12,407 research outputs found
The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Observations of redshifted 21cm radiation from HI at high redshifts is an
important future probe of reionization. We consider the Multi-frequency Angular
Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint
function of the angular multipole l and frequency separation \Delta\nu. The
signal at two different frequencies is expected to get decorrelated as
\Delta\nu is increased, and quantifying this decorrelation is particularly
important in deciding the frequency resolution for future HI observations. This
is also expected to play a very crucial role in extracting the signal from
foregrounds as the signal is expected to decorrelate much faster than the
foregrounds (which are largely continuum sources) with increasing \Delta\nu. In
this paper we develop formulae relating the MAPS to different components of the
three dimensional HI power spectrum taking into account HI peculiar velocities.
We show that the flat-sky approximation provides a very good representation
over the angular scales of interest, and a final expression which is very
simple to calculate and interpret. We present results considering two models
for the HI distribution, namely, (i) DM: where the HI traces the dark matter
and (ii) PR: where the effects of patchy reionization are incorporated through
two parameters. We find that while the DM signal is largely featureless, the PR
signal peaks at the angular scales of the individual bubbles, and the signal is
considerably enhanced for large bubble size. For most cases of interest at l
\sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less,
whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence
also carries an imprint of the bubble size and the bias, and is expected to be
an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted
versio
Exchange coupling between two ferromagnetic electrodes separated by a graphene nanoribbon
In this study, based on the self-energy method and the total energy
calculation, the indirect exchange coupling between two semi-infinite
ferromagnetic strips (FM electrodes) separated by metallic graphene nanoribbons
(GNRs) is investigated. In order to form a FM/GNR/FM junction, a graphitic
region of finite length is coupled to the FM electrodes along graphitic zigzag
or armchair interfaces of width . The numerical results show that, the
exchange coupling strength which can be obtained from the difference between
the total energies of electrons in the ferromagnetic and antiferromagnetic
couplings, has an oscillatory behavior, and depends on the Fermi energy and the
length of the central region.Comment: 4 pages, 6 figures, International Conference on Theoretical Physics
'Dubna-Nano2008
Contrasting Supersymmetry and Universal Extra Dimensions at Colliders
We contrast the experimental signatures of low energy supersymmetry and the
model of Universal Extra Dimensions and discuss various methods for their
discrimination at hadron and lepton colliders. We study the discovery reach of
hadron colliders for level 2 Kaluza-Klein modes, which would indicate the
presence of extra dimensions. We also investigate the possibility to
differentiate the spins of the superpartners and KK modes by means of the
asymmetry method of Barr. We then review the methods for discriminating between
the two scenarios at a high energy linear collider such as CLIC. We consider
the processes of Kaluza-Klein muon pair production in universal extra
dimensions in parallel to smuon pair production in supersymmetry. We find that
the angular distributions of the final state muons, the energy spectrum of the
radiative return photon and the total cross-section measurement are powerful
discriminators between the two models.Comment: 6 pages, 8 figures, to appear in the proceedings of the 2005
International Linear Collider Workshop, Stanford, US
Critical Behavior of J/psi across the Phase Transition from QCD sum rules
We study behavior of J/psi in hot gluonic matter using
QCD sum rules. Taking into account temperature dependences of the gluon
condensates extracted from lattice thermodynamics for the pure SU(3) system, we
find that the mass and width of J/psi exhibit rapid change across the critical
temperature.Comment: 5 pages, 3 figures. Poster contribution for Quark Matter 2008. To be
published in the proceeding
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