8,978 research outputs found
Gauge and Lorentz invariant pionic correlations in quasi-elastic electron scattering
The role of the pion in the parity-conserving and parity-violating
quasi-elastic nuclear response functions is analyzed within a relativistic
model which fulfills gauge invariance.Comment: 14 pages, 4 figures, Proceedings of the Varenna School "From Nuclei
and Thier Constituents to Stars", August 200
The Determination of the Star Formation Rate in Galaxies
A spectrophotometric model able to compute the integrated spectrum of a
galaxy, including the contribution both of the stellar populations and of the
ionized interstellar gas of the HII regions powered by young hot stars, has
been used to study several spectral features and photometric quantities in
order to derive calibrations of the star formation history of late type
galaxies. Attention has been paid to analyze the emission of the Balmer lines
and the [OII]3727 line to test their attitude at providing estimates
of the present star formation rate in galaxies. Other features, like D
and the equivalent width of the H line, influenced by the presence
of intermediate age stars, have been considered. Several ways of estimating the
star formation rates in normal galaxies are discussed and some considerations
concerning the applicability of the models are presented. Criteria have been
also studied for ascertaining the presence of a burst, current or ended not
long ago. Bursts usually hinder the determination of the past star formation
rate.Comment: 21 pages, needs A&A l-aa.sty, accepted for publication in Astronomy &
Astrophysic
On the analytic solution of the pairing problem: one pair in many levels
We search for approximate, but analytic solutions of the pairing problem for
one pair of nucleons in many levels of a potential well. For the collective
energy a general formula, independent of the details of the single particle
spectrum, is given in both the strong and weak coupling regimes. Next the
displacements of the solutions trapped in between the single particle levels
with respect to the unperturbed energies are explored: their dependence upon a
suitably defined quantum number is found to undergo a transition between two
different regimes.Comment: 30 pages, AMS Latex, 8 figures. Submitted to Phys. Rev.
Lepton mass effects in the Bethe-Heitler process
We develop the full finite lepton mass formalism for the production of real
photons via the Bethe-Heitler reaction of unpolarized leptons off unpolarized
nucleons. Genuine lepton mass effects are described, in particular their
dependence upon the lepton mass and the initial beam energy, as well as their
sensitivity to the nucleon isospin. In the minimum momentum transfer region,
these effects dominate the muon induced proton cross section and become
significant for electron scattering at small .Comment: Final version including errat
The pairing Hamiltonian for one pair of identical nucleons bound in a potential well
The problem of one pair of identical nucleons sitting in single
particle levels of a potential well and interacting through the pairing force
is treated introducing even Grassmann variables. The eigenvectors are
analytically expressed solely in terms of these with coefficients fixed by the
eigenvalues and the single particle energies. When the latter are those of an
harmonic oscillator well an accurate expression is derived for both the
collective eigenvalue and for those trapped in between the single particle
levels, for any strength of the pairing interaction and for any number of
levels. Notably the trapped solutions are labelled through an index upon which
they depend parabolically.Comment: 5 pages, 1 postscript figur
The many levels pairing Hamiltonian for two pairs
We address the problem of two pairs of fermions living on an arbitrary number
of single particle levels of a potential well (mean field) and interacting
through a pairing force. The associated solutions of the Richardson's equations
are classified in terms of a number , which reduces to the seniority
in the limit of large values of the pairing strength and yields the number
of pairs not developing a collective behaviour, their energy remaining finite
in the limit. We express analytically, through the moments of the
single particle levels distribution, the collective mode energy and the two
critical values and of the coupling which can
exist on a single particle level with no pair degeneracy. Notably and merge when the number of single particle levels
goes to infinity, where they coincide with the (when it exists) of
a one pair system, not envisioned by the Richardson theory. In correspondence
of the system undergoes a transition from a mean field to a
pairing dominated regime. We finally explore the behaviour of the excitation
energies, wave functions and pair transfer amplitudes finding out that the
former, for , come close to the BCS predictions, whereas the
latter display a divergence at , signaling the onset of a long
range off-diagonal order in the system.Comment: 35 pages, 6 figures, 2 tables, to be published in EPJ
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