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Register a new userProduction of K+ and of K- Mesons in Heavy-Ion Collisions from 0.6 to 2.0 AGeV Incident Energy
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This paper summarizes the yields and the emission patterns of K+ and of K- mesons measured in inclusive C+C, Ni+Ni and Au+Au collisions at incident energies from 0.6 AGeV to 2.0 AGeV using the Kaon Spectrometer KaoS at GSI. For Ni+Ni collisions at 1.5 and at 1.93 AGeV as well as for Au+Au at 1.5 AGeV detailed results of the multiplicities, of the inverse slope parameters of the energy distributions and of the anisotropies in the angular emission patterns as a function of the collision centrality are presented. When comparing transport-model calculations to the measured K+ production yields an agreement is only obtained for a soft nuclear equation of state (compression modulus KN ~ 200 MeV). The production of K- mesons at energies around 1 to 2 AGeV is dominated by the strangeness-exchange reaction K- N <-> pi Y (Y = Lambda, Sigma) which leads to a coupling between the K- and the K+ yields. However, both particle species show distinct differences in their emission patterns suggesting different freeze-out conditions for K+ and for K- mesons.
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Azimuthal distributions of pi+, K+ and K- mesons have been measured in Au+Au reactions at 1.5 AGeV and Ni+Ni reactions at 1.93 AGeV. In semi-central 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.
The production and the propagation of K+ and of K- mesons in heavy-ion collisions at beam energies of 1 to 2 AGeV 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 AGeV 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 for K+. These observations indicate that K+ mesons decouple earlier from the reaction zone than K- mesons.
We have measured the sideward flow of neutral strange ($K^0_s$) mesons in 6 AGeV Au + Au collisions. A prominent anti-flow signal is observed for an impact parameter range (b $lesssim 7$ fm) which spans central and mid-central events. Since the $K^0_s$ scattering cross section is relatively small in nuclear matter, this observation suggests that the in-medium kaon vector potential plays an important role in high density nuclear matter.
At the Kaon Spectrometer KaoS at SIS, GSI the production of kaons and antikaons in heavy ion reactions at a beam energy of 1.5 AGeV has been measured for the collision systems Ni+Ni and Au+Au. The K-/K+ ratio is found to be constant for both systems and as a function of impact parameter but the slopes of K+ and K- spectra differ for all impact parameters. Furthermore the respective polar angle distributions will be presented as a function of centrality.
Azimuthal distributions of $eta$ and $pi^{0}$ mesons emitted at midrapidity in collisions of 1.9 AGeV $^{58}$Ni+$^{58}$Ni and 2 AGeV $^{40}$Ca+$^{nat}$Ca are studied as a function of the number of projectile-like spectator nucleons. The observed anisotropy corresponds to a negative elliptic flow signal for $eta$ mesons, indicating a preferred emission perpendicular to the reaction plane. The effect is smallest in peripheral Ni+Ni collisions. In contrast, for $pi^{0}$ mesons, elliptic flow is observed only in peripheral Ni+Ni collisions, changing from positive to negative sign with increasing pion transverse momentum.
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