1. Confirmation for a kaon-bound state K - pp observed in antiproton- 4 He annihilation at rest G. Bendiscioli 1, T. Bressani 2, A Fontana 1, L. Lavezzi 1, A Panzarasa 1, A. Rotondi 1 1 Dipartimento di Fisica Nucleare Università di Pavia and INFN, Sezione di Pavia 2 Dipartimento di Fisica Sperimentale Università di Torino and INFN, Sezione di Torino

2. 2 Deeply Bound Kaonic States Theoretical predictions assert that bound states can be formed in light systems [Akaishi and Yamazaki (2002)] Theoretical estimates of the potentials The attractive force is strong enough to produce a contraction of the surrounding nucleus  extremely dense nuclear system

3. 3 Experimental results Bound StateReactionRef. K - pnn 4 He(K - stop,p) Suzuki et al. [2005]  ≤ 21.6 MeV K - ppn 4 He(K - stop,n) Suzuki et al. [2005]  ≤ 21.6 MeV 15 OK -16 O(K -,n) Kishimoto et al. [2005] K - ppA(K - stop,N) Agnello et al. [2005]

4. 4 annihilation at rest Annihilations where several nucleons are involved produce an environment rich of strange quarks which is suitable for the creation of a bound state The bound state is searched through the decay The analyzed events belong to the set with two negative and three positive tracks, connected to the same vertex: The searched signal is a peak in the invariant mass distribution 2p  - (since   p  - ) Analyzed events have been collected by the OBELIX experiment

5. 5 Selected events Particle identification is performed through independent measurement of velocity (  ), momentum (p) and dE/dx Only pions and protons with these momenta are detected: p(  ) > 80 MeV/c p(p) > 300 MeV/c Total number of analyzed events 9829

6. 6 Inclusive distribution Moreover the entries in the histogram are twice the events The inclusive distribution of the invariant mass 2p  - has no hint for the bound state

7. 7  invariant mass cut From fit: mass = 2212.1 ± 4.9 MeV width = 27.0 ± 7.8 MeV A peak is visible in the subset of events with IM(p  - ) in [1085 – 1145] MeV

8. 8 Angular correlations and background reduction angle between proton and p  - (  ) If  and p come from the bound state  they are emitted back-to- back in the center of mass system of the bound state  cos  = -1 Here the system 2p  - is emitted together with other particles. Thus it has momentum different from 0. angle  IM(p  - ) in [1085 – 1145] MeV and IM(2p  - ) in [2200 - 2240] MeV -0.5

9. 9 angle  -0.65 angle between 2p  - (bound state) and  +  - (K 0 ) if  cos  = -1 Here: 1.nucleons are not initially at rest 2.neutral particles are present 3.final particles interact IM(p  - ) in [1085 - 1145] MeV and IM(2p  - ) in [2200 - 2240] MeV Angular correlations and background reduction

10. 10 angle  Cuts: IM(  ) in [1085 – 1145] MeV and cos  < -0.4 Cuts: IM(  ) in [1085 – 1145] MeV and cos  < -0.55 angle 

11. 11 statistical significance Background evaluation: The background contribution is calculated evaluating the difference between the results of fits with good chi2, then statistically undistinguishable, from different background models (polynomials, gaussian). IM(p  - ) in [1085 – 1145] MeV … and cos  < -0.4

12. 12 Conclusions A peak has been observed in antiproton – 4 He annihilation at rest in correspondence to the bound state K - pp, with statistical significance higher than 3 , with mass 2212.1 ± 4.9 MeV and width 27.0 ± 7.8 MeV The binding energy is 159 ± 5 MeV The B.R. for the production of the peak is 2.5 x10-4 per stopped antiproton The discrepancy with FINUDA results concerning the same bound state K - pp (, [Agnello et al.]) can be assigned to the fact that FINUDA findings is evaluated summing up the contributions from three different nuclear targets ( 6 Li, 7 Li, 12 C); moreover effects like the mass and width modification of a resonance in different nuclear media, the different inital states of the nuclear targets, etc. should be kept into account.