Search for K-pp – an antikaon nuclear bound state

We propose to search for the antikaon nuclear bound state K - pp. The antikaon is predicted to form nuclear bound states, because of the very strongly attractive interaction between the antikaon and nucleons. The system is expected to have unique features, namely high density at zero temperature, and due to this reason it is often called kaonic nuclear cluster (KNC). The K - pp is the most fundamental system of such bound states. The verification of its existence with a reliable experiment is of vital importance and will shed light on the nature of the antikaon- nucleon interaction. We investigate the reaction p + p K+ + K- pp at 3.0 GeV incident proton energy employing the FOPI detector at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany. With its excellent high-rate capabilities, its particle tracking system, the upgraded TOF barrel, dedicated beam-line counters, and a -trigger system it provides very good particle identification and allows to efficiently collect the full kinematical information of this reaction. Therefore, with this exclusive measurement the signature of the K-pp can be evaluated with a combined invariant-mass (IM) analysis of the decay particles (K-pp + p (p + - ) + p) and missing mass (MM) analysis of the K+ . For the specific needs of the p + p experiment the Stefan Meyer Institute (SMI) is developing a fast start counter, a veto detector, the liquid hydrogen target system, as well as a beam profile monitor to track and monitor the position and profile of the incident proton beam. The experiment has been proposed by the FOPI collaboration - of which SMI is member since 2005 - to the GSI Program Advisory Committee (G-PAC) in March 2007. It was ranked category A and was granted a total of 54 shifts of beam time.

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 DANE, to study the (1405) resonance. The strong attraction of the antikaon-nuclear interaction below threshold manifests in resonances like (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.