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تسجيل مستخدم جديدAzimuthal Anisotropy of $eta$ and $pi^{0}$ Mesons in Heavy-Ion Collisions at 2 AGeV
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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.
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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 anis
otropy 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.
The azimuthal anisotropy coefficients v_2 and v_4 of pi^0 and eta mesons are measured in Au+Au collisions at sqrt(s_NN)=200 GeV, as a function of transverse momentum p_T (1-14 GeV/c) and centrality. The extracted v_2 coefficients are found to be cons
istent between the two meson species over the measured p_T range. The ratio of v_4/v_2^2 for pi^0 mesons is found to be independent of p_T for 1-9 GeV/c, implying a lack of sensitivity of the ratio to the change of underlying physics with p_T. Furthermore, the ratio of v_4/v_2^2 is systematically larger in central collisions, which may reflect the combined effects of fluctuations in the initial collision geometry and finite viscosity in the evolving medium.
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.
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.
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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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