M. Menzel

First measurement of antikaon phase-space distributions in nucleus–nucleus collisions at subthreshold beam energies

Abstract Differential production cross sections of K − and K + mesons have been measured as function of the polar emission angle in Ni + Ni collisions at a beam energy of 1.93xa0 A GeV. In near-central collisions, the spectral shapes and the widths of the rapidity distributions of K − and K + mesons are in agreement with the assumption of isotropic emission. In non-central collisions, the K − and K + rapidity distributions are broader than expected for a single thermal source. In this case, the polar angle distributions are strongly forward–backward peaked and the nonisotropic contribution to the total yield is about one third both for K + and K − mesons. The K − /K + ratio is found to be about 0.03 independent of the centrality of the reaction. This value is significantly larger than predicted by microscopic transport calculations if in-medium modifications of K mesons are neglected.

Production of K{sup +} and of K{sup -} mesons in heavy-ion collisions from 0.6A to 2.0A GeV incident energy

This paper summarizes the yields and the emission patterns of K{sup +} and of K{sup -} mesons measured in inclusive C+C, Ni+Ni, and Au+Au collisions at incident energies from 0.6A to 2.0A GeV using the Kaon Spectrometer KaoS at GSI. For Ni+Ni collisions at 1.5A and at 1.93A GeV as well as for Au+Au at 1.5A GeV, detailed results are presented of the multiplicities, of the inverse slope parameters, and of the anisotropies in the angular emission patterns as a function of the collision centrality. When comparing transport-model calculations to the measured K{sup +} production yields, an agreement is only obtained for a soft nuclear equation of state (compression modulus K{sub N}{approx_equal}200 MeV). The production of K{sup -} mesons at energies around 1A to 2A GeV is dominated by the strangeness-exchange reaction K{sup -}N {pi}Y (Y={lambda},{sigma}) which leads to a coupling between the K{sup -} and K{sup +} yields. However, both particle species show distinct differences in their emission patterns suggesting different freeze-out conditions for K{sup +} and K{sup -} mesons.

First evidence for different freeze-out conditions for kaons and antikaons observed in heavy-ion collisions.

Differential production cross sections of K- and K+ mesons have been measured in Ni+Ni and Au+Au collisions at a beam energy of 1.5 A GeV. The K(-)/K(+) ratio is found to be nearly constant as a function of the collision centrality and system size. The spectral slopes and the polar emission pattern differ for K- and K+ mesons. These observations indicate that K+ mesons decouple earlier from the fireball than K- mesons.

Observation of different azimuthal emission patterns of K+and of K- mesons in heavy-ion collisions at 1-2A GeV

Azimuthal distributions of pi+, K+, and K- mesons have been measured in Au+Au reactions at 1.5A GeV and Ni+Ni reactions at 1.93 A GeV. In semicentral collisions at midrapidity, pi+ and K+ mesons are emitted preferentially perpendicular to the reaction plane in both collision systems. In contrast for K- mesons in Ni+Ni reactions, an in-plane elliptic flow was observed for the first time at these incident energies.

In-Medium Effects on Phase Space Distributions of Antikaons Measured in Proton-Nucleus Collisions

Differential production cross sections of K+/- mesons have been measured in p + C and p + Au collisions at 1.6, 2.5, and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the K- multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction NY --> K- NN and an attractive in-medium K- N potential at saturation density.

Low-energy cross section of the {sup 7}Be(p,{gamma}){sup 8}B solar fusion reaction from the Coulomb dissociation of {sup 8}B

An exclusive measurement of the Coulomb breakup of {sup 8}B into {sup 7}Be+p at 254A MeV was used to infer the low-energy {sup 7}Be(p,{gamma}){sup 8}B cross section. The radioactive {sup 8}B beam was produced by projectile fragmentation of 350A MeV {sup 12}C and separated with the FRagment Separator (FRS) at Gesellschaft fuer Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S{sub 17} factors exhibit good agreement with the most recent direct {sup 7}Be(p,{gamma}){sup 8}B measurements. By using the energy dependence of S{sub 17} according to the recently refined cluster model for {sup 8}B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S{sub 17}(0)=20.6{+-}0.8(stat){+-}1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Abstract:Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π+)/ is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K+ multiplicities per participating nucleon M(K+)/ are independent of the target size at 1 AGeV and at 1.8 AGeV. The K- multiplicity per participating nucleon M(K-)/ is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K-/K+ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected.

Nuclear deformation and neutron excess as competing effects for dipole strength in the pygmy region.

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A=124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the γELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIγS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

K+- and K−-production in heavy-ion collisions at SIS energies

The production and the propagation of K+- and of K−-mesons in heavy-ion collisions at beam energies of 1 to 2 A GeV have systematically been investigated with the Kaon Spectrometer (KaoS) at the SIS at the GSI. The ratio of the K+-production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. At 1.5 A GeV a comprehensive study of the K+- and of the K−-emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K−/K+ ratio is found to be nearly constant as a function of the collision centrality. The spectral slopes and the polar emission patterns are different for K− and K+. These observations indicate that K+-mesons decouple earlier from the reaction zone than K−-mesons.

Review of the results of the KaoS Collaboration

The production of K+ and of K− mesons in heavy-ion collisions at beam energies of 1 to 2 A GeV has systematically been investigated with the kaon spectrometer KaoS. The ratio of the K+ production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. A comprehensive study of the K+ and of the K− emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K−/K+ ratio is found to be nearly constant as a function of the collision centrality and can be explained by the dominance of strangeness exchange. On the other hand, the spectral slopes and the polar emission patterns are different for K− and for K+. Furthermore the azimuthal distribution of the particle emission has been investigated. K+ mesons and pions are emitted preferentially perpendicular to the reaction plane as well in Au+Au as in Ni+Ni collisions. In contrast for K− mesons in Ni+Ni reactions an in-plane flow was observed for the first time at these incident energies.