Münster 2002 – scientific programme

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HK: Physik der Hadronen und Kerne

HK 20: Heavy Ions II

HK 20.4: Talk

Tuesday, March 12, 2002, 18:00–18:15, E

Direct Measurement of Delta Production in ⁴⁰Ar + ^natCa at 0.8A GeV — •Ana Marin — Gesellschaft für Schwerionenforschung, D-64291 Darmstadt, Germany

Charged pions and protons can be identified in the modularized photon spectrometer TAPS if one restricts the analysis to isolated BaF₂ crystals in the hit response and if one correlates time–of–flight and deposited energy. This method is exploited in ⁴⁰Ar+^natCa at 0.8A GeV to investigate the production of Δ(1232) resonances in the rapidity range 0.2<y<0.4 and for transverse momenta 90 MeV/c<p_t<800 MeV/c via the invariant–mass reconstruction of the pπ decay products. The inclusive Δ mass distribution (83% Δ⁺⁺, 17% Δ⁰) is shifted to 1206 MeV, while we obtain a Δ⁺⁺ cross section of 190 ± 30 mbarn in 4π, given a temperature of 50±8 MeV of the fireball derived from the Δ p_t distribution and assuming isotropic emission. Comparison to the simultaneously measured π⁰ yield results in Δ⁺⁺/π⁰ = 0.155±0.013. This is well below the value of 0.675 expected from the isobar model and shows that only a minority of the pions is still bound in resonances at thermal freeze–out.