7,868 research outputs found
Fermion propagators in space-time
The one- and the two-particle propagators for an infinite non-interacting
Fermi system are studied as functions of space-time coordinates. Their
behaviour at the origin and in the asymptotic region is discussed, as is their
scaling in the Fermi momentum. Both propagators are shown to have a divergence
at equal times. The impact of the interaction among the fermions on their
momentum distribution, on their pair correlation function and, hence, on the
Coulomb sum rule is explored using a phenomenological model. Finally the
problem of how the confinement is reflected in the momentum distribution of the
system's constituents is briefly addressed.Comment: 26 pages, 9 figures, accepted for publication on Phys. Rev.
Relativistic Hamiltonians in many-body theories
We discuss the description of a many-body nuclear system using Hamiltonians
that contain the nucleon relativistic kinetic energy and potentials with
relativistic corrections. Through the Foldy-Wouthuysen transformation, the
field theoretical problem of interacting nucleons and mesons is mapped to an
equivalent one in terms of relativistic potentials, which are then expanded at
some order in 1/m_N. The formalism is applied to the Hartree problem in nuclear
matter, showing how the results of the relativistic mean field theory can be
recovered over a wide range of densities.Comment: 14 pages, uses REVTeX and epsfig, 3 postscript figures; a postscript
version of the paper is available by anonymous ftp at
ftp://carmen.to.infn.it/pub/depace/papers/951
New Measurements of Nucleon Structure Functions from the CCFR/NuTeV Collaboration
We report on the extraction of the structure functions F_2 and Delta xF_3 =
xF_3nu-xF_3nubar from CCFR neutrino-Fe and antineutrino-Fe differential cross
sections. The extraction is performed in a physics model independent (PMI) way.
This first measurement for Delta xF_3, which is useful in testing models of
heavy charm production, is higher than current theoretical predictions. The F_2
(PMI) values measured in neutrino and muon scattering are in good agreement
with the predictions of Next to Leading Order PDFs (using massive charm
production schemes), thus resolving the long-standing discrepancy between the
two sets of data.Comment: 5 pages. Presented by Arie Bodek at the CIPNAP2000 Conference, Quebec
City, May 200
Meson-exchange currents and quasielastic predictions for charged-current neutrino-12C scattering in the superscaling approach
We evaluate and discuss the impact of meson-exchange currents (MECs) on
charged-current quasielastic neutrino cross sections. We consider the nuclear
transverse response arising from two-particle two-hole states excited by the
action of electromagnetic, purely isovector meson-exchange currents in a fully
relativistic framework based on the work by the Torino Collaboration [A. D.
Pace, M. Nardi, W. M. Alberico, T. W. Donnelly, and A. Molinari, Nucl. Phys.
A726, 303 (2003)]. An accurate parametrization of this MEC response as a
function of the momentum and energy transfers involved is presented. Results of
neutrino-nucleus cross sections using this MEC parametrization together with a
recent scaling approach for the one-particle one-hole contributions (named
SuSAv2) are compared with experimental data (MiniBooNE, MINERvA, NOMAD and T2K
Collaborations).Comment: 16 pages, 19 figure
Inclusive versus Exclusive EM Processes in Relativistic Nuclear Systems
Connections are explored between exclusive and inclusive electron scattering
within the framework of the relativistic plane-wave impulse approximation,
beginning with an analysis of the model-independent kinematical constraints to
be found in the missing energy--missing momentum plane. From the interplay
between these constraints and the spectral function basic features of the
exclusive and inclusive nuclear responses are seen to arise. In particular, the
responses of the relativistic Fermi gas and of a specific hybrid model with
confined nucleons in the initial state are compared in this work. As expected,
the exclusive responses are significantly different in the two models, whereas
the inclusive ones are rather similar. By extending previous work on the
relativistic Fermi gas, a reduced response is introduced for the hybrid model
such that it fulfills the Coulomb and the higher-power energy-weighted sum
rules. While incorporating specific classes of off-shellness for the struck
nucleons, it is found that the reducing factor required is largely
model-independent and, as such, yields a reduced response that is useful for
extracting the Coulomb sum rule from experimental data. Finally, guided by the
difference between the energy-weighted sum rules of the two models, a version
of the relativistic Fermi gas is devised which has the 0, 1 and 2 moments of the charge response which agree rather well
with those of the hybrid model: this version thus incorporates {\em a priori}
the binding and confinement effects of the stuck nucleons while retaining the
simplicity of the original Fermi gas.Comment: LaTex file with 15 .ps figure
Inelastic electron-nucleus scattering and scaling at high inelasticity
Highly inelastic electron scattering is analyzed within the context of the
unified relativistic approach previously considered in the case of quasielastic
kinematics. Inelastic relativistic Fermi gas modeling that includes the
complete inelastic spectrum - resonant, non-resonant and Deep Inelastic
Scattering - is elaborated and compared with experimental data. A
phenomenological extension of the model based on direct fits to data is also
introduced. Within both models, cross sections and response functions are
evaluated and binding energy effects are analyzed. Finally, an investigation of
the second-kind scaling behavior is also presented.Comment: 39 pages, 13 figures; formalism extended and slightly reorganized,
conclusions extended; to appear in Phys. Rev.
HCG 16 Revisited: Clues About Galaxy Evolution in Groups
We present new spectroscopic observations of 5 galaxies, members of the
unusually active compact group HCG 16, observed using the Palomar 5m telescope.
The high signal to noise ratios (S/N ) of the spectra allow us to
study the variation of the emission line characteristics and the stellar
populations in the nucleus and the circumnuclear regions of the galaxies. The
emission line characteristics of these galaxies are complex, varying between
Seyfert 2 and LINERs or between LINERs and starbursts. All of the galaxies show
traces of intermediate age stellar populations, supporting our previous result
that post-starburst galaxies are common in compact groups. The galaxies
HCG16--4 and HCG16--5 show double nuclei and therefore could be two cases of
recent merger. Our observations support a scenario where HCG 16 was formed by
the successive merger of metal poor, low mass galaxies. The galaxies HCG16--1
and HCG16--2, which are more evolved, form the old core of the group. Galaxies
HCG16--4 and HCG16--5 are two more recent additions still in a merging phase.
Galaxy HCG16--5 is a starburst galaxy which is just beginning to fall into the
core. If HCG 16 is representative of compact groups in their early stage, the
whole set of observations implies that the formation of compact groups is the
result of hierarchical galaxy formation. HCG 16 could be one example of this
process operating in the local universe.Comment: tar file containing text and figures is available at
http://www.daf.on.br/~reinaldo/paper.htm
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