The main goal of this project is to search for the kaonic nuclear bound states, K^–pp, with the FOPI detector at GSI exploiting an exclusive measurement of p + p → K^–pp + K⁺ → p + Λ + K⁺ reaction at T_p = 3.0 GeV. The possible existence of the kaonic nuclear states has been extensively discussed in the last years, especially the issue of the K^–pp system.

The K–pp system is a prototype of the kaonic nuclear state and its binding energy and width was first calculated by Akaishi and Yamazaki based on the so-called Λ(1405) Ansatz [the Λ(1405) is assumed to be a I = 0, ―KN quasi-bound state with B_K = 27 MeV]. In their approach they found a total binding energy of B_K = 48 MeV, resulting in a mass M = 2322 MeV/c² and width G_Spp = 61 MeV for the K–pp. Recent Faddeev calculations also predict the K^–pp to be deeply-bound. On the other hand, a theory based on chiral dynamics and the “two-pole structure” Ansatz of the Λ(1405) prefers “weak” binding. An experiment providing conclusive data to answers this issue is strongly awaited.

We therefore started to investigate the NN reaction, a unique and promising approach. Based on the results and lessons learned from a test beam time in fall 2005, we submitted in March 2007 a new proposal to the Program Advisory Committee of GSI to investigate the p + p → K–pp + K⁺ reaction at an incident proton energy of T_p = 3.0 GeV where the K^–pp undergoes a successive decay K^–pp → Λ + p [p + π] + p. A theoretical study triggered by this project suggested that the strongly bound K^–pp system with a short p-p distance is populated quite favorably in a p + p → K–pp + K⁺ reaction since at large momentum transfer a high sticking probability of Λ(1405) + p → K–pp is predicted. The proposal obtained a very positive evaluation.

With the FOPI apparatus, which has a ~4π acceptance, we are able to measure the missing mass MM(pp-K+) and the invariant mass M_inv(Λ-p) simultaneously. In this way, the background is strongly suppressed, an ambiguity of identifying the exotic object is avoided and the ambitious goal of this project, namely to obtain a conclusive information on the issue of the deeply bound kaonic state, can be achieved.

Fig. 1 shows schematically the setup of the experiment on which a typical event example is overlaid. The charged kaon is emitted and detected mostly in the backward detector region with a combination of the central drift chamber and the new RPC TOF wall in the 0.6 T magnetic field. The decay product of the K^–pp is measured in the forward region with a combination of the silicon tracker, the forward drift chamber and the forward TOF wall. A pair of multiplicity counters (Lambda trigger) effectively selects an event which involves a short-lived neutral Lambda particle at online level.

 

Simultaneously we started to analyze a vast data set of p + p → p + Λ + K+ process collected at Tp = 2.85 GeV, 2.5 GeV and 2.15 GeV by the DISTO experiment. The reaction aimed by the FOPI experiment shares the same final states. The DISTO data allows a similar analysis strategy, namely the MM(pp-K+) and the Minv(Λ-p). A preliminary result was presented at the EXA08 conference, showing an identical deep and broad structure in both MM(pp-K+) and the Minv(Λ-p) spectra at Tp = 2.85 GeV, which is unlikely to be explained by a final state interaction of the direct p + p → p + Λ + K+ Dalitz process, implying a possible interpretation of an existence of the K–pp.

Though an analysis of the DISTO data is continuing, from this lesson and coping with some analysis difficulties, we obtained several invaluable inputs to optimize further the forthcoming FOPI experiment. Not-yet-analyzed DISTO data sets of T_p = 2.5 GeV and 2.15 GeV will deepen our understanding of the p + p → p + Λ + K⁺ background process.

The activities of SMI during 2008 were largely devoted to the development of the various subsystems. SMI is responsible for the development and operation of a new start counter, a beam profile monitor, a veto detector, and finally also the liquid hydrogen target. After several test beam times at the Beam Test Facility at LNF, Frascati (January), at the Proton Irradiation Facility at PSI, Switzerland (March) and at the COSY-TOF experiment at the Forschungszentrum Jülich (July), we integrated all the detectors into FOPI at GSI and tested the system in September 2008 (Fig. 2). Except for the target system which was not built in during this test, all subsystems have been operated.

 

Outlook

In April 2009 it is planned to run a further test with the entire detector configuration to test the whole setup and check the background situation (15 shifts). The production run is scheduled for August 2009 (60 shifts)