A kinematic refit for the analysis of the reaction p p → p K+ Λ at 3.1 GeV with FOPI

In order to study the existence of the ppK- kaonic bound state, the FOPI experiment at GSI took data with a 3.1 GeV/c proton beam hitting a LH2 target in August 2009. The ppK-, which might decay into a lambda and a proton, is searched for, applying the missing mass and invariant mass technique. The reaction of interest will be p p → p K+ Λ, where the lambda further decays into a proton and a pion. In order to improve the mass resolution of the reconstructed lambda, a kinematic refit was developed. The refit imposes the knowledge of several physical processes on the track-fitting by introducing certain constraints on the reconstructed tracks of the final reaction p p → p K+ p π-. In addition to several non-vertex constraints (e.g. energy/momentum conservation), also a secondary-vertex constraint is applied. The basic functioning as well as preliminary results of the kinematic refit are presented.