(1440) transition
Parity-conserving and -violating response functions are computed for the
inclusive electroexcitation of the N*(1440)(Roper) resonance in nuclear matter
modeled as a relativistic Fermi gas. Using various empirical parameterizations
and theoretical models of the N-N*(1440) transition form factors, the
sensitivity of the response functions to details of the structure of the Roper
resonance is investigated. The possibility of disentangling this resonance from
the contribution of Delta electroproduction in nuclei is addressed. Finally,
the contributions of the Roper resonance to the longitudinal scaling function
and to the Coulomb sum rule are also explored.Comment: 25 pages, 10 figures. Minor changes in the Introduction. Accepted in
NP
The 2p-2h electromagnetic response in the quasielastic peak and beyond
The contribution to the nuclear transverse response function R_T arising from
two particle-two hole (2p-2h) states excited through the action of
electromagnetic meson exchange currents (MEC) is computed in a fully
relativistic framework. The MEC considered are those carried by the pion and by
Delta degrees of freedom, the latter being viewed as a virtual nucleonic
resonance. The calculation is performed in the relativistic Fermi gas model in
which Lorentz covariance can be maintained. All 2p-2h many-body diagrams
containing two pionic lines that contribute to R_T are taken into account and
the relative impact of the various components of the MEC on R_T is addressed.
The non-relativistic limit of the MEC contributions is also discussed and
compared with the relativistic results to explore the role played by relativity
in obtaining the 2p-2h nuclear response.Comment: 27 pages, 12 figures, revtex4; minor modifications in the discussion
of the results, references adde
The Pion in Electromagnetic and Weak Neutral Current Nuclear Response Functions
The impact of pionic correlations and meson--exchange currents in determining
the (vector) response functions for electroweak quasielastic lepton scattering
from nuclei is discussed. The approach taken builds on previous work where the
Fermi gas model is used to maintain consistency in treating forces and currents
(gauge invariance) and to provide a Lorentz covariant framework. Results
obtained in first-order perturbation theory are compared with infinite-order
summation schemes (HF and RPA) and found to provide quite successful
approximations for the quasielastic response functions. The role of pionic
correlations in hardening the responses R_L and R_T is investigated in some
detail, including studies of the relative importance of central and tensor
pieces of the force and of exchange and self-energy diagrams; in addition,
their role in significantly modifying the longitudinal parity-violating
response R_{AV}^L is explored. The MEC are shown to provide a small, but
non-negligible, contribution in determining the vector responses.Comment: TeX, 21 figures (Postscript, available from the authors), MIT
preprint CTP\#219
Inclusive quasielastic scattering of polarized electrons from polarized nuclei
The inclusive quasielastic response functions that appear in the scattering
of polarized electrons from polarized nuclei are computed and analyzed for
several closed-shell-minus-one nuclei with special attention paid to 39K.
Results are presented using two models for the ejected nucleon --- when
described by a distorted wave in the continuum shell model or by a plane wave
in PWIA with on- and off-shell nucleons. Relativistic effects in kinematics and
in the electromagnetic current have been incorporated throughout. Specifically,
the recently obtained expansion of the electromagnetic current in powers only
of the struck nucleon's momentum is employed for the on-shell current and the
effects of the first-order terms (spin-orbit and convection) are compared with
the zeroth-order (charge and magnetization) contributions. The use of polarized
inclusive quasielastic electron scattering as a tool for determining
near-valence nucleon momentum distributions is discussed.Comment: 51 LaTeX pages, 14 Postscript figure
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