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HK: Hadronen und Kerne
HK 23: Plenar
HK 23.1: Group Report
Tuesday, March 17, 1998, 11:30–12:00, P
Evolution of resonance matter studied by neutral meson production at SIS energies — •P.H. Vogt1, M. Appenheimer2, R. Averbeck3, Y. Charbonnier4, H. Delagrange4, J. Díaz5, V. Hejny2, S. Hlavac3, R. Holzmann3, A. Kugler6, H. Löhner1, A. Marin3, G. Martínez4, T. Matulewicz4, V. Metag3,2, R. Novotny2, R.W. Ostendorf1, R. Pleskac6, Y. Schutz4, R.H. Siemssen1, R.S. Simon3, P. Tlustý6, W. Wagner6, J. Weiss2, H.W. Wilschut1, A.R. Wolf2, and M. Wolf2 — 1KVI Groningen — 2Univ. Giessen — 3GSI Darmstadt — 4GANIL Caen — 5IFIC Valencia — 6NPI Řež u Prahy
The formation and evolution of compressed nuclear matter is studied in heavy-ion collisions at relativistic energies. Production probabilities and energy spectra of π0 end η mesons have been measured at midrapidity for light and heavy colliding systems and bombarding energies between 0.8 and 2 GeV/nucleon. Since mesons arise dominantly from the decay of nucleon resonances, the population of resonances becomes accessible. Neutral mesons have been measured with the Two-Arm Photon Spectrometer TAPS by detecting their decay photons. Event characterization was achieved by using the small-angle hodoscope of the KaoS collaboration and a large-angle scintillator array. In this way the meson production probabilities and energy spectra could be analyzed as a function of the impact parameter and the number of participating nucleons.
The meson multiplicities depend linearly on the number of participating nucleons in a light system. A more than linear increase in a heavy reaction system is interpreted as evidence for a multistep production process. A remarkable scaling of the transverse mass distributions for π0 and η mesons is observed even for energies where η production is subthreshold. Mass and energy dependence of the scaling behaviour is investigated. The low transverse mass enhancement of π0 yield can be explained by the decay of Δ resonances from the late stage of the collision process. Alternative explanations will be discussed. The meson production yields can be described assuming a hadronic gas in thermal and chemical equilibrium. The energy dependence of the resonance population and the evolution towards resonance matter is discussed. Limits on the production of η′ and ω mesons will be compared to expectations from a thermal model.