Partial wave analysis of the reaction p(3.5GeV)+p→pK⁺Λ to search for the "ppK^-" bound state

Employing the Bonn-Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5GeV)+p→pK⁺Λ. This reaction might contain information about the kaonic cluster "ppK^-" (with quantum numbers J^P=0^- and total isospin I=1/2) via its decay into pΛ. Due to interference effects in our coherent description of the data, a hypothetical K-NN (or, specifically "ppK^-") cluster signal need not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectrum like pΛ. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a K-NN cluster. At a confidence level of CL_s=95% such a cluster cannot contribute more than 2-12% to the total cross section with a pK⁺Λ final state, which translates into a production cross-section between 0.7 μb and 4.2 μb, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.