7,656 research outputs found
QCD Thermodynamics with Improved Actions
The thermodynamics of the SU(3) gauge theory has been analyzed with tree
level and tadpole improved Symanzik actions. A comparison with the continuum
extrapolated results for the standard Wilson action shows that improved actions
lead to a drastic reduction of finite cut-off effects already on lattices with
temporal extent . Results for the pressure, the critical temperature,
surface tension and latent heat are presented. First results for the
thermodynamics of four-flavour QCD with an improved staggered action are also
presented. They indicate similarly large improvement factors for bulk
thermodynamics.Comment: Talk presented at LATTICE96(finite temperature) 4 pages, LaTeX2e
file, 6 eps-file
Disorder effects on infrared reflection spectra of InN films
The effect of plasmon and LO-phonon damping on the optical measurements of InN films is discussed. Phonon and plasmon damping dramatically modifies the spectral features of the optical spectra and destabilizes the coupled modes of the system. Phonon damping affects the optical properties in a qualitatively different way from plasmon damping. Increased phonon damping leads to a merging of the coupled modes for a certain range of carrier density. Plasmon damping broadens the spectral linewidths of both of the coupled modes when plasmon energies are of the order of LO-phonon energies. However, when plasmon energies are larger than LO-phonon energies, increasing plasmon damping makes the higher-energy mode completely degenerate with plasmon, and may even have lower energies than plasmon. In weakly damped situations, we also discuss the low-energy optical transmission region that forms in between the coupled modes. Finally, the effect of plasmon and LO-phonon coupling on the analyses of the experimental infrared reflection data of InN films (for different electron densities) is discussed. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71000/2/JAPIAU-95-9-4795-1.pd
On the origin of the various types of radio emission in GRS 1915+105
We investigate the association between the radio ``plateau'' states and the
large superluminal flares in GRS 1915+105 and propose a qualitative scenario to
explain this association. We identify several candidate superluminal flare
events from available monitoring data on this source and analyze the
contemporaneous RXTE pointed observations. We detect a strong correlation
between the average X-ray flux during the ``plateau'' state and the total
energy emitted in radio during the subsequent radio flare. We find that the
sequence of events is similar for all large radio flares with a fast rise and
exponential decay morphology. Based on these results, we propose a qualitative
scenario in which the separating ejecta during the superluminal flares are
observed due to the interaction of the matter blob ejected during the X-ray
soft dips, with the steady jet already established during the ``plateau''
state. This picture can explain all types of radio emission observed from this
source in terms of its X-ray emission characteristics.Comment: Corrected typo in the author names, contents unchanged, accepted in
Ap
Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4
Polarization measurements in X-rays can provide unique opportunity to study
the behavior of matter and radiation under extreme magnetic fields and extreme
gravitational fields. Unfortunately, over past two decades, when X-ray
astronomy witnessed multiple order of magnitude improvement in temporal,
spatial and spectral sensitivities, there is no (or very little) progress in
the field of polarization measurements of astrophysical X-rays. Recently, a
proposal has been submitted to ISRO for a dedicated small satellite based
experiment to carry out X-ray polarization measurement, which aims to provide
the first X-ray polarization measurements since 1976. This experiment will be
based on the well known principle of polarization measurement by Thomson
scattering and employs the baseline design of a central low Z scatterer
surrounded by X-ray detectors to measure the angular intensity distribution of
the scattered X-rays. The sensitivity of such experiment is determined by the
collecting area, scattering and detection efficiency, X-ray detector
background, and the modulation factor. Therefore, it is necessary to carefully
select the scattering geometry which can provide the highest modulation factor
and thus highest sensitivity within the specified experimental constraints. The
effective way to determine optimum scattering geometry is by studying various
possible scattering geometries by means of Monte Carlo simulations. Here we
present results of our detailed comparative study based on Geant4 simulations
of five different scattering geometries which can be considered within the
weight and size constraints of the proposed small satellite based X-ray
polarization measurement experiment.Comment: 14 pages, 6 figures, accepted for publication in "Nuclear Inst. and
Methods in Physics Research, A
Flavor-Symmetry Restoration and Symanzik Improvement for Staggered Quarks
We resolve contradictions in the literature concerning the origins and size
of unphysical flavor-changing strong interactions generated by the
staggered-quark discretization of QCD. We show that the leading contributions
are tree-level in \order(a^2) and that they can be removed by adding three
correction terms to the link operator in the standard action. These corrections
are part of the systematic Symanzik improvement of the staggered-quark action.
We present a new improved action for staggered quarks that is accurate up to
errors of \order(a^4,a^2\alpha_s) --- more accurate than most, if not all,
other discretizations of light-quark dynamics.Comment: 7 page
Measuring nanomechanical motion with an imprecision far below the standard quantum limit
We demonstrate a transducer of nanomechanical motion based on cavity enhanced
optical near-fields capable of achieving a shot-noise limited imprecision more
than 10 dB below the standard quantum limit (SQL). Residual background due to
fundamental thermodynamical frequency fluctuations allows a total imprecision 3
dB below the SQL at room temperature (corresponding to 600 am/Hz^(1/2) in
absolute units) and is known to reduce to negligible values for moderate
cryogenic temperatures. The transducer operates deeply in the quantum
backaction dominated regime, prerequisite for exploring quantum backaction,
measurement-induced squeezing and accessing sub-SQL sensitivity using
backaction evading techniques
Detection of hard X-ray pulsations and a strong iron K_beta emission line during an extended low state of GX 1+4
We present here results obtained from a detailed timing and spectral analysis
of three BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in
August 1996, March 1997, and August 2000. In the middle of the August 2000
observation, the source was in a rare low intensity state that lasted for about
30 hours. Though the source does not show pulsations in the soft X-ray band
(1.0-5.5 keV) during the extended low state, pulsations are detected in
5.5-10.0 keV energy band of the MECS detector and in hard X-ray energy bands
(15-150 keV) of the PDS instrument. Comparing the 2-10 keV flux during this low
state with the previously reported low states in GX 1+4, we suggest that the
propeller regime in GX 1+4 occurs at a lower mass accretion rate than reported
earlier. Broad-band (1.0-150 keV) pulse averaged spectroscopy reveals that the
best-fit model comprises of a Comptonized continuum along with an iron K_alpha
emission line. A strong iron K_beta emission line is detected for the first
time in GX 1+4 during the extended low state of 2000 observation with
equivalent width of ~550 eV. The optical depth and temperature of the
Comptonizing plasma are found to be identical during the high and low intensity
states whereas the hydrogen column density and the temperature of the seed
photons are higher during the low state. We also present results from pulse
phase resolved spectroscopy during the high and low flux episodes.Comment: Accepted for publication in The Astrophysical Journa
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