Measurement of (1385) in Al+Al collisions at 1.9 A GeV

The FOPI Collaboration performed a high statistic experiment to study the strangeness production in Al+Al collisions at 1.9A GeV. The high statistics data (300 million of events) allow the reconstruction of the Σ(1385) hyperon resonance at SIS energies for the first time. This measurement corresponds to sub-threshold production since the threshold beam energy for Σ∗ production in the elementary reaction is Ethr. = 2.33 GeV. Recent theoretical calculations predict an important coupling of the K− with Σ(1385) (calledΣ∗ in the following),Λ(1405) andΛ(1520) resonances in the medium [1, 2]. Therefore, measurement of the sub-thresholdΣ∗ production cross-section may carry important information regarding the complex reactions involved in the sub-threshold K− production. The Σ∗ is an isospin triplet state and while the Σ∗0 cannot be reconstructed with our apparatus (decay into Λ and π), the charged meson decay products π± of Σ∗± are identified with the FOPI detector. TheΣ∗ are reconstructed from a topological analysis of their double two body decay: