7,671 research outputs found
Probing the momentum dependence of medium modifications of the nucleon-nucleon elastic cross sections
The momentum dependence of the medium modifications on nucleon-nucleon
elastic cross sections is discussed with microscopic transport theories and
numerically investigated with an updated UrQMD microscopic transport model. The
semi-peripheral Au+Au reaction at beam energy MeV is adopted as an
example. It is found that the uncertainties of the momentum dependence on
medium modifications of cross sections influence the yields of free nucleons
and their collective flows as functions of their transverse momentum and
rapidity. Among these observables, the elliptic flow is sensitively dependent
on detailed forms of the momentum dependence and more attention should be paid.
The elliptic flow is hardly influenced by the probable splitting effect of the
neutron-neutron and proton-proton cross sections so that one might pin down the
mass splitting effect of the mean-field level at high beam energies and high
nuclear densities by exploring the elliptic flow of nucleons or light clusters.Comment: 13 pages, 6 figures, 1 tabl
Scalings of Elliptic Flow for a Fluid at Finite Shear Viscosity
Within a parton cascade approach we investigate the scaling of the
differential elliptic flow with eccentricity and system
size and its sensitivity to finite shear viscosity. We present calculations for
shear viscosity to entropy density ratio in the range from up
to , finding that the saturation value varies by about a factor 2.
Scaling of is seen also for finite which
indicates that it does not prove a perfect hydrodynamical behavior, but is
compatible with a plasma at finite . Introducing a suitable freeze-out
condition, we see a significant reduction of especially at
intermediate and for more peripheral collisions. This causes a breaking
of the scaling for both and the averaged , while keeping
the scaling of v_2(p_T)/\la v_2\ra. This is in better agreement with the
experimental observations and shows as a first indication that the
should be significantly lower than the pQCD estimates. We finally point out the
necessity to include the hadronization via coalescence for a definite
evaluation of from intermediate data.Comment: 5 pages, 5 figures. Two points in fig.4 has been change
Anisotropies in momentum space at finite Shear Viscosity in ultrarelativistic heavy-ion collisions
Within a parton cascade we investigate the dependence of anisotropies in
momentum space, namely the elliptic flow and the
, on both the finite shear viscosity and the
freeze-out (f.o.) dynamics at the RHIC energy of 200 AGeV. In particular it is
discussed the impact of the f.o. dynamics looking at two different procedures:
switching-off the collisions when the energy density goes below a fixed value
or reducing the cross section according to the increase in from a QGP
phase to a hadronic one. We address the relation between the scaling of
with the eccentricity and with the integrated elliptic
flow. We show that the breaking of the scaling is not
coming mainly from the finite but from the f.o. dynamics and that the
is weakly dependent on the f.o. scheme. On the other hand the
is found to be much more dependent on both the and the f.o.
dynamics and hence is indicated to put better constraints on the properties of
the QGP. A first semi-quantitative analysis show that both and
(with the smooth f.o.) consistently indicate a plasma with .Comment: 7 pages. Proceedings of the International School of Nuclear Physics
in Erice, Sicily, to appear in Progress in Particle and Nuclear Physic
Numerical simulation of liquid sloshing in a partially filled container with inclusion of compressibility effects
A numerical scheme of study is developed to model compressible two-fluid flows simulating liquid sloshing in a partially filled tank. For a two-fluid system separated by an interface as in the case of sloshing, not only a Mach-uniform scheme is required, but also an effective way to eliminate unphysical numerical oscillations near the interface. By introducing a preconditioner, the governing equations expressed in terms of primitive variables are solved for both fluids (i.e. water, air, gas etc.) in a unified manner. In order to keep the interface sharp and to eliminate unphysical numerical oscillations in unsteady fluid flows, the non-conservative implicit Split Coefficient Matrix Method (SCMM) is modified to construct a flux difference splitting scheme in the dual time formulation. The proposed numerical model is evaluated by comparisons between numerical results and measured data for sloshing in an 80% filled rectangular tank excited at resonance frequency. Through similar comparisons, the investigation is further extended by examining sloshing flows excited by forced sway motions in two different rectangular tanks with 20% and 83% filling ratios. These examples demonstrate that the proposed method is suitable to capture induced free surface waves and to evaluate sloshing pressure loads acting on the tank walls and ceiling
Does the NJL chiral phase transition affect the elliptic flow of a fluid at fixed ?
We have derived and solved numerically the Boltzmann-Vlasov transport
equations that includes both two-body collisions and the chiral phase
transition by mean of NJL-field dynamics. The scope is to understand if the
field dynamics supply new genuine effects on the build-up of the elliptic flow
, a measure of the asymmetry in the momentum space, and in particular if
it can affect the relation between and the shear viscosity to entropy
ratio . Solving the transport equation with a constant cross section
for the condition of collisions at AGeV it is shown
a sizable suppression of due to the attractive nature of the field
dynamics that generates the constituent mass. However the key result is that if
of the system is kept fixed by an appropriate local renormalization of
the cross section the does not depend on the details of the collisional
and/or field dynamics and in particular it is not affected significantly by the
chiral phase transition.Comment: 5 pages, 5 figure
Fast nucleon emission as a probe of the isospin momentum dependence
In this article we investigate the structure of the non-local part of the
symmetry term, that leads to a splitting of the effective masses of protons and
neutrons in asymmetric matter. Based on microscopic transport simulations we
suggest some rather sensitive observables in collisions of neutron-rich
(unstable) ions at intermediate () energies. In particular we focus the
attention on pre-equilibrium nucleon emissions. We discuss interesting
correlations between the N/Z content of the fast emitted particles and their
rapidity or transverse momentum, that show a nice dependence on the
prescription used for the effective mass splitting.Comment: 5 pages, 6 figures, revtex
Uncertainty Estimates and Prediction Interval Development for Internal Strain Gage Balance Calibration Systems
Currently, there is a lack of the use of mathematically rigorous methods to evaluate the performance of multivariate force measurement systems. The specific problem addressed in the research stems from the practical issues faced by test engineers when wind tunnel models with internal strain gage balances are readied for test. Check loads are applied and the question that needs to be answered is whether or not the balance is reading within acceptable limits. These systems tend to be difficult to characterize uncertainty, primarily due to their multivariate nature, but also due to the desire for an estimate on the explanatory variable of the system, instead of the response. This estimation of the explanatory variable is inherent to the calibration problem. For systems that are modeled using non-linear terms, no closed form solution will exist for the explanatory variable. This research details the development of a prediction interval which includes the measurement error in the calibration and check systems. The 20,000 lb. manual stand for calibrating balances used in the National Transonic Facility (NTF) is employed by NASA Langley Research Center and the case study for the work. The uncertainty estimates were developed using the propagation of error method on derived physics equations for the system. The uncertainty estimates were integrated into the developed prediction interval, which demonstrated a capture rate of 96% for a trial set of check loads using a 95% level of condence. Comparisons are made to prediction interval capture rates for the Single Vector System using a common set of check loads on an NTF balance
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