Formation of the S = − 1 resonance X ( 2265 ) in the reaction pp → X + K + at 2 . 50 and 2 . 85 GeV June 7 , 2012

Analyzing DISTO data of pp → pΛK at Tp = 2.50 and 2.85 GeV to populate a previously reported X(2265) resonance with MX = 2267 MeV/c 2 and ΓX = 118 MeV at 2.85 GeV, we found that the production of X(2265) at 2.50 GeV is much less than that at 2.85 GeV (less than 10%), though it is expected from a kinematical consideration to be produced as much as 33% of that at 2.85 GeV. The small population of X(2265) at 2.50 GeV is consistent with the very weak production of Λ(1405) at the same incident energy toward its production threshold, thus indicating that Λ(1405) plays an important role as a doorway state for the formation of X(2265). PACS. 21.45.+v – 21.90.+f – 24.10.-i – 21.30.Fe Recently, analyzing a set of the DISTO data of an exclusive reaction, pp → pΛK, taken at an incident kinetic energy of Tp = 2.85 GeV, we found [1] a broad resonance with a mass of MX = 2267 ± 2(stat) ± 5(syst) MeV/c and a width of ΓX = 118± 8(stat)± 10(syst) MeV, in the invariant-mass spectrum M(pΛ), and also in the missingmass spectrum ∆M(K). For the time being, we call this resonance X(2265). An indication for a similar resonance in K− absorption by light nuclei was reported from FINUDA [2]. For further understanding the nature of X(2265) we studied the entrance-channel behavior of the pp reaction, and analyzed more reaction data from DISTO taken at 2.50 GeV, at which energy the formation ofX(2265) should still be kinematically allowed (the nominal threshold energy: T thres p (X(2267)) = 2.19 GeV), whereas the formation of the Λ(1405) resonance (abbreviated here as Λ∗) is expected to become very weak toward its production threshold (T thres p (Λ ∗) = 2.42 GeV). This will clarify the nature of X(2265) and let us know if the Λ∗ plays an essential role in the formation process. The DISTO experiment was carried out with the SATURNE accelerator at Saclay [3,4]. Here, we have analyzed the data set of the exclusive reaction products pΛK at Tp = 2.50 GeV, and compared the results with those at 2.85 GeV [1,5] using the same analysis method and checking the acceptance corrections at both incident energies. About 125k exclusive events were selected from a neutral hyperon missing-mass spectrum ∆M(pK), using the previous procedures described in detail in [1,5]. 2 P. Kienle et al.: Formation of the S = −1 resonance X(2265) in the reaction pp → X +K at 2.50 and 2.85 GeV 1.0 1.5