Abstract : The stiffness of the hadronic equation of state has been extracted from the production rate of $K^+$ mesons in heavy ion collisions around 1 $A$ GeV incident energy. The data are best described with a compressibility coefficient $\kappa$ around 200 MeV, a value which is usually called soft''. This is concluded from a detailed comparison of the results of transport theories with the experimental data using two different procedures: (i) the energy dependence of the ratio of $K^+$ from Au+Au and C+C collisions and (ii) the centrality dependence of the $K^+$ multiplicities. It is demonstrated that input quantities of these transport theories which are not precisely known, like the kaon-nucleon potential, the $\Delta N \to N K^+ \Lambda$ cross section or the life time of the $\Delta$ in matter do not modify this conclusion.
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Journal articles

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Submitted on : Friday, February 3, 2006 - 4:12:18 PM
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### Citation

Christoph Hartnack, H. Oeschler, Joerg Aichelin. Hadronic Matter is Soft. Physical Review Letters, American Physical Society, 2006, 96, pp.012302. ⟨10.1103/PhysRevLett.96.012302⟩. ⟨in2p3-00025558⟩

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