1,128 research outputs found
Comparing different freeze-out scenarios in azimuthal hadron correlations induced by fast partons
I review the linearized hydrodynamical treatment of a fast parton traversing
a perturbative quark-gluon plasma. Using numerical solutions for the medium's
response to the fast parton, I obtain the medium's distribution function which
is then used in a Cooper-Frye freeze-out prescription to obtain an azimuthal
particle spectrum. Two different freeze-out scenarios are considered which
yield significantly different results. I conclude that any meaningful
comparison of azimuthal hadron correlation functions to RHIC data requires
implementing a realistic freeze-out scenario in an expanding medium.Comment: Contribution to the Proceedings for 2008 Hot Quarks in Estes Park,
CO, as accepted for publication in EPJ-
Operation of a Bloch oscillator
We report the operation of a Bloch oscillator. The active medium was a
staticvoltage driven, doped GaAs/AlAs superlattice which was
electromagnetically coupled to a resonator. The oscillator produced tuneable
microwave radiation (frequency ~ 60 GHz; power ~ 0.5 mW; efficiency ~ 4 %). The
gain (~ 10^4 /cm) was due to the nonlinearity mediated by miniband electrons.
We also present a theory of the oscillator. The Bloch oscillator should in
principle be feasible for generation of radiation up to frequencies of 10 THz
and more.Comment: 9 pages, 4 figure
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
System size dependence of nuclear modification and azimuthal anisotropy of jet quenching
We investigate the system size dependence of jet-quenching by analyzing
transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at
=200 GeV for different centralities. The fast partons
are assumed to lose energy by radiating gluons as they traverse the plasma and
undergo multiple collisions. The energy loss per collision, , is
taken as proportional to (where is the energy of the parton),
proportional to , or a constant depending on whether the formation
time of the gluon is less than the mean path, greater than the mean free path
but less than the path length, or greater than the path length of the partons,
respectively. NLO pQCD is used to evaluate pion production by modifying the
fragmentation function to account for the energy loss. We reproduce the nuclear
modification factor by treating as the only free
parameter, depending on the centrality and the mechanism of energy loss. These
values are seen to explain the nuclear modification of prompt photons, caused
by the energy lost by final state quarks before they fragment into photons.
These also reproduce the azimuthal asymmetry of transverse momentum
distribution for pions within a factor of two and for prompt photons in a fair
agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded.
Typographical corrections done, several references added. To appear in
Journal of Physics
Effect of finite chemical potential on QGP-Hadron phase transition in a statistical model of fireball formation
We study the effect of finite chemical potential for the QGP constituents in
the Ramanathan et al. statistical model (Phys.Rev.C70, 027903,2004). While the
earlier computations using this model with vanishing chemical potentials
indicated a weakly first order phase transition for the system in the vicinity
of 170 MeV (Pramana, 68, 757, 2007), the introduction of finite values for the
chemical potentials of the constituents makes the transition a smooth roll over
of the phases, while allowing fireball formation with radius of a few "fermi"
to take place. This seems to be in conformity with the latest consensus on the
nature of the QGP-Hadron phase transition.
Keywords: Quark Gluon Plasma, Quark Hadron Phase TransitionComment: LaTex 20 pages, 20 figure
Quark Matter 2006: high-pT and jets
An overview of new experimental results on high-\pT{} particle production and
jets in heavy ion collisions from the Quark Matter 2006 conference is
presented.Comment: Presented at Quark Matter 200
Implementation of a medium-modified parton shower algorithm
We present a Monte Carlo implementation of medium-induced gluon radiation in
the final-state branching process. Medium effects are introduced through an
additive term in the splitting functions. We have implemented such modification
within PYTHIA. We show the medium effects on the hump-backed plateau, and the
transverse momentum and angular distributions with respect to the parent
parton. As expected, with increasing medium densities there is an increase
(decrease) of partons with small (large) momentum fraction, and angular
broadening is observed. The effects on the transverse-momentum distributions
are more involved, with an enhancement of low- and intermediate- partons
and a decrease at large , which is related to energy conservation, and to
the lack of momentum exchange with the medium in our approach.Comment: LaTeX, 6 pages, 2 eps figures; proceedings of the 3rd International
Conference on Hard and Electromagnetic Probes in High-Energy Nuclear
Collisions - Hard Probes 2008 (Illa de A Toxa, Spain, June 8th-14th 2008
Terahertz superlattice parametric oscillator
We report a GaAs/AlAs superlattice parametric oscillator. It was pumped by a
microwave field (power few mW) and produced 3rd harmonic radiation (frequency
near 300 GHz). The nonlinearity of the active superlattice was due to Bragg
reflections of conduction electrons at the superlattice planes. A theory of the
nonlinearity indicates that parametric oscillation should be possible up to
frequencies above 10 THz. The active superlattice may be the object of further
studies of predicted extraordinary nonlinearities for THz fields.Comment: 10 pages, 4 figure
Medium dependence of multiplicity distributions in MLLA
We study the modification of the multiplicity distributions in MLLA due to
the presence of a QCD medium. The medium is introduced though a multiplicative
constant () in the soft infrared parts of the kernels of QCD evolution
equations. Using the asymptotic ansatz for quark and gluons mean multiplicities
and respectively, we study two
cases: fixed as previously considered in the literature, and fixed
. We find opposite behaviors of the dispersion of the multiplicity
distributions with increasing in both cases. For fixed the
dispersion decreases, while for fixed it increases.Comment: LaTeX, 9 pages, 4 eps figures; proceedings of the 3rd International
Conference on Hard and Electromagnetic Probes in High-Energy Nuclear
Collisions - Hard Probes 2008 (Illa de A Toxa, Spain, June 8th-14th 2008
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