Performance of the FOPI Λ - Trigger ∗

Recently, the search for deeply bound nuclear states with antikaons has attracted large interest. It was recently found [1] that the p+p reaction might be well suited for the production of (K−pp) nuclear bound states. We have proposed to perform an exclusive measurement using the FOPI detector [2] exploiting the reaction p+p at 3.5GeV and to build for this purpose a dedicated trigger device. The final state of the pp→(Kpp)K → ΛpK reaction involves Λ hyperons which can be detected using their decay into p + π− (64% branching ratio). Thus, the FOPI detector has been extended by a Λ trigger system, in order to enrich events containing Λ candidates. The scheme of the Λ trigger (SIΛVIO – SIlicon Λ Vertexing and Identification Online) is shown in figure 1. It consists of two detector layers downstream of the target with distances such that the bulk part (about 60%) of the produced Λs decay in between the two layers. The first layer