C. Sturm

MEASUREMENT OF THE COULOMB DISSOCIATION OF 8B AT 254 MEV/NUCLEON AND THE 8B SOLAR NEUTRINO FLUX

We have measured the Coulomb dissociation of 8B into 7Be and proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S17 factor for the 7Be(p,gamma)8B reaction at E{c.m.} = 0.25-2.78 MeV is deduced yielding S17(0)=20.6 pm 1.2 (exp.) pm 1.0 (theo.) eV-b. nThis result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb-dissociation studies performed at 46.5 and 51.2 MeV/nucleon.

The kaon spectrometer at SIS

A double-focussing QD magnetic spectrometer has been developed and installed at the SIS heavy-ion facility at GSI in Darmstadt. The spectrometers primary purpose is to study meson production in energetic nucleus-nucleus collisions in detail. Its compact design is matched to the requirements of kaon detection with short flight path (5–6.5 m), large solid angle (up to 35 msr), wide momentum acceptance (pmaxpmin ⋍ 2), maximum momentum 1.6 GeV/c (1.9 GeV/c at reduced solid angle) and reasonable momentum resolution (⋍ 1% without and ⋍ 10−3 with ray tracing). A focal-plane length of about 1.5 m allows the efficient use of the detectors necessary for particle identification and ray-tracing, involving wire chambers, time-of-flight scintillators and Cherenkov detectors. Collisions can be characterized by two multiple-module plastic-scintillator hodoscopes detecting reaction fragments in the forward hemisphere. While the primary purpose for the construction of the spectrometer is the measurement of kaons, it can serve as a general purpose magnetic spectrometer. Its large solid angle also allows the study of two-particle correlations.

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.

Kaon production in heavy ion collisions and the nuclear equation of state

Abstract Double differential cross sections for protons, π + and K + are measured in Au + Au collisions at 1.0 GeV/u and Θ lab =44°±4° with a new magnetic spectrometer at SIS. The pion spectra show an enhancement at low momenta with respect to a thermal distribution and little dependence on the collision centrality. The high-energetic pions measured in semi-central collisions at approximately midrapidity exhibit an anisotropic azimuthal angular distribution, peaking at 90° and 270° with respect to the reaction plane. The same feature is observed for protons. The kaon to proton ratio increases with the number of participating nucleons, i.e. with the centrality of the collision, whereas the pion to proton ratio stays constant. The first data on subthreshold K + production in a heavy mass collision system are discussed with respect to nuclear matter properties by a comparison to microscopic transport calculations.

Pion and kaon emission from the fireball formed in Ne + NaF collisions at 1–2 GeV/nucleon☆

Abstract Double differential cross sections for proton, π + and K + meson production have been measured in Ne + NaF collisions at 1 and 2 GeV/nucleon. Parameterizations of the meson spectra with Maxwell-Boltzmann distributions yield common inverse slope parameters for kaons and high-energy pions at each beam energy. No evidence for different freeze out temperatures can be deduced from the spectral slopes of the emitted particles.

Kaon production in hadronic matter

Subthreshold kaon production has been studied in symmetric nucleus-nucleus collisions as a function of the nucleus mass, beam energy and centrality. In Au+Au collsions at 1 AGeV theK + multiplicity increases more than linearly with increasing number of participating nucleons. Transport calculations have to assume a soft equation of state in order to reproduce the data. The in-mediumK − cross section measured in Ni+Ni collisions is enhanced by about a factor of 7 as compared to the free cross section when using theK + cross section at equivalent beam energies as a normalization.

Coulomb dissociation of 8B at 254 MeV/u

We have measured the Coulomb-dissociation cross section for 8 B→ 7 Be+p in the field of 208 Pb for a wide range of 0.15–2.95 MeV at E in =254 MeV/u . The preliminary results agree with Monte Carlo simulations that used the 7 Be(p,γ) 8 B cross sections measured by Filippone et al., and Vaughn et al. and assuming E1 (+M1) Coulomb dissociation process. Further analysis is in progress.

Meson Production in Heavy-Ion Collisions at 1 GEV/Nucleon

Relati vistic heavy-ion collisions provide a unique tool to study the properties of hot and dense nuclear matter. The high stopping power of nuclei in nuclear matter leads to a rapid heating and to a compression of the medium. Two classes of observables are available. One ofthem tests the dynamics of the nuclear matter expressed by pictorial words like flow, squeeze out and side splash, whereas the other class of observables explores the properties of hot and dense nuclear matter via particle production.