4,226 research outputs found

### Quasielastic K+ scattering in nuclei

The quasielastic scattering kaon-nucleus experiment performed at BNL is
analyzed in a finite nucleus continuum random phase approximation framework,
treating the reaction mechanism in Glauber theory up to two-step inelastic
processes. A good description of the data is achieved, also providing a useful
constraint on the strength of the effective particle-hole interaction in the
scalar-isoscalar channel at intermediate momentum transfers.Comment: 4 pages, 3 figures, needs espcrc1 and epsfig; presented at the
International Conference on Hypernuclear and Strange Particle Physics, BNL
October 13-18, 199

### Randomness in nuclei and in the quark-gluon plasma

The issue of averaging randomness is addressed, mostly in nuclear physics,
but shortly also in QCD. The Feshbach approach, so successful in dealing with
the continuum spectrum of the atomic nuclei ("optical model"), is extended to
encompass bound states as well ("shell model"). Its relationship with the
random-matrix theory is discussed and the bearing of the latter on QCD,
especially in connection with the spectrum of the Dirac operator, is briefly
touched upon. Finally the question of whether Feshbach's theory can cope with
the averaging required by QCD is considered.Comment: 24 pages, 6 figures; to appear in the Proceedings of the Workshop
"Quark-Gluon Plasma and Relativistic Heavy Ions", Frascati, 14-18 January
200

### Nuclear response functions with finite range Gogny force: tensor terms and instabilities

A fully-antisymmetrized random phase approximation calculation employing the
continued fraction technique is performed to study nuclear matter response
functions with the finite range Gogny force. The most commonly used parameter
sets of this force, as well as some recent generalizations that include the
tensor terms are considered and the corresponding response functions are shown.
The calculations are performed at the first and second order in the continued
fraction expansion and the explicit expressions for the second order tensor
contributions are given. Comparison between first and second order continued
fraction expansion results are provided. The differences between the responses
obtained at the two orders turn to be more pronounced for the forces including
tensor terms than for the standard Gogny ones. In the vector channels the
responses calculated with Gogny forces including tensor terms are characterized
by a large heterogeneity, reflecting the different choices for the tensor part
of the interaction. For sake of comparison the response functions obtained
considering a G-matrix based nuclear interaction are also shown. As first
application of the present calculation, the possible existence of spurious
finite-size instabilities of the Gogny forces with or without tensor terms has
been investigated. The positive conclusion is that all the Gogny forces, but
the GT2 one, are free of spurious finite-size instabilities. In perspective,
the tool developed in the present paper can be inserted in the fitting
procedure to construct new Gogny-type forces

### Statistical theory of the many-body nuclear system

A recently proposed statistical theory of the mean fields associated with the
ground and excited collective states of a generic many-body system is extended
by increasing the dimensions of the P-space. In applying the new framework to
nuclear matter, in addition to the mean field energies we obtain their
fluctuations as well, together with the ones of the wavefunctions, in first
order of the expansion in the complexity of the Q-space states. The physics
described by the latter is assumed to be random. To extract numerical
predictions out of our scheme we develop a schematic version of the approach,
which, while much simplified, yields results of significance on the size of the
error affecting the mean fields, on the magnitude of the residual effective
interaction, on the ground state spectroscopic factor and on the mixing
occurring between the vectors spanning the P-space.Comment: 27 pages, 3 figures; Dedicated to the memory of Herman Feshbac

### Spreading Widths of Doorway States

As a function of energy E, the average strength function S(E) of a doorway
state is commonly assumed to be Lorentzian in shape and characterized by two
parameters, the peak energy E_0 and the spreading width Gamma. The simple
picture is modified when the density of background states that couple to the
doorway state changes significantly in an energy interval of size Gamma. For
that case we derive an approximate analytical expression for S(E). We test our
result successfully against numerical simulations. Our result may have
important implications for shell--model calculations.Comment: 13 pages, 7 figure

### Interaction of Regular and Chaotic States

Modelling the chaotic states in terms of the Gaussian Orthogonal Ensemble of
random matrices (GOE), we investigate the interaction of the GOE with regular
bound states. The eigenvalues of the latter may or may not be embedded in the
GOE spectrum. We derive a generalized form of the Pastur equation for the
average Green's function. We use that equation to study the average and the
variance of the shift of the regular states, their spreading width, and the
deformation of the GOE spectrum non-perturbatively. We compare our results with
various perturbative approaches.Comment: 26 pages, 9 figure

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