A search for strange tribaryonic states in the reaction

1. A search for strange tribaryonic states in the reaction Very Preliminary ΛπNN ΛπNN ΣNN Λnp ΣNN Λnp E549 The neutron measurement required trifle coincidence. 1. Incident K- and its track by beamline detectors. 2. Neutron and its stop timing(TOF) by NC. 3. Secondary charged particle andits track by TC-Tracking Chamber. This is semi-inclusive measurement like as E471. σ=0.030 E549 Experimental Setup Preliminary TC 1/β ① ② NC Resolution variation on a Missing Mass T0 & BDC VDC ③ Black – previous (E471) Red – Present (E549) TC Missing Mass resolution in σ [MeV/c2] Missing Mass [MeV/c2] 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. 1/β resolution was improved from 0.044 to 0.030. (left figure) With this improvement, 1.5 times better missing mass resolution than E471 case was expectecd. (right figure) Threshold=2MeVee Σ decay 1/β ~ 5.18 1/β ADC (MeVee) 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. 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. Semi-inclusive neutron Background subtracted Black – Data Red – Estimated background Very Preliminary Counts / (5 MeV/c2) Efficiency corrected pion coincidence missing mass 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) E471(nucl-ex/0310018) E549 Counts / (10 MeV/c) Preliminary Missing Mass [MeV/c2] Black – 7 MeVee threshold Red – 10 MeVee threshold Blue – 13 MeVee threshold Neutron detection efficiency estimation by DEMON code [1] Black – 7 MeVee threshold Red – 10 MeVee threshold Blue – 13 MeVee threshold Arbitrary Unit Neutron momentum (MeV/c) Neutron momentum (MeV/c) Missing Mass [MeV/c2] Efficiency [ratio] Summary • Neutron spectra was measured for the reaction with upgraded setup. • At this analysis stage, Narrow (Γ~20 MeV/c2) peak structure did not appear on present result. • Further analysis is in progress for the search for kaonic tribaryon state. Missing Mass [MeV/c2] [1] R. C. Byrd et al. Nucl. Inst. Meth. A313 (1992) 437. 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 E471nucl-ex/0310018 was performed via reaction. [1] Y. Akaishi and T. Yamazaki Phy. Rev. C 65 (2002) 044005. [2]M. Iwasaki et al, nucl-ex/0310018 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.