Confirmation for a kaon-bound state K - pp observed in antiproton- 4 He annihilation at rest

1. Confirmation for a kaon-bound state K-pp observed in antiproton-4He 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. Deeply Bound Kaonic States Theoretical predictions assert that bound states can be formed in light systems Theoretical estimates of the potentials [Akaishi and Yamazaki (2002)] The attractive force is strong enough to produce a contraction of the surrounding nucleus  extremely dense nuclear system

3. Experimental results

4. annihilation at rest Analyzed events have been collected by the OBELIX experiment 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 2pp- (since Lpp-)

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

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

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

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

9. Angular correlations and background reduction angle Q angle between 2pp- (bound state) and p+p- (K0) • if  cosQ = -1 • Here: • nucleons are not initially at rest • neutral particles are present • final particles interact IM(pp-) in [1085 - 1145] MeV and IM(2pp-) in [2200 - 2240] MeV -0.65

10. angle F angle Q Cuts: IM(L) in [1085 – 1145] MeV and cosF < -0.55 Cuts: IM(L) in [1085 – 1145] MeV and cosQ < -0.4

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(pp-) in [1085 – 1145] MeV … and cosQ < -0.4

12. Conclusions • A peak has been observed in antiproton – 4He annihilation at rest in correspondence to the bound state K-pp, with statistical significance higher than 3s, 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 (6Li, 7Li, 12C); 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.