During the last year the analysis of the data on K⁺ and K^- production in proton-nucleus collisions has continued and now first results can be presented. The experiment already described in the last annual report had taken place 1998 at SIS/GSI Darmstadt. In the kaon spectrometer a new 2^nd level trigger enabled beam intensities above 10⁹ s^-1 to be applied.

Fig. 1 Inclusive double differential K-meson production cross sections versus momentum in the laboratory frame at a polar angle of 40 ^°. Only statistical errors are plotted. Open symbols denote K⁺, full symbols K^-.

Fig. 1 shows double differential cross sections for K^± production. The 3.5 GeV data exhibit structures which might be caused by a yet unknown problem with the trigger efficiency (the trigger was different for the 2.5 GeV data).
In the 2.5 GeV K⁺ spectra the maximum of the differential cross section shifts to lower momenta with increasing mass number. Taking into account the observation angle of 40^°, this is an evidence for the emission system being different in these targets. The K⁺ are emitted more towards target rapidity in the p+Au system. Consequently they are either produced in secondary reactions or rescattered before leaving the nucleus.
Heavy ion data at comparable energies can well be described by Boltzmann distributions (e. g. in [1]). When trying this for the proton data, one has to know the kaon emission frame. Fig. 1 contains the fits of the 2.5 GeV spectra with a Boltzmann function in the proton-nucleus center-of-mass frame. An attempt to adjust this distribution in the nucleon-nucleon frame failed. This can be regarded as a hint for the K⁺ being produced in multi-step processes. The inverse slope parameters are T = 123.5(9) MeV (Au) and T = 115.5(8) MeV (C).
K⁺ are scarcely absorbed in nuclear matter because strangeness exchange reactions are not possible. Antikaons do these reactions, e. g.  K^-+N®p+L, so they are strongly absorbed. In Fig. 2 two K^-/K⁺ ratios at 3.5 GeV are plotted. The difference between carbon and gold target might be a hint for K^- absorption, but can also arise from a difference in the emission systems between K⁺ and K^-.

Fig. 2 K-/K+ ratio at 3.5 GeV, from the spectra of Fig. 1.		Fig. 3 Compilation [2] of total cross sections on K+ (left curve) and K- (right curve) production in p+p collisions. The curves represent parametrizations [4].

Over the shown momentum range this ratio is in the same order of magnitude as the ratio of total p+p cross sections, as can be seen in Fig. 3. So, the 3.5 GeV data seem not to exhibit obvious in-medium effects besides rescattering and absorption. If there are such effects, they must be visible more clearly in near-threshold data, i. e. 1.6 and 2.5 GeV. The future analysis will show this and will allow a comparison to complementary data from ITEP[3] and COSY/ANKE.

1	FZR		6	Jagiellonian University Cracow, Institute of Physics, Poland
2	GSI Darmstadt		7	FZR and TU Dresden, Institut für Kern- und Teilchenphysik
3	TU Darmstadt, Institut für Kernphysik		*	FZR, now at Argonne National Laboratory, Argonne IL, USA
4	Universität Marburg, Physikalisches Institut		**	GSI, now at Ohio State University, Columbus OH, USA
5	Universität Frankfurt, Institut für Kernphysik

References

[1]	C. Sturm et al., Phys. Rev. Lett. 86/1 (2001) 39
[2]	P. Senger, H. Ströbele, J. Phys. G 25 (1999) R59
[3]	Y. T. Kiselev et al., J. Phys. G 25 (1999) 381
[4]	A. Sibirtsev, Phys. Lett. B 359 (1995) 29,
	A. Sibirtsev et al., Z. Phys. A 358 (1997)

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