784 research outputs found
Rates for the reactions antiproton-proton --> pi phi and gamma phi
We study antiproton-proton annihilation at rest into and
. Rescattering by and
for states is sizable, of
order in the branching ratio, but
smaller than experiment. For the
rescattering contributions are negligible, but the channel is well
explained by a intermediate state combined with vector meson
dominance.Comment: 12 pages, plain latex, 2 postscript figures available upon request,
PSI-PR-93-2
--Dependence of Bond Energies in Double--- Hypernuclei
The -dependence of the bond energy of the
hypernuclear ground states is calculated in a three-body
model and in the Skyrme-Hartree-Fock approach.
Various and -nucleus or potentials
are used and the sensitivity of to the interactions
is discussed. It is shown that in medium and heavy
hypernuclei, is a linear function of
, where is rms radius of the hyperon orbital. It
looks unlikely that it will be possible to extract
interaction from the double- hypernuclear energies only, the
additional information about the -core interaction, in particular, on
is needed.Comment: 11 pages, LaTex, 3 figure
Hyperon-nucleon scattering and hyperon masses in the nuclear medium
We analyze low-energy hyperon-nucleon scattering using an effective field
theory in next-to-leading order. By fitting experimental cross sections for
laboratory hyperon momenta below 200 MeV/c and using information from the
hypertriton we determine twelve contact-interaction coefficients. Based on
these we discuss the low-density expansion of hyperon mass shifts in the
nuclear medium.Comment: 10 pages, 2 figure
Single-Particle Spin-Orbit Strengths of the Nucleon and Hyperons by SU6 Quark-Model
The quark-model hyperon-nucleon interaction suggests an important
antisymmetric spin-orbit component. It is generated from a color analogue of
the Fermi-Breit interaction dominating in the one-gluon exchange process
between quarks. We discuss the strength S_B of the single-particle spin-orbit
potential, following the Scheerbaum's prescription. Using the SU6 quark-model
baryon-baryon interaction which was recently developed by the Kyoto-Niigata
group, we calculate NN, Lambda N and Sigma N G-matrices in symmetric nuclear
matter and apply them to estimate the strength S_B. The ratio of S_B to the
nucleon strength S_N =~ -40 MeV*fm^5 is (S_Lambda)/(S_N) =~ 1/5 and
(S_Sigma)/(S_N) =~ 1/2 in the Born approximation. The G-matrix calculation of
the model FSS modifies S_Lambda to (S_Lambda)/(S_N) =~ 1/12. For S_N and
S_Sigma, the effect of the short-range correlation is comparatively weak
against meson-exchange potentials with a short-range repulsive core. The
significant reduction of the Lambda single-particle potential arises from the
combined effect of the antisymmetric LS force, the flavor-symmetry breaking
originating from the strange to up-down quark-mass difference, as well as the
effect of the short-range correlation. The density dependence of S_B is also
examined.Comment: 26 page
Hyperonic mixing in five-baryon double-strangeness hypernuclei in a two-channel treatment
Properties of hypernuclei H and He are studied in a two-channel approach with explicit treatment of
coupling of channels ^3\text{Z}+\Lambda+\Lambda and \alpha+\Xi. Diagonal
\Lambda\Lambda and coupling \Lambda\Lambda-\Xi N interactions are derived
within G-matrix procedure from Nijmegen meson-exchange models. Bond energy
\Delta B_{\Lambda\Lambda} in He exceeds significantly
that in H due to the channel coupling. Diagonal \Xi\alpha
attraction amplifies the effect, which is sensitive also to \Lambda-core
interaction. The difference of the \Delta B_{\Lambda\Lambda} values can be an
unambiguous signature of the \Lambda\Lambda-\Xi N coupling in \Lambda\Lambda
hypernuclei. However, improved knowledge of the hyperon-nucleus potentials is
needed for quantitative extraction of the coupling strength from future data on
the \Lambda\Lambda hypernuclear binding energies.Comment: 11 pages with 3 figures; Phys. Rev. C, accepte
Brueckner Rearrangement Effects in He and He
Rearrangement effects in light hypernuclei are investigated in the framework
of the Brueckner theory. We can estimate without detailed numerical
calculations that the energy of the -core is reduced by more than 2.5
MeV when the adheres to He to form He. Similar
assessment of rearrangement contributions is essential to deduce the strength
of interaction from experimentally observed . The recently observed experimental value of 1 MeV
for the of \hll suggests that the matrix element of
in \hll is around -2 MeV.Comment: 7 pages, to appear in Phys. Rev.
Neutron Star Constraints on the H Dibaryon
We study the influence of a possible H dibaryon condensate on the equation of
state and the overall properties of neutron stars whose population otherwise
contains nucleons and hyperons. In particular, we are interested in the
question of whether neutron stars and their masses can be used to say anything
about the existence and properties of the H dibaryon. We find that the equation
of state is softened by the appearance of a dibaryon condensate and can result
in a mass plateau for neutron stars. If the limiting neutron star mass is about
that of the Hulse-Taylor pulsar a condensate of H dibaryons of vacuum mass 2.2
GeV and a moderately attractive potential in the medium could not be ruled out.
On the other hand, if the medium potential were even moderately repulsive, the
H, would not likely exist in neutron stars. If neutron stars of about 1.6 solar
mass were known to exist, attractive medium effects for the H could be ruled
out. Certain ranges of dibaryon mass and potential can be excluded by the mass
of the Hulse-Taylor pulsar which we illustrate graphically.Comment: Revised by the addition of a figure showing the region of dibaryon
mass and potential excluded by the Hulse-Taylor pulsar. 18 pages, 11 figures,
latex (submitted to Phys. Rev. C
Modeling Cluster Production at the AGS
Deuteron coalescence, during relativistic nucleus-nucleus collisions, is
carried out in a model incorporating a minimal quantal treatment of the
formation of the cluster from its individual nucleons by evaluating the overlap
of intial cascading nucleon wave packets with the final deuteron wave function.
In one approach the nucleon and deuteron center of mass wave packet sizes are
estimated dynamically for each coalescing pair using its past light-cone
history in the underlying cascade, a procedure which yields a parameter free
determination of the cluster yield. A modified version employing a global
estimate of the deuteron formation probability, is identical to a general
implementation of the Wigner function formalism but can differ from the most
frequent realisation of the latter. Comparison is made both with the extensive
existing E802 data for Si+Au at 14.6 GeV/c and with the Wigner formalism. A
globally consistent picture of the Si+Au measurements is achieved. In light of
the deuteron's evident fragility, information obtained from this analysis may
be useful in establishing freeze-out volumes and help in heralding the presence
of high-density phenomena in a baryon-rich environment.Comment: 31 pages REVTeX, 19 figures (4 oversized included as JPEG). For full
postscript figures (LARGE): contact [email protected]
Light Lambda-Lambda Hypernuclei and the Onset of Stability for Lambda-Xi Hypernuclei
New Faddeev-Yakubovsky calculations for light Lambda-Lambda hypernuclei are
presented in order to assess the self consistency of the Lambda-Lambda
hypernuclear binding-energy world data and the implied strength of the
Lambda-Lambda interaction, in the wake of recent experimental reports on
Lambda-Lambda-4H and Lambda-Lambda-6He. Using Gaussian soft-core simulations of
Nijmegen one-boson-exchange model interactions, the Nijmegen soft-core model
NSC97 simulations are found close to reproducing the recently reported binding
energy of Lambda-Lambda-6He, but not those of other species. For stranger
systems, Faddeev calculations of light Lambda-Xi hypernuclei, using a
simulation of the strongly attractive Lambda-Xi interactions due to the same
model, suggest that Lambda-Xi-6He marks the onset of nuclear stability for Xi
hyperons.Comment: 5 pages, 3 postscript figures; fig.2 replaced, minor changes,
accepted as Rapid Communication in PR
Genioglossal muscle response to CO2 stimulation during NREM sleep
STUDY OBJECTIVES: The objective was to evaluate the responsiveness of upper airway muscles to hypercapnia with and without intrapharyngeal negative pressure during non-rapid eye movement (NREM) sleep and wakefulness. DESIGN: We assessed the genioglossal muscle response to CO2 off and on continuous positive airway pressure (CPAP) (to attenuate negative pressure) during stable NREM sleep and wakefulness in the supine position. SETTING: Laboratory of the Sleep Medicine Division, Brigham and Women's Hospital. PATIENTS OR PARTICIPANTS: Eleven normal healthy subjects. INTERVENTIONS: During wakefulness and NREM sleep, we measured genioglossal electromyography (EMG) on and off CPAP at the normal eupneic level and at levels 5 and 10 mm Hg above the awake eupneic level. MEASUREMENTS AND RESULTS: We observed that CO2 could increase upper-airway muscle activity during NREM sleep and wakefulness in the supine position with and without intrapharyngeal negative pressure. The application of nasal CPAP significantly decreased genioglossal EMG at all 3 levels of PETCO2 during NREM sleep (13.0 +/- 4.9% vs. 4.6 +/- 1.6% of maximal EMG, 14.6 +/- 5.6% vs. 7.1 +/- 2.3% of maximal EMG, and 17.3 +/- 6.3% vs. 10.2 +/- 3.1% of maximal EMG, respectively). However, the absence of negative pressure in the upper airway did not significantly affect the slope of the pharyngeal airway dilator muscle response to hypercapnia during NREM sleep (0.72 +/- 0.30% vs. 0.79 +/- 0.27% of maximal EMG per mm Hg PCO2, respectively, off and on CPAP). CONCLUSIONS: We conclude that both chemoreceptive and negative pressure reflex inputs to this upper airway dilator muscle are still active during stable NREM sleep
- …