8,592 research outputs found
Kaon squeeze-out in heavy ion reactions
The squeeze-out phenomenon of and mesons, i.e. the azimuthal
asymmetry of and mesons emitted at midrapidity in heavy ion
reactions, is investigated for beam energies of 1-2 A.GeV. It is found that the
squeeze-out signal is strongly affected by in-medium potentials of these
mesons. The repulsive -nucleus potential gives rise to a pronounced
out-of-plane emission of 's at midrapidity. With the potential we
reproduce well the experimental data of the azimuthal distribution. It is
found that the attractive -nucleus potential cancels to a large extent the
influence of rescattering and reabsorption of the mesons on the
projectile and target residuals (i.e. shadowing). This results in an
azimuthally isotropic emission of the midrapidity mesons with transverse
momentum up to 0.8 GeV/c. Since it is well accepted that the shadowing alone
would lead to a significant out-of-plane preference of particle emission, in
particular at high transverse momenta, the disappearance of the out-of-plane
preference for the mesons can serve as an unambiguous signal of the
attractive potential. We also apply a covariant formalism of the kaon
dynamics to the squeeze-out phenomenon. Discrepancies between the theory and
the experiments and possible solutions are discussed.Comment: 24 pages Latex using Elsevier style, 7 PS figures, accepted for
publication in Euro. Phys. Jour.
Consequences of covariant kaon dynamics in heavy ion collisions
The influence of the chiral mean field on the kaon dynamics in heavy ion
reactions is investigated. Inside the nuclear medium the kaons are described as
dressed quasi-particles carrying effective masses and momenta. A momentum
dependent part of the interaction which resembles a Lorentz force originates
from spatial components of the vector field and provides an important
contribution to the in-medium kaon dynamics. This contribution is found to
counterbalance the influence of the vector potential on the in-plane flow
to a strong extent. Thus it appears to be difficult to restrict the in-medium
potential from the analysis of the corresponding transverse flow.Comment: 14 pages, RevTex, 3 PS figures, accepted for publication in Phys.
Lett.
Radial flow of kaon mesons in heavy ion reactions
This work investigates the collective motion of kaons in heavy ion reactions
at SIS energies (about 1-2 GeV/nucleon). A radial collective flow of
mesons is predicted to exist in central Au + Au collisions, which manifests in
a characteristic "shoulder-arm" shape of the transverse mass spectrum of the
midrapidity mesons. The radial flow arises from the repulsive
mean field in nuclear matter. In spite of a strong reabsorption and
rescattering the attractive mean field leads as well to a collective
radial flow of mesons. The radial flow, however, is different from
that of mesons and can be observed by a characteristic "concave"
structure of the transverse mass spectrum of the mesons emitted at
midrapidity. The kaon radial flows can therefore serve as a novel tool for the
investigation of kaon properties in dense nuclear matter.Comment: 30 pages RevTex, 5 PS figures, accepted for publication in Phys. Rev.
In-medium dependence and Coulomb effects of the pion production in heavy ion collisions
The properties of the high energy pions observed in heavy ion collisions, in
particular in the system Au on Au at 1 GeV/nucleon are investigated. The
reaction dynamics is described within the Quantum Molecular Dynamics (QMD)
approach. It is shown that high energy pions freeze out early and originate
from the hot, compressed matter. --resonances are found to give an
important contribution toward the high energy tail of the pion. Further the
role of in-medium effects in the description of charged pion yields and spectra
is investigated using a microscopic potential derived from the Brueckner
G-matrix which is obtained with the Reid soft-core potential. It is seen that
the high energy part of the spectra is relatively more suppressed due to
in-medium effects as compared to the low energy part. A comparision to
experiments further demonstrates that the present calculations describe
reasonably well the neutral (TAPS) and charged (FOPI) pion spectra. The
observed energy dependence of the ratio, i.e. deviations from the
isobar model prediction, is due to Coulomb effects and again indicate that high
energy pions probe the hot and dense phase of the reaction. These findings are
confirmed independently by a simple phase space analysis.Comment: 28 pages Latex, prepared with elsevier-style, 13 PS-figure
Strangeness production in heavy ion collisions at SPS and RHIC within two-source statistical model
The experimental data on hadron yields and ratios in central Pb+Pb and Au+Au
collisions at SPS and RHIC energies, respectively, are analysed within a
two-source statistical model of an ideal hadron gas. These two sources
represent the expanding system of colliding heavy ions, where the hot central
fireball is embedded in a larger but cooler fireball. The volume of the central
source increases with rising bombarding energy. Results of the two-source model
fit to RHIC experimental data at midrapidity coincide with the results of the
one-source thermal model fit, indicating the formation of an extended fireball,
which is three times larger than the corresponding core at SPS.Comment: Talk at "Strange Quarks in Matter" Conference (Strangeness'2001),
September 2001, Frankfurt a.M., German
Feasibility study concerning the actual implementation of a joint cross-border procurement procedure by public buyers from different Member States
Joint procurement refers to a situation in which two or more contracting authorities conduct a procurement procedure together. The key characteristic of this specific procedure is that only one tender is published on behalf of all participating contracting authorities. The notion of joint procurement does not automatically imply any cross-border element as such. Joint cross-border procurement refers to the particular procurement procedure which involves contracting authorities from different Member States conducting a common tender by bundling their demands and acting jointly in the award of the contract. There have already been some attempts to conduct JCBPP procedures in the European Union, despite the fact that until the implementation of Directive 2014/24/EU there were no explicit legal provisions to facilitate such forms of cooperation. Therefore contracting authorities faced both legal and practical difficulties, mostly due to conflicts between national public procurement rules and barriers preventing recourse to other Member Statesâ central purchasing body or the joint cross-border award of public contracts. Currently existing literature still offers only a few contributions describing the use and impact of JCBPP and the limited practical experience there is in conducting such procedures has not been presented in an aggregated form so far. This analysis is intended to help identify the best solutions for implementing JCBPP projects and offers concrete recommendations which should serve as a guideline for all interested stakeholders. The aim of the study is not to assess theoretical scenarios of JCBPP between contracting authorities in different Member States or to offer a legal analysis of factors that can influence the implementation of such projects. Its scope is to practically analyse projects that have been implemented in the past months or years and to highlight how they have been conducted, the obstacles or difficulties experienced by the participating contracting authorities and how they managed to overcome them. Centralised â and therefore âjointâ â purchasing techniques are successfully used in most Member States and the idea of exploring a cross-border dimension is generating more and more interest as it facilitates cooperation between contracting authorities across Europe and at the same time enhances the benefits of the Internal Market by creating better business opportunities for economic operators. Thus, this feasibility study on the possible implementation of a JCBPP procedure consists of an analysis of relevant JCPPP projects, including a cost benefit analysis, and draws conclusions based on the main issues which need to be considered when conducting a JCBPP. The results of the feasibility study have allowed the drafting of recommendations for the implementation of JCBPP procedures. The feasibility study focuses on the legal, administrative and procedural aspects encountered in four selected JCBPP projects, taking into consideration country and sector-specific characteristics relevant to the implementation of such procedures
Origin of subthreshold K^+ production in heavy ion collisions
We investigate the origin of subthreshold production in heavy ion
collisions at intermediate energies. In particular we study the influence of
the pion induced creation processes. We find that this channel shows a
strong dependence on the size of the system, i.e., the number of participating
nucleons as well as on the incident energy of the reaction. In an energy region
between 1--2 GeV/nucleon the pion induced processes essentially contribute to
the total yield and can even become dominant in reactions with a large number
of participating nucleons. Thus we are able to reproduce recent measurements of
the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a
realistic momentum dependent nuclear mean field.Comment: 6 pages Latex using RevTex, revised version accepted for publication
in Phys. Rev.
Probing the nuclear equation of state by production in heavy ion collisions
The dependence of production on the nuclear equation of state is
investigated in heavy ion collisions. An increase of the excitation function of
multiplicities obtained in heavy () over light () systems
when going far below threshold which has been observed by the KaoS
Collaboration strongly favours a soft equation of state. This observation holds
despite of the influence of an in-medium kaon potential predicted by effective
chiral models which is necessary to reproduce the experimental yields.Comment: 4 pages Revtex, 4 PS figures, to appear in Phys. Rev. Let
- âŠ