159,338 research outputs found

### Difficulties in probing density dependent symmetry potential with the HBT interferometry

Based on the updated UrQMD transport model, the effect of the symmetry
potential energy on the two-nucleon HBT correlation is investigated with the
help of the coalescence program for constructing clusters, and the CRAB
analyzing program of the two-particle HBT correlation. An obvious non-linear
dependence of the neutron-proton (or neutron-neutron) HBT correlation function
($C_{np,nn}$) at small relative momenta on the stiffness factor $\gamma$ of the
symmetry potential energy is found: when $\gamma \lesssim 0.8$, the $C_{np,nn}$
increases rapidly with increasing $\gamma$, while it starts to saturate if
$\gamma \gtrsim 0.8$. It is also found that both the symmetry potential energy
at low densities and the conditions of constructing clusters at the late stage
of the whole process influence the two-nucleon HBT correlation with the same
power.Comment: 11 pages, 4 figure

### Numerical study of mach number effects in compressible wall-bounded turbulence

The aim of this work is to improve the present understanding of compressibility effects in wall-bounded turbulence and to provide data for improving models. A family of wall-bounded compressible flows has been investigated using direct numerical simulation (DNS). The research is divided into two aspects: a study of the intrinsic compressibility effects in isothermal-wall channel flow and a study of the impinging shock/turbulent boundary-layer interaction. For the channel flow, an energy sink is introduced in the energy equation to effectively eliminate the compressibility effects caused by mean-property variation, isolating the intrinsic compressibility effects induced by fluctuations of the density and temperature (and pressure, dilatation, etc) fields. Centreline Mach numbers, Mcl, up to 6.2 have been considered, for which we find that both explicit compressibility terms in the TKE equation such as the pressure-dilatation and dilatational dissipation, and the implicit compressibility such as Reynolds-stress-anisotropy tensor begin to become important. An oblique shock/turbulent boundary-layer interaction at free stream Mach number M ? = 2 is also investigated. Central to this work is the need to quickly obtain a fully developed turbulent boundary-layer over the shortest possible downstream distance, for which a quasi-deterministic inflow strategy is used. Then an oblique shock is impinged on to the fully developed turbulence boundary-layer and the flow separates. Explicit and implicit compressibility effects become important and the TKE budget is altered completely within the interaction zone. The interactions of shock with the separation bubble and the velocity are also addressed

### A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on $\pi-\pi$ correlations at FAIR/AGS energies

HBT correlations of $\pi^--\pi^-$ pairs at FAIR/AGS energies are investigated
by using the UrQMD transport model and the CRAB analyzing program. Three
different possible sources (treatment of resonance lifetimes, a soft equation
of state and non-Gaussian effects) to understand the HBT $R_O/R_S$ puzzle are
investigated. Firstly, we find that different treatments of the resonance decay
time can not resolve the HBT time-related puzzle, however it can modify the HBT
radii at low transverse momenta to some extent to explain the data slightly.
Secondly, with a soft equation of state with momentum dependence, the measured
transverse momentum dependent HBT radii and $R_O/R_S$ ratio can be described
fairly well. Thirdly, non-Gaussian effects are visible in the calculated
correlation function. Using the Edgeworth expansion, one finds that the
non-Gaussian effect is strongest in the longitudinal direction and weakest in
the sideward direction.Comment: 18 pages, 6 figures. To be published in J.Phys.

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### Comparative study of wall shear stress at the ascending aorta for different mechanical heart valve prostheses

An experimental study is reported which investigates the wall shear stress (WSS) distribution in a transparent model of the human aorta comparing a bileaflet mechanical heart valve (BMHV) with a trileaflet mechanical heart valve (TMHV) in physiological pulsatile flow. Elastic micro-pillar WSS sensors, calibrated by micro-Particle-Image-Velocimetry measurement, are applied to the wall along the ascending aorta. Peak WSS values are observed almost twice in BMHV compared to TMHV. Flow field analyses illuminate that these peaks are linked to the jet-like flows generated in the valves interacting with the aortic wall. Not only the magnitude but also the impact regions are specific for the different valve designs. The side-orifice jets generated by BMHV travel along the aortic wall in the ascending aorta and cause a whole range impact, while the jets generated by TMHV impact further downstream in the ascending aortic generating less severe WSS

### The role of $q\bar q$ components in the N(1440) resonance

The role of 5-quark components in the pion and electromagnetic decays and
transition form factors of the N(1440) is explored. The $qqqq\bar q$
components, where the 4-quark subsystem has the flavor-spin symmetries
$[4]_{FS}[22]_F[22]_S$ and $[4]_{FS}[31]_F[31]_S$, which are expected to have
the lowest energy of all $qqqq\bar q$ configurations, are considered in detail
with a nonrelativistic quark model. The matrix elements between the 5-quark
components of the N(1440) and the nucleon, $qqqq\bar q\to qqqq\bar q$, play a
minor role in these decays, while the transition matrix elements $qqqq\bar q\to
qqq$ and $qqq\to qqqq\bar q$ that involve quark antiquark annihilation are very
significant. Both for the electromagnetic and strong decay the change from the
valence quark model value is dominated by the confinement triggered $q\bar q$
annihilation transitions. In the case of pion decay the calculated decay width
is enhanced substantially both by the direct $q\bar q \to \pi$ and also by the
confinement triggered $q\bar q\to \pi$ transitions. Agreement with the
empirical value for the pion decay width may be reached with a $\sim$ 30%
$qqqq\bar q$ component in the N(1440).Comment: 23 pages revte

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