(K−,π−) production of nuclear K̄ bound states in proton-rich systems via doorways
Introduction
Very recently we predicted the possible presence of discrete nuclear bound states of K̄ in few-body nuclear systems [1], [2], [3]. The K̄-nucleus interaction was derived from K̄N interactions which were constructed so as to account for the K̄N scattering lengths, the K−p atomic shift and the energy and width of Λ(1405). In these systems the strong attraction of the I=0 K̄N interaction () plays an important role. It accommodates a deeply bound state, while contracting the surrounding nucleus, thus producing an unusually dense nuclear system. Since the binding energies are so large that the main decay channel of the I=0 K̄N to Σ+π is closed energetically (and additionally, the channel to Λ+π is forbidden by the isospin selection rule), these deeply bound states are expected to have small widths. Narrow bound states, and , with K̄ binding energies of 108 and 86 MeV, respectively, were predicted. We studied a 4He(stopped K−,n) process to produce , in which the ejected neutron is used as a spectator. Its experimental feasibility has been discussed by Iwasaki et al. [4]. Another type of reactions, in-flight (K−,N), was discussed by Kishimoto [5]. In the present Letter we propose an alternative method, namely, (K−,π−) and (π+,K+) reactions, to produce more exotic K̄ bound states.
In view of the situation that Λ(1405) is a bound state accommodated in a calculated K−p potential, as shown in Fig. 1, we readily recognize that a nuclear K̄ system is nothing but “dissolved” states. Therefore, the formation of a in a nucleus as a “seed” will lead to the production of K̄ bound states. In other words, the produced in a nucleus can serve as a “doorway” toward K̄ bound states. The problem is how to produce in a nucleus and how to identify produced K̄ bound states. Here, we point out that the “strangeness exchange reactions” (K−,π−) (or similarly, (π+,K+)) would lead to the production and detection of K̄ bound states [6]. Although it resembles the ordinary method for Λ and Σ hypernuclear spectroscopy, no attention has ever been paid to the excitation region, which is much higher than MΣc2=1190 MeV. One of the advantages of this reaction is to produce very exotic K̄ bound systems on proton-rich “nuclei”, such as p–p, that are unbound without the presence of K−. We first discuss the structure of such exotic systems that can be formed only by the (K−,π−) reaction and then consider their production processes.
Section snippets
Structure of proton-rich bound states
Table 1 shows what kinds of exotic species of K̄ bound states are formed following (K−,π−) reactions. The I=0 K̄N pair, which possesses a strong attraction, gives an essential clue to lower the energy of a bound system. Thus, K−pp, K−ppp and K−pppn systems on non-existing nuclei, which can be produced from d(K−,π−), 3He(K−,π−) and 4He(K−,π−) reactions, respectively, are of particular interest. The doorway states are expressed as , and in the hypernuclear nomenclature, which are
Production of nuclei via formation
Let us now discuss how to produce K̄ nuclei. In the special case for we have proposed to use the 4He(stopped K−,n) reaction [2], [3], [4], which is a nuclear Auger process. The use of in-flight (K−,N) reactions was proposed by Kishimoto [5], who considered a knock-on type mechanism. In principle, we need a kind of trapping process for an incoming energetic K− into a nucleus. In this respect the abundant production of in K−-induced reactions, which was observed in past bubble-chamber
Production via formation
The process we have considered above is a “direct reaction” type. There is another process, that is, the “resonant formation of a compound nucleus”. The production cross section for Λ(1405) observed in a bubble-chamber experiment [12], [13] shows a resonance-like plateau at a c.m. energy of MeV (Fig. 8 of Ref. [12]). This indicates the resonant formation of a compound state, which decays in vacuum as as shown in Fig. 4. When this resonant
Past experiments
It is an intriguing question to ask whether or not the K−+d bubble-chamber experiment [12], [13] showed any evidence for the presence of a bound K−pp state. In fact, the invariant mass spectra of (Σπ)0 in the K−+d→(Σπ)0+π−+ps reaction channel revealed not only Σ(1385), Λ(1405) and Λ(1520) peaks but also a substantial continuum (Fig. 11(a) of Ref. [12]), which might be accounted for as due to the three-body final states in the decay of a hypothetical K−pp bound state, as The
Concluding remarks
In the present Letter we have shown theoretically that very exotic species of K̄ bound states can be formed in (K−,π−) and (π+,K+) reactions, where Λ(1405) and Λ(1520) play important roles as doorways. Such “bound-K̄ nuclear spectroscopy” will become a new paradigm in strangeness nuclear physics. Of particular interest is the possibility that a high-density nuclear medium will be created around a K−. Whether or not the K− and the surrounding nucleons keep their identities and obey the present
Acknowledgements
The authors would like to thank Dr. A. Dote and Prof. H. Horiuchi for informing us of their AMD results prior to publication, Professor H. Ströbele for informing us of their bubble chamber results, and Professor P. Kienle for stimulating discussions. The present work is supported by the Grant-in-Aid for Scientific Research of Monbukagakusho of Japan.
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