3,405 research outputs found
suppression in the threshold model at RHIC and LHC energy
In the QGP based threshold model \cite{Blaizot:2000ev,Blaizot:1996nq}, in
addition to the normal nuclear absorption, 's are subjected to an
'anomalous' suppression such that above a threshold density , all
the 's are melted. In the threshold model we have analysed the recent
PHENIX data on the centrality dependence of suppression in Au+Au
collisions at RHIC. Feedback from decay of the state is accounted for.
's are anomalously suppressed above a threshold density,
. Threshold density for anomalous
suppression of the state is uncertain to a large extent, . The fraction of the state can not be determined
unambiguously, depending on the nuclear absorption, it can vary from 20% to
40%. We have also predicted for the suppression in Pb+Pb collisions at LHC
energy. In central Pb+Pb collisions, 's are suppressed by a factor of
3-4. Suppression pattern is rather similar to that in Au+Au collisions, if not
slighty less in central collisions. Using the PHENIX data on the participant
number dependence of the Bjorken energy density, we have also estimated the QGP
formation time. For critical temperature =192 MeV, estimated QGP formation
time ranges between 0.07-0.09 fm/c.Comment: 9 pages, 9 figure
Sequential melting of charmonium states in an expanding Quark Gluon Plasma and suppression at RHIC and LHC energy collisions
We have developed a hydrodynamic model to study sequential melting of
charmonium states in an expanding QGP medium. According to the initial fluid
temperature profile, 's are randomly distributed in the transverse
plane. As the fluid evolve in time, the free streaming 's are
suppressed if the local fluid temperature exceed a critical temperature. PHENIX
data on the centrality dependence of suppression in Au+Au collisions
at mid-rapidity are explained by sequential melting of the charmonium states,
, and , in the expanding medium. The critical
temperatures and agree with lattice motivated calculations. The feed-down
fraction depend on whether the cold nuclear matter effect is included or
not. It changes from with cold nuclear matter effect included to
when the effect is neglected. Model fails to reproduce the PHENIX data
on the centrality dependence of suppression in Cu+Cu collisions at
mid-rapidity, indicating that the mechanism of suppression is
different in Au+Au and in Cu+Cu collisions. We also use the model to predict
for the centrality dependence of suppression in Pb+Pb collisions at
LHC energy, =5500 GeV. In LHC energy, 's are more suppressed
in mid central collisions than in Au+Au collisions at RHIC energy.Comment: 9 pages, 9 figures. Revised version to be published in J. Phys.
ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036
ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals
coherent X-ray pulsation from this source for the first time. The source was
first detected in the serendipitous Einstein observation and later identified
as a radio pulsar. Possible pulsation in the gamma-ray band has been detected
from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA
band is characterized by a hard power-law type emission with a photon index of
about 2.3, when fitted with a single power-law function modified with
absorption. An additional blackbody component of about 0.14 keV increases the
quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in
the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are
similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR
B0633+17 (Geminga), and PSR B0656+14.Comment: To appear in ApJ Letter
Evidence for TeV gamma-ray emission from the shell type SNR RXJ1713.7-3946
We report the results of TeV gamma-ray observations of the shell type SNR
RXJ1713.7-3946 (G347.3-0.5). The discovery of strong non-thermal X-ray emission
from the northwest part of the remnant strongly suggests the existence of
electrons with energies up to 100 TeV in the remnant, making the SNR a good
candidate TeV gamma-ray source. We observed RXJ1713.7-3946 from May to August
1998 with the CANGAROO 3.8m atmospheric imaging Cerenkov telescope and obtained
evidence for TeV gamma-ray emission from the NW rim of the remnant with the
significance of 5.6 sigma. The observed TeV gamma-ray flux from the NW rim
region was estimated to be (5.3 +/- 0.9[statistical] +/- 1.6[systematic]) *
10^{-12} photons cm^{-2} s^{-1} at energies >= 1.8 +/- 0.9 TeV. The data
indicate that the emitting region is much broader than the point spread
function of our telescope. The extent of the emission is consistent with that
of hard X-rays observed by ASCA. This TeV gamma-ray emission can be attributed
to the Inverse Compton scattering of the Cosmic Microwave Background Radiation
by shock accelerated ultra-relativistic electrons. Under this assumption, a
rather low magnetic field of 11 micro gauss is deduced for the remnant from our
observation.Comment: Accepted for publication by Astronomy and Astrophysics (5 pages, 2
figures
Observations of the supernova remnant W28 at TeV energies
The atmospheric Cerenkov imaging technique has been used to search for
point-like and diffuse TeV gamma-ray emission from the southern supernova
remnant, W28, and surrounding region. The search, made with the CANGAROO 3.8m
telescope, encompasses a number of interesting features, the supernova remnant
itself, the EGRET source 3EG J1800-2338, the pulsar PSR J1801-23, strong 1720
MHz OH masers and molecular clouds on the north and east boundaries of the
remnant. An analysis tailored to extended and off-axis point sources was used,
and no evidence for TeV gamma-ray emission from any of the features described
above was found in data taken over the 1994 and 1995 seasons. Our upper limit
(E>1.5 TeV) for a diffuse source of radius 0.25deg encompassing both molecular
clouds was calculated at 6.64e-12 photons cm^-2 s^-1 (from 1994 data), and
interpreted within the framework of a model predicting TeV gamma-rays from
shocked-accelerated hadrons. Our upper limit suggests the need for some cutoff
in the parent spectrum of accelerated hadrons and/or slightly steeper parent
spectra than that used here (-2.1). As to the nature of 3EG J1800-2338, it
possibly does not result entirely from pi-zero decay, a conclusion also
consistent with its location in relation to W28.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysic
TeV observations of Centaurus A
We have searched for TeV gamma-rays from Centaurus A and surrounding region
out to +/- 1.0 deg using the CANGAROO 3.8m telescope. No evidence for TeV
gamma-ray emission was observed from the search region, which includes a number
of interesting features located away from the tracking centre of our data. The
3 sigma upper limit to the flux of gamma-rays above 1.5 TeV from an extended
source of radius 14' centred on Centaurus A is 1.28e-11 photons cm^-2 s^-1.Comment: 4 pages. Astroparticle Physics, accepted for publication. Some upper
limits overestimated by factor 2-4 in original version astro-ph/9901316. Now
correcte
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