341 research outputs found
Polarization Phenomena by Deuteron Fragmentation into Pions
The fragmentation of deuterons into pions emitted forward in the kinematic
region forbidden for free nucleon-nucleon collisions is analyzed. The inclusive
relativistic invariant spectrum of pions and the tensor analyzing power T_{20}
are investigated within the framework of an impulse approximation using
different kinds of the deuteron wave function. The influence of P-wave
inclusion in the deuteron wave function is studied, too. The invariant spectrum
is shown to be more sensitive to the amplitude of the process
than the tensor analyzing power T_{20}. It is shown that the inclusion of the
non-nucleon degrees of freedom in a deuteron results a satisfactory description
of experimental data about the inclusive pion spectrum and improves the
description of data about T_{20}. According to the experimental data, T_{20}
has the positive sign and very small values, less than 0.2, what contradicts to
the theoretical calculations ignoring these degrees of freedom.Comment: 18 pages, 8 eps figures, 1 picture - svjour.cls required; enlarged
new version with corrections and additional figure. The Abstract and the
section "Summary and outlook" have been also corrected. Final version to
appear in Eur.Phys.J. A. A talk given at the International Workshop
"Symmetries and Spin" (July 17-22, Prague, Czech Republic
Quark model description of the tetraquark state X(3872) in a relativistic constituent quark model with infrared confinement
We explore the consequences of treating the X(3872) meson as a tetraquark
bound state. As dynamical framework we employ a relativistic constituent quark
model which includes infrared confinement in an effective way. We calculate the
decay widths of the observed channels X-> Jpsi+2\pi (3\pi) and X-> \bar
D0+D0+\pi0 via the intermediate off--shell states X-> Jpsi+\rho(\omega) and X->
\bar D + D*. For reasonable values of the size parameter of the X(3872) we find
consistency with the available experimental data. We also discuss the possible
impact of the X(3872) in a s-channel dominance description of the
Jpsi-dissociation cross section.Comment: 9 pages, 5 figures; discussion and references added, accepted in
Phys. Rev.
Probing the deuteron structure at small N-N distances by cumulative pion production
The fragmentation of deuterons into pions emitted forward in the kinematic
region forbidden for free nucleon-nucleon collisions is analyzed. It is shown
that the inclusion of the non-nucleonic degrees of freedom in a deuteron
results in a satisfactory description of the data for the inclusive pion
spectrum and improves the description of the data about . According to
the data, has very small positive values, less than 0.2, which
contradicts the theoretical calculations ignoring these degrees of freedom.Comment: 3 pages, 2 postscript figures; to appear in the proceedings of
Conference on Quarks and Nuclear Physics (QNP 2002), Julich, Germany, 9-14
Jun 200
Hard X-ray emitting black hole fed by accretion of low angular momentum matter
Observed spectra of Active Galactic Nuclei (AGN) and luminous X-ray binaries
in our Galaxy suggest that both hot (~10^9 K) and cold (~10^6 K) plasma
components exist close to the central accreting black hole. Hard X-ray
component of the spectra is usually explained by Compton upscattering of
optical/UV photons from optically thick cold plasma by hot electrons.
Observations also indicate that some of these objects are quite efficient in
converting gravitational energy of accretion matter into radiation. Existing
theoretical models have difficulties in explaining the two plasma components
and high intensity of hard X-rays. Most of the models assume that the hot
component emerges from the cold one due to some kind of instability, but no one
offers a satisfactory physical explanation for this. Here we propose a solution
to these difficulties that reverses what was imagined previously: in our model
the hot component forms first and afterward it cools down to form the cold
component. In our model, accretion flow has initially a small angular momentum,
and thus it has a quasi-spherical geometry at large radii. Close to the black
hole, the accreting matter is heated up in shocks that form due to the action
of the centrifugal force. The hot post-shock matter is very efficiently cooled
down by Comptonization of low energy photons and condensates into a thin and
cold accretion disk. The thin disk emits the low energy photons which cool the
hot component.Comment: 15 pages, 2 figures, submitted to ApJ Let
Deuteron distribution in nuclear matter
We analyze the properties of deuteron-like structures in infinite, correlated
nuclear matter, described by a realistic hamiltonian containing the Urbana
two-nucleon and the Urbana TNI many-body potentials. The distribution
of neutron-proton pairs, carrying the deuteron quantum numbers, is obtained as
a function of the total momentum by computing the overlap between the nuclear
matter in its ground state and the deuteron wave functions in correlated basis
functions theory. We study the differences between the S- and D-wave components
of the deuteron and those of the deuteron-like pair in the nuclear medium. The
total number of deuteron type pairs is computed and compared with the
predictions of Levinger's quasideuteron model. The resulting Levinger's factor
in nuclear matter at equilibrium densityis 11.63. We use the local density
approximation to estimate the Levinger's factor for heavy nuclei, obtaining
results which are consistent with the available experimental data from
photoreactions.Comment: 22 pages, 7 figures, typeset using REVTe
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