354 research outputs found
Chemical Equilibrium in Collisions of Small Systems
The system-size dependence of particle production in heavy-ion collisions at
the top SPS energy is analyzed in terms of the statistical model. A systematic
comparison is made of two suppression mechanisms that quantify strange particle
yields in ultra-relativistic heavy-ion collisions: the canonical model with
strangeness correlation radius determined from the data and the model
formulated in the canonical ensemble using chemical off-equilibrium strangeness
suppression factor. The system-size dependence of the correlation radius and
the thermal parameters are obtained for p-p, C-C, Si-Si and Pb-Pb collisions at
sqrt(s_NN) = 17.3 AGeV. It is shown that on the basis of a consistent set of
data there is no clear difference between the two suppression patterns. In the
present study the strangeness correlation radius was found to exhibit a rather
weak dependence on the system size.Comment: 9 pages, 8 figures, submitted to Physical Review
Quantum Mechanical Localization Effects for Bose-Einstein Correlations
For a set of N identical massive boson wavepackets with optimal initial quantum mechanical localization, we derive the Hanbury-Brown/Twiss (HBT) two-particle correlation function. Our result provides finite multiplicity corrections to the coherent state formalism and allows to trace back an error in the so-called cos-prescription. It suggests that what the HBT radius parameters in very small boson emitting systems (e.g. Z_0-decays, p-pbar annihilation) measure is essentially the initial spatial wavepacket width. Both one- and two-particle spectra depend explicitly on this width. Our derivation gives an algorithm for calculating one-particle spectra and two-particle correlations from an arbitrary phase space occupation (q_i,p_i,t_i) as e.g. returned by event generators of heavy ion collisions
Status of Chemical Equilibrium in Relativistic Heavy Ion Collisions
Recent work on chemical equilibrium in heavy ion collisions is reviewed. The
energy dependence of thermal parameters is discussed. The centrality dependence
of thermal parameters at SPS energies is presented.Comment: 7 pages, 7 Postscript figure
Thermal analysis of hadron multiplicities from relativistic quantum molecular dynamics
Some questions arising in the application of the thermal model to hadron
production in heavy ion collisions are studied. We do so by applying the
thermal model of hadron production to particle yields calculated by the
microscopic transport model RQMD(v2.3). We study the bias of incomplete
information about the final hadronic state on the extraction of thermal
parameters.It is found that the subset of particles measured typically in the
experiments looks more thermal than the complete set of stable particles. The
hadrons which show the largest deviations from thermal behaviour in RQMD(v2.3)
are the multistrange baryons and antibaryons. We also looked at the influence
of rapidity cuts on the extraction of thermal parameters and found that they
lead to different thermal parameters and larger disagreement between the RQMD
yields and the thermal model.Comment: 12 pages, 2 figures, uses REVTEX, only misprint and stylistic
corrections, to appear in Physical Review
Gluon Radiation and Coherent States in Ultrarelativistic Nuclear Collisions
We explore the correspondence between classical gluon radiation and quantum
radiation in a coherent state for gluons produced in ultrarelativistic nuclear
collisions. The expectation value of the invariant momentum distribution of
gluons in the coherent state is found to agree with the gluon number
distribution obtained classically from the solution of the Yang-Mills
equations. A criterion for the applicability of the coherent state formalism to
the problem of radiation in ultrarelativistic nucleus-nucleus collisions is
discussed. This criterion is found to be fulfilled for midrapidity gluons with
perturbative transverse momenta larger than about 1-2 GeV and produced in
collisions between valence partons.Comment: 15 pages, 6 figures, RevTeX (with epsf, psfig style files
Using conceptual metaphor and functional grammar to explore how language used in physics affects student learning
This paper introduces a theory about the role of language in learning
physics. The theory is developed in the context of physics students' and
physicists' talking and writing about the subject of quantum mechanics. We
found that physicists' language encodes different varieties of analogical
models through the use of grammar and conceptual metaphor. We hypothesize that
students categorize concepts into ontological categories based on the
grammatical structure of physicists' language. We also hypothesize that
students over-extend and misapply conceptual metaphors in physicists' speech
and writing. Using our theory, we will show how, in some cases, we can explain
student difficulties in quantum mechanics as difficulties with language.Comment: Accepted for publication in Phys. Rev. ST:PE
Strange Messages: Chemical and Thermal Freeze-out in Nuclear Collisions
Thermal models are commonly used to interpret heavy-ion data on particle
yields and spectra and to extract the conditions of chemical and thermal
freeze-out in heavy-ion collisions. I discuss the usefulness and limitations of
such thermal model analyses and review the experimental and theoretical
evidence for thermalization in nuclear collisions. The crucial role of
correlating strangeness production data with single particle spectra and
two-particle correlation measurements is pointed out. A consistent dynamical
picture for the heavy-ion data from the CERN SPS involves an initial
prehadronic stage with deconfined color and with an appreciable isotropic
pressure component. This requires an early onset of thermalization.Comment: 15 pages, 2 figures, talk given at Strange Quark Matter '98, Padova,
Italy, 20-24 July 1998, to be published in J. Phys. G 25; final version with
updated reference
Quantum radiation in external background fields
A canonical formalism is presented which allows for investigations of quantum
radiation induced by localized, smooth disturbances of classical background
fields by means of a perturbation theory approach. For massless,
non-selfinteracting quantum fields at zero temperature we demonstrate that the
low-energy part of the spectrum of created particles exhibits a non-thermal
character. Applied to QED in varying dielectrics the response theory approach
facilitates to study two distinct processes contributing to the production of
photons: the squeezing effect due to space-time varying properties of the
medium and of the velocity effect due to its motion. The generalization of this
approach to finite temperatures as well as the relation to sonoluminescence is
indicated.Comment: 20 page
PRIDE: Quality control in a proteomics data repository
The PRoteomics IDEntifications (PRIDE) database is a large public proteomics data repository, containing over 270 million mass spectra (by November 2011). PRIDE is an archival database, providing the proteomics data supporting specific scientific publications in a computationally accessible manner. While PRIDE faces rapid increases in data deposition size as well as number of depositions, the major challenge is to ensure a high quality of data depositions in the context of highly diverse proteomics work flows and data representations. Here, we describe the PRIDE curation pipeline and its practical application in quality control of complex data depositions
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