240 research outputs found
The impact of energy conservation in transport models on the multiplicity ratio in heavy-ion collisions and the symmetry energy
The charged pion multiplicity ratio in intermediate energy central heavy-ion
collisions has been proposed as a suitable observable to constrain the high
density dependence of the isovector part of the equation of state, with
contradicting results. Using an upgraded version of the T\"ubingen QMD
transport model, which allows the conservation of energy at a local or global
level by accounting for the potential energy of hadrons in two-body collisions
and leading thus to particle production threshold shifts, we demonstrate that
compatible constraints for the symmetry energy stiffness can be extracted from
pion multiplicity and elliptic flow observables. Nevertheless, pion
multiplicities are proven to be highly sensitive to the yet unknown isovector
part of the in-medium (1232) potential which hinders presently the
extraction of meaningful information on the high density dependence of the
symmetry energy. A solution to this problem together with the inclusion of
contributions presently neglected, such as in-medium pion potentials and
retardation effects, are needed for a final verdict on this topic.Comment: 12 pages, 7 figure
Feasability of constraining the curvature parameter of the symmetry energy using elliptic flow data
A QMD type transport model supplemented by a phase-space coalescence model
fitted to FOPI experimental multiplicities of free nucleons and light clusters
has been used to study the density dependence of the symmetry energy above the
saturation point by a comparison with experimental elliptic flow ratios
measured by the FOPI-LAND and ASYEOS collaborations in Au+Au
collisions at 400 MeV/nucleon impact energy. A previous calculation has proven
that neutron-to-proton and neutron-to-charged particles elliptic flow ratios
probe on average different densities allowing in principle the extraction of
both the slope and curvature parameters of the symmetry energy.
Consequently a Gogny interaction inspired potential has been modified to allow
independent changes of and . Comparing theoretical predictions
with experimental data for neutron-to-proton and neutron-to-charged particles
elliptic flow ratios the following constraints have been extracted:
=8522(exp)20(th)12(sys) MeV and
=96315(exp)170(th)166(sys) MeV. Residual model
dependence is accounted for in the magnitude of the quoted theoretical error.
Systematical uncertainties are generated by the inability of the transport
model to reproduce experimental light-cluster-to-proton multiplicity ratios. A
value for , free of systematical theoretical uncertainties, can be extracted
from the neutron-to-proton elliptic flow ratio alone:
=8430(exp)19(th) MeV. It has also been demonstrated that elliptic
flow ratios reach a maximum sensitivity on the parameter in heavy-ion
collisions of about 250 MeV/nucleon impact energy, allowing a reduction of the
experimental component of uncertainty to about 150 MeV for this parameter.Comment: 22 pages, 13 figure
Dilepton production at HADES: theoretical predictions
Dileptons represent a unique probe for nuclear matter under extreme
conditions reached in heavy-ion collisions. They allow to study meson
properties, like mass and decay width, at various density and temperature
regimes. Present days models allow generally a good description of dilepton
spectra in ultra-relativistic heavy ion collision. For the energy regime of a
few GeV/nucleon, important discrepancies between theory and experiment, known
as the DLS puzzle, have been observed. Various models, including the one
developed by the T\"{u}bingen group, have tried to address this problem, but
have proven only partially successful. High precision spectra of dilepton
emission in heavy-ion reactions at 1 and 2 GeV/nucleon will be released in the
near future by the HADES Collaboration at GSI. Here we present the predictions
for dilepton spectra in C+C reactions at 1 and 2 GeV/nucleon and investigate up
to what degree possible scenarios for the in-medium modification of vector
mesons properties are accessible by the HADES experiment.Comment: 12 pages, 4 figures; submitted to Phys.Lett.
Towards a model-independent constraint of the high-density dependence of the symmetry energy
Neutron-proton elliptic flow difference and ratio have been shown to be
promising observables in the attempt to constrain the density dependence of the
symmetry energy above the saturation point from heavy-ion collision data. Their
dependence on model parameters like microscopic nucleon-nucleon cross-sections,
compressibility of nuclear matter, optical potential, and symmetry energy
parametrization is thoroughly studied. By using a parametrization of the
symmetry energy derived from the momentum dependent Gogny force in conjunction
with the T\"{u}bingen QMD model and comparing with the experimental FOPI/LAND
data for 197Au+197Au collisions at 400 MeV/nucleon, a moderately stiff, x=-1.35
+/- 1.25, symmetry energy is extracted, a result that agrees with that of a
similar study that employed the UrQMD transport model and a momentum
independent power-law parametrization of the symmetry energy. This contrasts
with diverging results extracted from the FOPI ratio
available in the literature.Comment: 18 pages, 3 figures, accepted for publication in Phys. Rev.
In-medium potential, pion production in heavy-ion collisions and the symmetry energy
Using the dcQMD transport model, the isoscalar and isovector in-medium
potentials of the (1232) baryon are studied and information regarding
their effective strength is obtained from a comparison to experimental pion
production data in heavy-ion collisions below 800 MeV/nucleon impact energy.
The best description is achieved for an isoscalar potential moderately more
attractive than the nucleon optical potential and a rather small isoscalar
relative effective mass m 0.45. For the isovector component
only a constraint between the potential's strength at saturation and the
isovector effective mass difference can be extracted, which depends on
quantities such as the slope of the symmetry energy and the neutron-proton
effective mass difference. These results are incompatible with the usual
assumption, in transport models, that the (1232) and nucleon potentials
are equal. The density dependence of symmetry energy can be studied using the
high transverse momentum tail of pion multiplicity ratio spectra. Results are
however correlated with the value of neutron-proton effective mass difference.
This region of spectra is shown to be affected by uncertain model ingredients
such as the pion potential or in-medium correction to inelastic scattering
cross-sections at levels smaller than 10. Extraction of precise constraints
for the density dependence of symmetry energy above saturation will require
experimental data for pion production in heavy-ion collisions below 800
MeV/nucleon impact energy and experimental values for the high transverse
momentum tail of pion multiplicity ratio spectra accurate to better than 5.Comment: 22 pages, 13 figure
Flow probe of symmetry energy in relativistic heavy-ion reactions
Flow observables in heavy-ion reactions at incident energies up to about 1
GeV per nucleon have been shown to be very useful for investigating the
reaction dynamics and for determining the parameters of reaction models based
on transport theory. In particular, the elliptic flow in collisions of
neutron-rich heavy-ion systems emerges as an observable sensitive to the
strength of the symmetry energy at supra-saturation densities. The comparison
of ratios or differences of neutron and proton flows or neutron and hydrogen
flows with predictions of transport models favors an approximately linear
density dependence, consistent with ab-initio nuclear-matter theories.
Extensive parameter searches have shown that the model dependence is comparable
to the uncertainties of existing experimental data. Comprehensive new flow data
of high accuracy, partly also through providing stronger constraints on model
parameters, can thus be expected to improve our knowledge of the equation of
state of asymmetric nuclear matter.Comment: 20 pages, 24 figures, review to appear in EPJA special volume on
nuclear symmetry energ
Therapeutic alternatives with CPAP in obstructive sleep apnea
Obstructive Sleep Apnea (OSA), characterized by airflow cessation (apnea) or reduction (hypopnea) due to repeated pharyngeal obstructions during sleep, causes frequent disruption of sleep and hypoxic events. The condition is linked to many adverse health related consequences, such as neurocognitive and cardiovascular disorders, and metabolic syndrome. OSA is a chronic condition requiring long-term treatment, so treatment using continuous positive airway pressure (CPAP) has become the gold standard in cases of moderate or severe OSA. However, its effectiveness is influenced by patients’ adherence. Surgery for OSA or treatment with oral appliances can be successful in selected patients, but for the majority, lifestyle changes such as exercise and dietary control may prove useful. However, exercise training remains under-utilized by many clinicians as an alternative treatment for OSA. Other interventions such as oral appliance (OA), upper way stimulation, and oropharyngeal exercises are used in OSA. Because the benefit of all these techniques is heterogeneous, the major challenge is to associate specific OSA therapies with the maximum efficacy and the best patient compliance
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