1,427 research outputs found
Rapidity Distributions of Dileptons from a Hadronizing Quark-Gluon Plasma
It has been predicted that dilepton production may be used as a quark-gluon
plasma probe. We calculate the rapidity distributions of thermal dileptons
produced by an evolving quark-gluon plasma assuming a longitudinal scaling
expansion with initial conditions locally determined from the hadronic rapidity
density. These distributions are compared with Drell-Yan production and
semileptonic charm decays at invariant mass , 4, and 6 GeV.Comment: 17 pages (standard LaTeX), 6 figures (available as topdraw files or
printed versions upon request), GSI-93-6
Scanning the Quark-Gluon Plasma with Charmonium
We suggest the variation of charmonium suppression with Feynman x_F in heavy
ion collisions as a novel and sensitive probe for the properties of the matter
created in such reactions. In contrast to the proton-nucleus case where nuclear
suppression is weakest at small x_F, final state interactions with the comoving
matter create a minimum at x_F=0, which is especially deep and narrow if a
quark-gluon plasma is formed. While a particularly strong effect is predicted
at SPS, at the higher RHIC energy it overlaps with the expected sharp variation
with x_F of nuclear effects and needs comparison with proton-nucleus data. If
thermal enhancement of J/\Psi production takes over at the energies of RHIC and
LHC, it will form an easily identified peak, rather than dip in x_F dependence.
We predict a steep dependence on centrality and suggest that this new probe is
complementary to the dependence on transverse energy, and is more sensitive to
a scenario of final state interactions.Comment: 5 pages including 3 figures. Stylistic and clarifying corrections are
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An Absolute Flux Density Measurement of the Supernova Remnant Casseopia A at 32 GHz
We report 32 GHz absolute flux density measurements of the supernova remnant
Cas A, with an accuracy of 2.5%. The measurements were made with the 1.5-meter
telescope at the Owens Valley Radio Observatory. The antenna gain had been
measured by NIST in May 1990 to be .
Our observations of Cas A in May 1998 yield . We also report absolute flux density measurements of 3C48, 3C147, 3C286,
Jupiter, Saturn and Mars.Comment: 30 pages, 4 figures; accepted for publication by AJ. Revised
systematic error budget, corrected typos, and added reference
QUaD: A High-Resolution Cosmic Microwave Background Polarimeter
We describe the QUaD experiment, a millimeter-wavelength polarimeter designed
to observe the Cosmic Microwave Background (CMB) from a site at the South Pole.
The experiment comprises a 2.64 m Cassegrain telescope equipped with a
cryogenically cooled receiver containing an array of 62 polarization-sensitive
bolometers. The focal plane contains pixels at two different frequency bands,
100 GHz and 150 GHz, with angular resolutions of 5 arcmin and 3.5 arcmin,
respectively. The high angular resolution allows observation of CMB temperature
and polarization anisotropies over a wide range of scales. The instrument
commenced operation in early 2005 and collected science data during three
successive Austral winter seasons of observation.Comment: 23 pages, author list and text updated to reflect published versio
IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL
IGR J18483-0311 is a poorly known transient hard X-ray source discovered by
INTEGRAL during observations of the Galactic Center region performed between
23--28 April 2003. Aims: To detect new outbursts from IGR J18483-0311 using
INTEGRAL and archival Swift XRT observations and finally to characterize the
nature of this source using the optical/near-infrared (NIR) information
available through catalogue searches. Results: We report on 5 newly discovered
outbursts from IGR J18483-0311 detected by INTEGRAL.For two of them it was
possible to constrain a duration of the order of a few days. The strongest
outburst reached a peak flux of 120 mCrab (20--100 keV): its broad band
JEM--X/ISGRI spectrum (3--50 keV) is best fitted by an absorbed cutoff power
law with photon index=1.4+/-0.3, cutoff energy of ~22 keV and Nh ~9x10^22
cm^-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of
18.52 days and 21.0526 seconds which are likely the orbital period of the
system and the spin period of the X-ray pulsar respectively. Swift XRT
observations of IGR J184830311 provided a very accurate source position
which strongly indicates a highly reddened star in the USNO--B1.0 and 2MASS
catalogues as its possible optical/NIR counterpart. Conclusions: The X-ray
spectral shape, the periods of 18.52 days and 21.0526 seconds, the high
intrinsic absorption, the location in the direction of the Scutum spiral arm
and the highly reddened optical object as possible counterpart, all favour the
hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact
companion. The system is most likely a Be X-ray binary, but a Supergiant Fast
X-ray Transient nature can not be entirely excluded.Comment: accepted for publication in A&A, 10 pages, 17 figures, 4 table
The Robinson Gravitational Wave Background Telescope (BICEP): a bolometric large angular scale CMB polarimeter
The Robinson Telescope (BICEP) is a ground-based millimeter-wave bolometric
array designed to study the polarization of the cosmic microwave background
radiation (CMB) and galactic foreground emission. Such measurements probe the
energy scale of the inflationary epoch, tighten constraints on cosmological
parameters, and verify our current understanding of CMB physics. Robinson
consists of a 250-mm aperture refractive telescope that provides an
instantaneous field-of-view of 17 degrees with angular resolution of 55 and 37
arcminutes at 100 GHz and 150 GHz, respectively. Forty-nine pair of
polarization-sensitive bolometers are cooled to 250 mK using a 4He/3He/3He
sorption fridge system, and coupled to incoming radiation via corrugated feed
horns. The all-refractive optics is cooled to 4 K to minimize polarization
systematics and instrument loading. The fully steerable 3-axis mount is capable
of continuous boresight rotation or azimuth scanning at speeds up to 5 deg/s.
Robinson has begun its first season of observation at the South Pole. Given the
measured performance of the instrument along with the excellent observing
environment, Robinson will measure the E-mode polarization with high
sensitivity, and probe for the B-modes to unprecedented depths. In this paper
we discuss aspects of the instrument design and their scientific motivations,
scanning and operational strategies, and the results of initial testing and
observations.Comment: 18 pages, 11 figures. To appear in Millimeter and Submillimeter
Detectors and Instrumentation for Astronomy III, Proceedings of SPIE, 6275,
200
Thermal history of the plasma and high-frequency gravitons
Possible deviations from a radiation-dominated evolution, occurring prior the
synthesis of light nuclei, impacted on the spectral energy density of
high-frequency gravitons. For a systematic scrutiny of this situation, the
CDM paradigm must be complemented by (at least two) physical
parameters describing, respectively, a threshold frequency and a slope. The
supplementary frequency scale sets the lower border of a high-frequency domain
where the spectral energy grows with a slope which depends, predominantly, upon
the total sound speed of the plasma right after inflation. While the infra-red
region of the graviton energy spectrum is nearly scale-invariant, the expected
signals for typical frequencies larger than 0.01 nHz are hereby analyzed in a
model-independent framework by requiring that the total sound speed of the
post-inflationary plasma be smaller than the speed of light. Current (e.g.
low-frequency) upper limits on the tensor power spectra (determined from the
combined analysis of the three large-scale data sets) are shown to be
compatible with a detectable signal in the frequency range of wide-band
interferometers. In the present context, the scrutiny of the early evolution of
the sound speed of the plasma can then be mapped onto a reliable strategy of
parameter extraction including not only the well established cosmological
observables but also the forthcoming data from wide band interferometers.Comment: 47 pages, 31 included figures, to appear in Classical and Quantum
Gravit
Energy loss of fast quarks in nuclei
We report an analysis of the nuclear dependence of the yield of Drell-Yan
dimuons from the 800 GeV/c proton bombardment of , C, Ca, Fe, and W
targets. Employing a new formulation of the Drell-Yan process in the rest frame
of the nucleus, this analysis examines the effect of initial-state energy loss
and shadowing on the nuclear-dependence ratios versus the incident proton's
momentum fraction and dimuon effective mass. The resulting energy loss per unit
path length is GeV/fm. This is the first
observation of a nonzero energy loss of partons traveling in nuclear
environment.Comment: 5 pages, including 4 figure
Parton energy loss limits and shadowing in Drell-Yan dimuon production
A precise measurement of the ratios of the Drell-Yan cross section per
nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is
reported. The behavior of the Drell-Yan ratios at small target parton momentum
fraction is well described by an existing fit to the shadowing observed in
deep-inelastic scattering. The cross section ratios as a function of the
incident parton momentum fraction set tight limits on the energy loss of quarks
passing through a cold nucleus
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