31 research outputs found
Thermalization and the chromo-Weibel instability
Despite the apparent success of ideal hydrodynamics in describing the
elliptic flow data which have been produced at Brookhaven National Lab's
Relativistic Heavy Ion Collider, one lingering question remains: is the use of
ideal hydrodynamics at times t < 1 fm/c justified? In order to justify its use
a method for rapidly producing isotropic thermal matter at RHIC energies is
required. One of the chief obstacles to early isotropization/thermalization is
the rapid longitudinal expansion of the matter during the earliest times after
the initial nuclear impact. As a result of this expansion the parton
distribution functions become locally anisotropic in momentum space. In
contrast to locally isotropic plasmas anisotropic plasmas have a spectrum of
soft unstable modes which are characterized by exponential growth of transverse
chromo-magnetic/-electric fields at short times. This instability is the QCD
analogue of the Weibel instability of QED. Parametrically the chromo-Weibel
instability provides the fastest method for generation of soft background
fields and dominates the short-time dynamics of the system.Comment: 8 pages, 4 figures, Invited plenary talk given at the 19th
International Conference on Ultrarelativistic Nucleus-Nucleus Collisions:
Quark Matter 2006 (QM 2006), Shanghai, China, 14-20 Nov 200
QGP collective effects and jet transport
We present numerical simulations of the SU(2) Boltzmann-Vlasov equation
including both hard elastic particle collisions and soft interactions mediated
by classical Yang-Mills fields. We provide an estimate of the coupling of jets
to a hot isotropic plasma, which is independent of infrared cutoffs. In
addition, we investigate jet propagation in anisotropic plasmas, as created in
heavy-ion collisions. The broadening of jets is found to be stronger along the
beam line than in azimuth due to the creation of field configurations with
B_t>E_t and E_z>B_z via plasma instabilities.Comment: 4 pages, 5 figures. Presented at the 20th International Conference on
Ultra-Relativistic Nucleus-Nucleus Collisions: Quark Matter 2008 (QM2008),
Jaipur, India, 4-10 Feb 200
Search for a Ridge Structure Origin with Shower Broadening and Jet Quenching
We investigate the role of jet and shower parton broadening by the strong
colour field in the - correlation of high
particles. When anisotropic momentum broadening () is
given to jet and shower partons in the initial stage, a ridge-like structure is
found to appear in the two hadron correlation. The ratio of the peak to the
pedestal yield is overestimated.Comment: Talk given at 20th Int. Conf. on Ultra-Relativistic Nucleus-Nucleus
Collisions, Jaipur, India, Feb.4-10, 200
QCD plasma thermalization, collective flow and extraction of shear viscosity
Fast thermalization and elliptic flow of QCD matter found at the Relativistic
Heavy Ion Collider (RHIC) are understood as the consequence of perturbative QCD
(pQCD) interactions within a (3+1) dimensional parton cascade. The main
contributions stem from pQCD-inspired bremsstrahlung. We extract the shear
viscosity to entropy ratio, which is between 0.08 and 0.15.Comment: 8 pages, 5 figures, Plenary talk at 20th International Conference on
Ultra-Relativistic Nucleus Nucleus Collisions (Quark Matter 2008), Jaipur,
India, 4-10 Feb. 200
Locally anisotropic momentum distributions of hadrons at freeze-out in relativistic heavy-ion collisions
A spheroidal anisotropic local momentum distribution is implemented in the
statistical model of hadron production. We show that this form leads to exactly
the same ratios of hadronic abundances as the equilibrium distributions, if the
temperature is identified with a characteristic transverse-momentum scale.
Moreover, to a very good approximation the transverse-momentum spectra of
hadrons are the same for isotropic and anisotropic systems, provided the size
of the system at freeze-out is appropriately adjusted. We further show that
this invariance may be used to improve the agreement between the model and
experimental HBT results.Comment: 7 pages, 4 figure
Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet
In a central nucleus-nucleus collision at high-energies, medium partons
kicked by a near-side jet acquire a momentum along the jet direction and
subsequently materialize as the observed ridge particles. They carry direct
information on the early parton momentum distribution which can be extracted by
using the ridge data for central AuAu collisions at \sqrt{s_{NN}}=200 GeV. The
extracted parton momentum distribution has a thermal-like transverse momentum
distribution but a non-Gaussian, relatively flat rapidity distribution at
mid-rapidity with sharp kinematic boundaries at large rapidities that depend on
the transverse momentum.Comment: In Proceedings of 20th International Conference on Ultra-Relativistic
Nucleus Nucleus Collisions, Jaipur, India, Feb. 4-10, 200
Suppression of forward dilepton production from an anisotropic quark-gluon plasma
We calculate the rapidity dependence of leading-order medium dilepton yields
resulting from a quark-gluon plasma which has a local time-dependent anisotropy
in momentum space. We present a phenomenological model which includes temporal
evolution of the plasma anisotropy parameter, xi, and the hard momentum scale,
p_hard. Our model interpolates between 1+1 dimensional collisionally-broadened
expansion at early times and 1+1 dimensional ideal hydrodynamic expansion at
late times. Using our model, we find that at LHC energies, forward high-energy
medium dilepton production would be suppressed by up to a factor of 3 if one
assumes an isotropization/thermalization time of 2 fm/c. Therefore, it may be
possible to use forward dilepton yields to experimentally determine the time of
onset of locally isotropic hydrodynamic expansion of the quark-gluon plasma as
produced in ultrarelativistic heavy-ion collisions.Comment: 12 pages, 5 figure
The First Magnetic Fields
We review current ideas on the origin of galactic and extragalactic magnetic
fields. We begin by summarizing observations of magnetic fields at cosmological
redshifts and on cosmological scales. These observations translate into
constraints on the strength and scale magnetic fields must have during the
early stages of galaxy formation in order to seed the galactic dynamo. We
examine mechanisms for the generation of magnetic fields that operate prior
during inflation and during subsequent phase transitions such as electroweak
symmetry breaking and the quark-hadron phase transition. The implications of
strong primordial magnetic fields for the reionization epoch as well as the
first generation of stars is discussed in detail. The exotic, early-Universe
mechanisms are contrasted with astrophysical processes that generate fields
after recombination. For example, a Biermann-type battery can operate in a
proto-galaxy during the early stages of structure formation. Moreover, magnetic
fields in either an early generation of stars or active galactic nuclei can be
dispersed into the intergalactic medium.Comment: Accepted for publication in Space Science Reviews. Pdf can be also
downloaded from http://canopus.cnu.ac.kr/ryu/cosmic-mag1.pd
From Glasma to Quark Gluon Plasma in heavy ion collisions
When two sheets of Color Glass Condensate collide in a high energy heavy ion
collision, they form matter with very high energy densities called the Glasma.
We describe how this matter is formed, its remarkable properties and its
relevance for understanding thermalization of the Quark Gluon Plasma in heavy
ion collisions. Long range rapidity correlations contained in the near side
ridge measured in heavy ion collisions may allow one to directly infer the
properties of the Glasma.Comment: Plenary Topical Overview Talk, Quark Matter 2008; 10 pages 8 figure