A search for strange tribaryonic states in the reaction H. Yim for KEK-E549 collaboration H. Bhang, J. Chiba, S. Choi, Y. Fukuda, T. Hanaki, R. S. Hayano, M. Iio, T. Ishikawa, M. Iwai, S. Ishimoto, T. Ishiwatari, K. Itahashi, M. Iwasaki, P. Kienle, J. Kim, Y. Matsuda, H. Ohnishi, S. Okada, H. Outa, M. Sato, S. Suzuki, T. Suzuki, D. Tomono, E. Widmann, T. Yamazaki, H. Yim Recently, Akaishi and Yamazaki have predicted strongly bound kaonic state with narrow width [1]. According to their prediction, bounding E is 108 MeV and width is 20 MeV and size of nucleus become shrink. To find the extraordinary state, KEK-PS E471 experiment was performed [2]. As a result, some indication of a narrow peak-like structure was found. But Due to low statistics, the narrow structure (Γ~20MeV) had not been confirmed. To confirm the situation, KEK-PS E549 E471 (nucl-ex/ ) Neutron momentum (MeV/c) Counts / (10 MeV/c) E549 Neutron momentum (MeV/c) From 1/β spectrum, we got a semi-inclusive neutron spectrum. Left one is from previous result (E471) and right one is our result (E549). The statistics is directly comparable, because same bin width is used. We achieved about 8 times better statistics than previous. Threshold=2MeVee ADC (MeVee) Σ decay 1/β ~ /β The scale of 1/β is checked by monochromatic neutron source what come from Σ + decay. Neutron component from Σ + decay is placed on 5.18 in 1/β. This correspond to 185MeV/c. Therefore we can trust the scale of 1/β. Missing mass spectrum is converted from semi-inclusive neutron momentum spectrum. Constant accidental background component is subtracted. The position of possible tribaryon state is indicated on X-axis as red colored bar. Counts / (5 MeV/c 2 ) Black – Data Red – Estimated background Semi-inclusive neutron Background subtracted ΛπNN Λnp ΣNN Missing Mass [MeV/c 2 ] Neutron spectra was measured for the reaction with upgraded setup. At this analysis stage, Narrow ( Γ ~20 MeV/c 2 ) peak structure did not appear on present result. Further analysis is in progress for the search for kaonic tribaryon state. Summary In the semi-inclusive spectrum, we couldn’t observe the narrow structure (Γ~20MeV). Though there is a possible existence of wide structure, the missing mass measurement is insensitive on the wide structure, because of huge amount of background. So we are going to estimate upper limit of formation of tribaryonic state. A further analysis step like a development of simulation for the pion coincidence and estimating number of stopped kaon in the target is in progress. The neutron measurement required trifle coincidence. 1. Incident K - and its tr ack by beamline detect ors. 2. Neutron and its stop timing(TOF) by NC. 3. Secondary charged p article and its track by TC-Tracking Chamber. This is semi-inclusive measurement like as E471. ① ② ③ TC NC T0 & BDC E549 Experimental Setup VD C Missing Mass [MeV/c 2 ] Missing Mass resolution in σ [MeV/c 2 ] Black – previous (E471) Red – Present (E549) Resolution variation on a Missing Mass 1/β σ=0.030 E549 1/β resolution was improved from to (left figure) With this improvement, 1.5 times better missing mass resolution than E471 case was expectecd. (right figure) was performed via reaction. [1] Y. Akaishi and T. Yamazaki Phy. Rev. C 65 (2002) [2] M. Iwasaki et al, nucl-ex/ E471 nucl-ex/ Black – 7 MeVee threshold Red – 10 MeVee threshold Blue – 13 MeVee threshold Missing Mass [MeV/c 2 ] Arbitrary Unit ΛπNN Λnp ΣNN As a mid-step for upper limit estimation for formation of tribaryonic state, pion coincidence is required, because estimation including proton coincidence makes difficult in simulation for the semi-inclusive measurement condition due to uncertainty of decay mechanism of tribaryonic state. Left figure is neutron detection efficiency which was estimated by DEMONS code. By applying this curve to missing mass spectrum, we got an efficiency corrected spectrum. (right figure) [1] R. C. Byrd et al. Nucl. Inst. Meth. A313 (1992) 437. Efficiency [ratio] Neutron detection efficiency estimation by DEMON code [1] Missing Mass [MeV/c 2 ] Black – 7 MeVee threshold Red – 10 MeVee threshold Blue – 13 MeVee threshold Efficiency corrected pion coincidence missing mass