FWF Project: Search for K-pp - an antikaon nuclear bound state

The 'strangeness' degree of freedom is one of the most fascinating aspects of the strong interaction physics. The strange quark has a mass larger than the mass of up and down quarks from which normal matter is build up, but is small on the energy scale of strong interactions (~1GeV). Its embedding in hadronic systems is a key element to probe the mass generation mechanism of hadrons that are the dominant source of the visible mass of the universe. Indications of the formation of strange objects - two or more baryons bound by an antikaon - are found in different experimental studies, but are not conclusive yet and new dedicated experiments are necessary to get a complete picture. The goal of this project is to search for possible di-baryon bound nuclear states glued together by an antikaon. Our effort was on one hand concentrated on the experimental search with the FOPI detector at the GSI Helmholtzzentrum in Darmstadt and on the other hand on the analysis of existing data measured with DISTO at the SATURNE National Laboratory. Both experiments were using proton-proton collisions to produce the theoretical predicted antikaon bound system. A complementary process will be investigated at J-PARC, using high momentum (~GeV/c) negative kaons bombarding a liquid helium-3 target.

Beside the search for strange di-baryon states valuable data on low-energy antikaon nucleon reactions will be extracted from FOPI and E15 data, as well as from stopped antikaon reactions on a carbon target, placed within the KLOE detector at DAΦNE, to study the Λ(1405) resonance. The strong attraction of the antikaon-nuclear interaction below threshold manifests in resonances like Λ(1405), which strength is still controversially discussed. In 2009 the project started with a measurement using the FOPI detector, which was upgraded with a liquid hydrogen target system and additional detector components to improve the search for strange di-baryon states. Complicated calibration procedures have to be applied, making final tests necessary, which have been performed in 2011. First results of the excellent performance to reconstruct lambda baryons have been published, but more analysis work has to be performed to extract the final result and to compare the FOPI data with the DISTO result.

Indeed, the analysis of the DISTO data yields an excess of events in their data set, which could be contributed to the formation of a dense object consisting of two baryons glued together by an antikaon. Results of this analysis work are published in different scientific papers. The experiment at J-PARC was delayed due to the earthquake and tsunami on March 2011 with damages of the J-PARC complex. But already in 2012 during a test beam the excellent performance of the detector system could be shown. First data taking has been started in May 2013 and will be continued in 2015 and 2016. Silicon Photo Multipliers (multi-pixel avalanche diodes) applied for the light read-out of scintillating fibres in this project, are very promising devices for medical applications.