26,854 research outputs found
Partonic Effects in Heavy Ion Collisions at RHIC
Effects of partonic interactions in heavy ion collisions at RHIC are studied
in a multiphase transport model (AMPT) that includes both initial partonic and
final hadronic interactions.It is found that a large parton scattering cross
section is needed to understand the measured elliptic flow of pions and
two-pion correlation function.Comment: 10 pages, 5 figures, Workshop on Quark and Hadron Dynamics, Budapest,
Hungary, March 3-7, 200
Studying Diquark Structure of Heavy Baryons in Relativistic Heavy Ion Collisions
We propose the enhancement of yield in heavy ion collisions at
RHIC and LHC as a novel signal for the existence of diquarks in the strongly
coupled quark-gluon plasma produced in these collisions as well as in the
. Assuming that stable bound diquarks can exist in the quark-gluon
plasma, we argue that the yield of would be increased by two-body
collisions between diquarks and quarks, in addition to normal
three-body collisions among , and quarks. A quantitative study of
this effect based on the coalescence model shows that including the
contribution of diquarks to production indeed leads to a
substantial enhancement of the ratio in heavy ion collisions.Comment: Prepared for Chiral Symmetry in Hadron and Nuclear Physics
(Chiral07), Nov. 13-16, 2007, Osaka, Japa
Parker-Jeans Instability of Gaseous Disks Including the Effect of Cosmic Rays
We use linear analysis to examine the effect of cosmic rays (CRs) on the
Parker-Jeans instability of magnetized self-gravitating gaseous disks. We adopt
a slab equilibrium model in which the gravity (including self-gravity) is
perpendicular to the mid-plane, the magnetic field lies along the slab. CR is
described as a fluid and only along magnetic field lines diffusion is
considered. The linearised equations are solved numerically. The system is
susceptible to Parker-Jeans instability. In general the system is less unstable
when the CR diffusion coefficient is smaller (i.e., the coupling between the
CRs and plasma is stronger). The system is also less unstable if CR pressure is
larger. This is a reminiscence of the fact that Jeans instability and Parker
instability are less unstable when the gas pressure is larger (or temperature
is higher). Moreover, for large CR diffusion coefficient (or small CR
pressure), perturbations parallel to the magnetic field are more unstable than
those perpendicular to it. The other governing factor on the growth rate of the
perturbations in different directions is the thickness of the disk or the
strength of the external pressure on the disk. In fact, this is the determining
factor in some parameter regimes.Comment: 19pages, 14figures submitted to Ap
Multi-wavelength Emission from the Fermi Bubble III. Stochastic (Fermi) Re-Acceleration of Relativistic Electrons Emitted by SNRs
We analyse the model of stochastic re-acceleration of electrons, which are
emitted by supernova remnants (SNRs) in the Galactic Disk and propagate then
into the Galactic halo, in order to explain the origin on nonthermal (radio and
gamma-ray) emission from the Fermi Bubbles (FB). We assume that the energy for
re-acceleration in the halo is supplied by shocks generated by processes of
star accretion onto the central black hole. Numerical simulations show that
regions with strong turbulence (places for electron re-acceleration) are
located high up in the Galactic Halo about several kpc above the disk. The
energy of SNR electrons that reach these regions does not exceed several GeV
because of synchrotron and inverse Compton energy losses. At appropriate
parameters of re-acceleration these electrons can be re-accelerated up to the
energy 10E12 eV which explains in this model the origin of the observed radio
and gamma-ray emission from the FB. However although the model gamma-ray
spectrum is consistent with the Fermi results, the model radio spectrum is
steeper than the observed by WMAP and Planck. If adiabatic losses due to plasma
outflow from the Galactic central regions are taken into account, then the
re-acceleration model nicely reproduces the Planck datapoints.Comment: 33 pages, 8 figures, accepted by Ap
A study of the effect of ionization and illumination on morphologies of planetary nebulae
We carry out a modelling study of the effect of ionization and illumination on the morphologies of planetary nebulae (PNs), aiming to investigate the hypothesis of Kwok (2010) that the bipolar and multipolar lobes of PNs can be produced by leakage of UV photons into those directions. Using photoinization code, we construct a series of nebular models consisting of a dense envelope and cones of low density. The results show that the visible morphologies of PNs may be fundamentally different from their intrinsic structures. © 2012 International Astronomical Union.published_or_final_versio
Strangeness equilibration in heavy ion collisions
Using a relativistic transport model for heavy ion collisions at energies that are below the threshold for kaon and antikaon production in nucleon-nucleon collisions, we study how their abundances approach the canonical equilibrium during the collisions. We find that kaons are far from chemical equilibrium at the initial and high density stage, and they approach equilibrium only during the expansion stage of the collisions when their production rate is small and becomes comparable to their annihilation rate. In contrast, antikaons approach chemical equilibrium much earlier but eventually fall out of equilibrium again as a result of their large annihilation cross sections in nuclear matter
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