1,415 research outputs found
J/Psi production in proton induced collisions at FAIR
We have examined the production of J/ mesons in high energy
proton-proton and proton-nucleus collisions at beam energies in the range from
158 GeV to 920 GeV, available from different fixed target experiments. In the
employed model J/ production in hadronic collisions is assumed to be a
factorisable two step process: (i) production of a pair which can be
reliably described by perturbative QCD, and (ii) formation of J/
resonance from the pair, which can be conveniently parameterized
incorporating different existing physical mechanisms of color neutralization.
We show that, for lower collision energies, J/ production through
quark-anti-quark annihilation gives larger contribution at higher , while
gluon-gluon fusion dominates the production at smaller . For
proton-nucleus collisions the model takes into account both the initial state
modification of parton distributions in nuclei and the final state interaction
of the produced pairs with the target nucleons. The model is found
to give reasonable description of data on J/ production in protonic and
proton-nucleus collisions, for different existing fixed target experiments. In
case of proton-nucleus collisions, our calculations show a non-negligible
dependence of the final state nuclear dissociation of J/ mesons on the
energy of the incident proton beam. The model has been applied to predict the
J/ production and suppression expected in proton-nucleus collisions at
energies relevant to FAIR, the upcoming accelerator facility at Darmstadt,
Germany. The amount of suppressions, for different mechanisms of J/
hadronization has been found to be distinguishably different which might help
an experimental settlement of the much controversial issue of color
neutralization.Comment: 17 pages, 37 figures. To appear in Physical Review
Population of the lower part of the instability strip: Delta Scuti stars and dwarf Cepheids (or AI Velorum)
Some of the properties of the atmospheric variations in delta Scuti stars were investigated with emphasis on the amplitude and the shape of both light curves and radial velocity curves. It is shown that these curves are small and rapidly variable in the case of dwarf Scuti stars; for the evolved stars the situation is more complex. The relation between variables and nonvariables, and also the results on abundances in the atmospheres of these stars were surveyed with respect to the hydrodynamics of their envelopes. The abundance anomalies of Am stars were qualitatively examined. The coexistence of abundance anomalies and variability among giants were also studied. Attempts were made to relate the variability to the hydrogen ionization zone in an envelope deprived of helium. Specific results are reported
Charmonium Suppression by Comover Scattering in Pb+Pb Collisions
The first reports of and production from experiment NA50 at
the CERN SPS are compared to predictions based on a hadronic model of
charmonium suppression. Data on centrality dependence and total cross sections
are in good accord with these predictions.Comment: 9 pages, latex, 6 figures, epsf, figure added and text modified to
clarify result
Asymptotic and measured large frequency separations
With the space-borne missions CoRoT and Kepler, a large amount of
asteroseismic data is now available. So-called global oscillation parameters
are inferred to characterize the large sets of stars, to perform ensemble
asteroseismology, and to derive scaling relations. The mean large separation is
such a key parameter. It is therefore crucial to measure it with the highest
accuracy. As the conditions of measurement of the large separation do not
coincide with its theoretical definition, we revisit the asymptotic expressions
used for analysing the observed oscillation spectra. Then, we examine the
consequence of the difference between the observed and asymptotic values of the
mean large separation. The analysis is focused on radial modes. We use series
of radial-mode frequencies to compare the asymptotic and observational values
of the large separation. We propose a simple formulation to correct the
observed value of the large separation and then derive its asymptotic
counterpart. We prove that, apart from glitches due to stellar structure
discontinuities, the asymptotic expansion is valid from main-sequence stars to
red giants. Our model shows that the asymptotic offset is close to 1/4, as in
the theoretical development. High-quality solar-like oscillation spectra
derived from precise photometric measurements are definitely better described
with the second-order asymptotic expansion. The second-order term is
responsible for the curvature observed in the \'echelle diagrams used for
analysing the oscillation spectra and this curvature is responsible for the
difference between the observed and asymptotic values of the large separation.
Taking it into account yields a revision of the scaling relations providing
more accurate asteroseismic estimates of the stellar mass and radius.Comment: accepted in A&
HD 41641: A classical Sct-type pulsator with chemical signatures of an Ap star
Among the known groups of pulsating stars, Sct stars are one of the
least understood. Theoretical models do not predict the oscillation frequencies
that observations reveal. Complete asteroseismic studies are necessary to
improve these models and better understand the internal structure of these
targets. We study the Sct star HD 41641 with the ultimate goal of
understanding its oscillation pattern. The target was simultaneously observed
by the CoRoT space telescope and the HARPS high-resolution spectrograph. The
photometric data set was analyzed with the software package PERIOD04, while
FAMIAS was used to analyze the line profile variations. The method of spectrum
synthesis was used for spectroscopically determining the fundamental
atmospheric parameters and individual chemical abundances. A total of 90
different frequencies was identified and analyzed. An unambiguous
identification of the azimuthal order of the surface geometry could only be
provided for the dominant p-mode, which was found to be a nonradial prograde
mode with m = +1. Using and , we estimated the mass,
radius, and evolutionary stage of HD 41641. We find HD 41641 to be a moderately
rotating, slightly evolved Sct star with subsolar overall atmospheric
metal content and unexpected chemical peculiarities. HD 41641 is a pure
Sct pulsator with p-mode frequencies in the range from 10 d to
20 d. This pulsating star presents chemical signatures of an Ap star and
rotational modulation due to surface inhomogeneities, which we consider
indirect evidence of the presence of a magnetic field.Comment: 11 pages, 11 figures, accepted for publication in A&
Modelling J/psi production and absorption in a microscopic nonequilibrium approach
Charmonium production and absorption in heavy ion collisions is studied with
the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the
scenario of universal and time independent color-octet dissociation cross
sections with one of distinct color-singlet J/psi, psi' and chi_c states,
evolving from small, color transparent configurations to their asymptotic
sizes. The measured J/psi production cross sections in pA and AB collisions at
SPS energies are consistent with both - purely hadronic - scenarios. The
predicted rapidity dependence of J/psi suppression can be used to discriminate
between the two experimentally. The importance of interactions with secondary
hadrons and the applicability of thermal reaction kinetics to J/psi absorption
are investigated. We discuss the effect of nuclear stopping and the role of
leading hadrons. The dependence of the psi' to J/psi ratio on the model
assumptions and the possible influence of refeeding processes is also studied.Comment: 35 pages, 16 figure
Cryosorber Studies for the LHC Long Straight Section Beam Screens with COLDEX
The cold bore experiment (COLDEX), that can be cooled below 3 K, has been fitted with a ~ 2 m long actively cooled beam screen equipped with cryosorber to simulate the LHC Long Straight Section (LSS) beam screens. Effects of both synchrotron radiation at grazing incidence with 194 eV critical energy and gas injections have been studied. Results as a function of temperature, gas species and gas coverage are presented. Possible implications to LHC LSS design and operation are discussed
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