3,098 research outputs found
Charmonium Suppression with cc~ Dissociation by Strings
We study the production of cc~ pairs in nuclear reactions at SPS energies
within the covariant transport approach HSD. The production of cc~ is treated
perturbatively employing experimental cross sections while the interactions of
cc~ pairs with baryons are included by conventional cascade-type two-body
collisions. Adopting 6 mb for the cc~-baryon cross sections the data on J/\Psi
suppression in p+A reactions are reproduced in line with calculations based on
the Glauber model. Additionally the dissociation of the cc~ pairs by strings is
included in a purely geometrical way. We find good agreement with experimental
data from the NA38 and NA50 collaboration with an estimate for the string
radius of ~0.2-0.3 fm.Comment: 9 pages (LaTeX), 5 PS figure
Fast Equilibration of Hadrons in an Expanding Fireball
Due to long chemical equilibration times within standard hadronic reactions
during the hadron gas phase in relativistic heavy ion collisions it has been
suggested that the hadrons are "born" into equilibrium after the quark gluon
plasma phase. Here we develop a dynamical scheme in which possible Hagedorn
states contribute to fast chemical equilibration times of baryon anti-baryon
pairs (as well as kaon anti-kaon pairs) inside a hadron gas and just below the
critical temperature. Within this scheme, we use master equations and derive
various analytical estimates for the chemical equilibration times. Applying a
Bjorken picture to the expanding fireball, the kaons and baryons as well as the
bath of pions and Hagedorn resonances can indeed quickly chemically equilibrate
for both an initial overpopulation or underpopulation of Hagedorn resonances.
Moreover, a comparison of our results to and
ratios at RHIC, indeed, shows a close match.Comment: 4 pages, 5 figure
ISM studies of GRB 030329 with high resolution spectroscopy
We present a series of early UVES/VLT high resolution spectra of the
afterglow of GRB 030329 at redshift z=0.16867+-0.00001. In contrast to other
spectra from this burst, both emission and absorption lines were detected. None
of them showed any temporal evolution. From the emission lines, we determine
the properties of the host galaxy which has a star formation rate (SFR) of
0.198 M_solar yr^-1 and a low metallicity of 1/7 Z_solar. Given the low total
stellar host mass M_star=10^7.75+-0.15 M_solar and an absolute luminosity
m_V=-16.37, we derive specific SFRs (SSFR) of log SFR/M = -8.5 yr^-1 and SFR/L
= 14.1 M_solar yr^-1 L_*^-1. This fits well into the picture of GRB hosts as
being low mass, low metallicity, actively star forming galaxies. The MgII and
MgI absorption lines from the host show multiple narrow (Doppler width b=5-10
km/s) components spanning a range of v about 260 km/s, mainly blueshifted
compared to the redshift from the emission lines. These components are likely
probing outflowing material of the host galaxy, which could arise from former
galactic superwinds, driven by supernovae from star forming regions. Similar
features have been observed in QSO spectra. The outflowing material is mainly
neutral with high column densities of log N(MgII)=14.0+-0.1 cm^-2 and log
N(MgI)=12.3+-0.1 cm^-2.Comment: 11 pages, 4 figures, submitted to Ap
Class Transitions and Two Component Accretion Flow in GRS 1915+105
The light curve of the galactic micro-quasar GRS 1915+105 changes in at least
thirteen different ways which are called classes. We present examples of the
transitions from one class to another as observed by the IXAE instrument aboard
the Indian Satellite IRS-P3. We find that the transitions are associated with
changes in photon counts over a time-scale of only a few hours and they take
place through unknown classes. Assuming that the transitions are caused by
variation of the accretion rates, this implies that a significant fraction of
the matter must be nearly freely falling in order to have such dramatic changes
in such a short time.Comment: 9 pages, 6 figures, Astronomy and Astrophys. (in press
Higgs boson pair production in gluon fusion at NLO with full top-quark mass dependence
We present the calculation of the cross section and invariant mass
distribution for Higgs boson pair production in gluon fusion at next-to-leading
order (NLO) in QCD. Top-quark masses are fully taken into account throughout
the calculation. The virtual two-loop amplitude has been generated using an
extension of the program GoSam supplemented with an interface to Reduze for the
integral reduction. The occurring integrals have been calculated numerically
using the program SecDec. Our results, including the full top-quark mass
dependence for the first time, allow us to assess the validity of various
approximations proposed in the literature, which we also recalculate. We find
substantial deviations between the NLO result and the different approximations,
which emphasizes the importance of including the full top-quark mass dependence
at NLO.Comment: Version published in PRL, v2: results at 13 TeV (v1 was at 14 TeV),
minor correction to virtual part included, conclusions unchange
Chromofields of Strings and Baryons
We calculate color electric fields of quark/antiquark () and 3
quark () systems within the chromodielectric model (CDM). We explicitly
evaluate the string tension of flux tubes in the --system and analyze
their profile. To reproduce results of lattice calculations we use a bag
pressure from which an effective strong coupling constant
follows. With these parameters we get a shaped
configuration for large --systems.Comment: Contributions to QNP 2002, Quarks and Nuclear Physics, Juelich,
Germany 3 pages, 10 eps figure
Electron and boson clusters in confined geometries: symmetry breaking in quantum dots and harmonic traps
We discuss the formation of crystalline electron clusters in semiconductor
quantum dots and of crystalline patterns of neutral bosons in harmonic traps.
In a first example, we use calculations for two electrons in an elliptic
quantum dot to show that the electrons can localize and form a molecular dimer.
The calculated singlet-triplet splitting (J) as a function of the magnetic
field (B) agrees with cotunneling measurements, with its behavior reflecting
the effective dissociation of the dimer for large B. Knowledge of the dot shape
and of J(B) allows determination of the degree of entanglement. In a second
example, we study strongly repelling neutral bosons in two-dimensional harmonic
traps. Going beyond the Gross-Pitaevskii (GP) mean-field approximation, we show
that bosons can localize and form polygonal-ring-like crystalline patterns. The
total energy of the crystalline phase saturates in contrast to the GP solution,
and its spatial extent becomes smaller than that of the GP condensate.Comment: LATEX, 9 pages with 6 figures. To appear in Proc. Natl. Acad. Sci.
(USA). For related papers, see http://www.prism.gatech.edu/~ph274cy
- âŠ