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Register a new userObservation of different azimuthal emission patterns for K+ and of K- mesons in Heavy Ion Collisions at 1-2 AGeV
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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.
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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.
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.
Azimuthal emission patterns of $K^pm$ mesons have been measured in Ni + Ni collisions with the FOPI spectrometer at a beam kinetic energy of 1.91 A GeV. The transverse momentum $p_{T}$ integrated directed and elliptic flow of $K^{+}$ and $K^{-}$ mesons as well as the centrality dependence of $p_{T}$ - differential directed flow of $K^{+}$ mesons are compared to the predictions of HSD and IQMD transport models. The data exhibits different propagation patterns of $K^{+}$ and $K^{-}$ mesons in the compressed and heated nuclear medium and favor the existence of a kaon-nucleon in-medium potential, repulsive for $K^{+}$ mesons and attractive for $K^{-}$ mesons.
The azimuthal angular distribution of K+ mesons has been measured in Au + Au collisions at 1 AGeV. In peripheral and semi-central collisions, K+ mesons preferentially are emitted perpendicular to the reaction plane. The strength of the azimuthal anisotropy of K+ emission is comparable to the one of pions. No in-plane flow was found for K+ mesons near projectile and target rapidity.
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. In contrast, only small azimuthal anisotropies are observed for $pi^{0}$ mesons. This may indicate that $eta$ mesons freeze out earlier from the fire ball than pions.
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