Production cross sections of K 1 and K 2 mesons have been measured in C 1 C collisions at beam energies per nucleon below and near the nucleon-nucleon threshold. At a given beam energy, the spectral slopes of the K 2 mesons are significantly steeper than the ones of the K 1 mesons. The excitation functions for K 1 and K 2 mesons nearly coincide when correcting for the threshold energy. In contrast, the K 1 yield exceeds the K 2 yield by a factor of about 100 in proton-proton collisions at beam energies near the respective nucleon-nucleon thresholds. [S0031-9007 (99)08547-6] PACS numbers: 25.75.DwThe properties of strange mesons in a medium of finite baryon density are essential for our understanding of the strong interaction. According to various theoretical approaches, antikaons feel strong attractive forces in the nuclear medium, whereas the in-medium kaon-nucleon potential is expected to be slightly repulsive [1][2][3][4][5].Predictions have been made that the effective mass of the K 2 meson decreases with increasing nuclear density leading to K 2 condensation in neutron stars above 3 times saturation density r 0 . This effect is expected to influence significantly the evolution of supernova explosions: the K 2 condensate softens the nuclear equation of state and thus causes a core with 1.5-2 solar masses to collapse into a black hole rather than to form a neutron star [6,7].Experimental evidence for the attractive in-medium K 2 N potential was found in K 2 nucleus scattering [8] and kaonic atoms [9]. Strong effects are expected in relativistic nucleus-nucleus collisions where baryonic densities of several times the saturation density r o can be reached. Under these conditions the K 2 effective mass will be reduced and thus the kinematical threshold for the process NN ! K 2 1 K 1 1 NN (which in free space corresponds to a kinetic beam energy of 2.5 GeV) will be lowered. As a consequence, the K 2 yield in A 1 A collisions at bombarding energies below the NN threshold will be enhanced significantly as compared to the case without in-medium mass reduction. In contrast, the yield of K 1 mesons is predicted to be decreased as the K 1 effective mass and thus the in-medium K 1 production threshold is slightly increased [3,10,11]. The in-medium KN potentials are also expected to affect the propagation of kaons and antikaons, hence modifying their emission pattern in nucleus-nucleus collisions.According to these considerations, the K 2 ͞K 1 ratio observed in nucleus-nucleus collisions at beam energies below the NN threshold is sensitive to the in-medium properties of kaons and antikaons. Moreover, relativistic transport calculations find distinct differences in the K 1 and K 2 spectral slopes, again due to medium effects. In particular, these calculations predict that the K 2 spectra fall off steeper than the K 1 spectra due to the decrease of the K 2 effective mass in the nuclear medium [12].Recent experimental studies of kaon and antikaon production in Ni 1 Ni collisions found a large K 2 yield at 1.8A GeV [13,14], a...

The production of pions and kaons has been measured in 197 Au+ 197 Au collisions at beam energies from 0.6 to 1.5 A·GeV with the Kaon Spectrometer at SIS/GSI. The K + meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions whereas the corresponding π ratio is reduced. The ratio of the K + meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.

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We have measured the Coulomb dissociation of 8 B into 7 Be and proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S17 factor for the 7 Be (p,γ) 8 B reaction at Ec.m. = 0.25 − 2.78 MeV is deduced yielding S17(0) = 20.6±1.2 (exp.) ± 1.0 (theo.) eV-b. This 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. PACs: 25.40.Lw, 25.70.De, 26.65.+t The precise knowledge of the solar thermonuclear fusion of 8 B (from 7 Be plus proton) is crucial for estimating the 8 B solar neutrino flux and the predicted neutrino rates in terrestrial neutrino measurements. The relevant 7 Be(p,γ) 8 B cross section σ(E) is parameterized in terms of the astrophysical factor S 17 (E) which is defined by2 /hv is the Sommerfeld parameter. The flux of 8 B solar neutrinos is particularly important for the results of the Homestake, Super Kamiokande, and SNO experiments [1] which measure high-energy solar neutrinos mainly or solely from the 8 B decay. Unfortunately, this cross section has not been known with sufficient accuracy for a long time, despite the fact that several comprehensive direct measurements were reported for the 7 Be(p,γ) 8 B reaction [2][3][4][5][6]. The main difficulty in such experiments is the determination of the effective target thickness of the radioactive 7 Be target. This difficulty is reflected in the fact that the results of these measurements can be grouped into two distinct data sets which agree in their energy dependence but disagree in their absolute normalization by about 30%. In view of this discrepancy, experimental studies with different methods are highly desirable.As an alternative approach one can measure the inverse process, the Coulomb dissociation (CD) of 8 B into 7 Be and proton [7]. The CD yields are enhanced because thicker targets can be used and a larger phase space is available for CD. This method uses stable targets and thus is free from the difficulty of determining the effective target thickness. On the other hand, direct (p,γ) and Coulomb dissociation measurements have different sensitivities to the multipole composition of the photon fields. The E2 amplitude is enhanced in CD due to the large flux of E2 virtual photons, whereas it can be neglected in the (p,γ) reaction.Recently, Motobayashi et al. have performed a CD experiment at E( 8 B) = 46.5 MeV/nucleon, yielding values for S 17 in the energy range 0.6−1.7 MeV [8]. The extracted (p,γ) cross section is consistent with the results from the lower group of direct-reaction data points [4][5][6]. Another measurement at 51.9 MeV/nucleon by the same group with improved accuracy led essentially to the same conclusion [9].In this article, we report on an experiment of the CD of 8 B at a higher energy of 254 MeV/nucleon performed at the SIS facility at GSI, Darmstadt, Germany. The present incident energy has several advantages compared to those used in Refs. ...

Abstract. The production of K + and of K − mesons in heavy-ion collisions at beam energies of 1 to 2 AGeV 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 enegies.