Search for Bound States of theηMeson in Light Nuclei

Abstract

A search for nuclear-bound states of the 77 meson has been carried out. Targets of lithium, carbon, oxygen, and aluminum were placed in a /r"^ beam at 800 MeV/c. A predicted T] bound state in ^^O* {Ex « 540 MeV) with a width of « 9 MeV was not observed. A bound state of a size j of the predicted cross section would have been seen in this experiment at a confidence level of 3CT {P> 0.9987).PACS numbers: 25.80.-e, 21.90.+f, 27.20.+n This Letter describes a search for a novel nuclear excitation involving the creation of a bound r] meson in the nuclear medium. The concept is similar in spirit to a number of ideas which have recently been vigorously pursued. Some familiar examples are A hypernuclear states, Z hypernuclear states, antiprotonic nuclear states, and various dibaryon resonances. In each case an attractive particle-nucleus potential is required together with some mechanism to inhibit the decay process, such as strangeness conservation in the case of the A.Several suggestions of the existence of bound states of the 7] meson in a wide range of nuclei have recently been published. ^"^ The suggestions of this novel nuclear excitation are based on bound-state formation through the attractive N-T] channel of the TV* (1535), where A^*(1535) is the {KN) resonance with (/,/'') = (T, y") and mass 1535 MeV/c^. This resonance dominates 77 production near threshold. Bhalerao and Liu'* have shown, by a coupled-channels analysis, that the lowenergy rfN interaction is attractive with a scattering length of 0.28 + 0.202/ fm. The attractive interaction is a consequence of the threshold being below the A^*(1535) resonance.Liu and his collaborators have examined the consequences of this attractive interaction in the formation of a bound-77 state as a function of mass number. Their study indicates that nuclear bound states could exist for mass numbers larger than A^\Q. At low mass numbers, only 5-state bound r/'s are predicted. At larger mass numbers, p and d states could become bound. Both binding energies and widths increase with A. The optimum case, in their analysis, is \^0, formed from the in^.p) reaction on *^0 at a momentum near 740 MeV/c. ^ At an angle near 15°, the momentum transfer is favorable for the transition involving the conversion of a p-shell neutron to an ^-shell ry. For higher mass numbers, the increase in predicted width would make this excitation more difficult to see over the continuum {K^,p) background which is present.An experiment to test these predictions was devised with the positive pion beam available at the low-energy separated beam I at the Brookhaven alternating-gradient synchrotron (AGS), and the Moby Dick spectrometer. The experimental arrangement is virtually identical with that used for the production and measurement of hypernuclei, and it has been described in detail in a number of publications (see Milner et al.^ and related references). The only differences involve the selection of pions, rather

Search for quasi boundηmesons

“…Furthermore, the linear combination of Eqs. (77) and (78) shows that the fitting of |B| and |C| is decoupled from that of A 0 and A 2 . The parameters resulting from fitting the data in this basis are given in Table 10, with the fit curves being shown in Fig.…”

Few-Body Aspects of the Near Threshold Pseudoscalar Meson Production

“…A decade later, a large values of the η-nucleon scattering length (1 fm) were extracted in some analysis reported in [99] which do not exclude the formation of bound η-nucleus states for such light nuclei as helium [100,101] or even for deuteron [102]. However, so far none of the experiments have confirmed univocally the existence of such a state neither in reactions induced by pions [103], protons [104], deuterons [105][106][107] or photons [93,108]. In the searches for the direct signal from the η-mesic helium the established so far upper limits amount to about 270 nb for the dp → ( 3 Heη) bound → pppπ − reaction [105], about 70 nb for the dp → ( 3 Heη) bound → 3 Heπ 0 reaction [105], and about 25 nb for the dd → ( 4 Heη) bound → 3 He pπ − reaction [106].…”

Threshold scattering of the η-meson off light nuclei

“…The ηN near-threshold interaction, therefore, remains an interesting field of investigation. Of particular relevance in this context is the possibility of η-nucleus bound states [9,10]. Their existence would provide us with an excellent opportunity to answer the above-mentioned questions in a reliable manner.…”