Low-energy ηd-resonance

Abstract

Elastic ηd scattering is considered within the Alt-Grassberger-Sandhas (AGS) formalism for various ηN input data. A three-body resonant state is found close to the ηd threshold. This resonance is sustained for different choices of the two-body ηN scattering length a ηN . The position of the resonance moves towards the ηd threshold when Re a ηN is increased, and turns into a quasi-bound state at Re a ηN = 0.733 fm.PACS numbers: 25.80.-e, 21.45.+v, 25.10.+s One of the main questions of η-meson physics concerns the existence of η-nuclei, a possibility predicted in a pioneering work by Haider and Liu [1]. In this context it is worth mentioning that according to [2] the mean free path of η mesons in a nuclear medium is about 2 fm, i.e., less than the size of a typical nucleus. A necessary condition for the existence of η-nuclei, hence, appears to be satisfied. A further indication is the observation of pionic nuclei (deeply bound pion states inside nuclei) [3], which were predicted theoretically in [4].Recently, in a number of theoretical investigations based on quite different approaches, such as the mean field approximation [5], the optical model [6], and few-body calculations [7,8], the idea of η-nuclei was given a rather firm ground. Experimentally, after the discouraging attempt of Ref. [9], certain evidence for the existence of η-nuclei was noticed by two groups [10,11]. An additional indication is given by the enhancement of near-threshold η production in the reaction np → dη as reported by the Uppsala group [12]. This observation is most likely to be understood as the effect of a near-threshold bound or resonant ηd state. Moreover, it was suggested [13] that even quite exotic systems containing an η meson and a hyperon (η-hypernuclei) may exist.Being of interest by itself, the existence of η-nuclei would also shed new light on various fundamental problems of particle physics. For example, the study of η-nuclei would give a clue for understanding the possible restoration of chiral symmetry in a nuclear medium, or its partial restoration which may occur at normal densities ∼ 0.17 fm −3 [14][15][16]. A further aspect, which makes such systems interesting for other fields of nuclear physics, is the modification of the two-body ηN force by the nuclear medium. Being enhanced due to the resonant character of the ηN interaction, general properties of such modifications should become particularly evident in this case. Especially, the structure of the S 11 resonance embedded in a nuclear medium may be studied. This would exhibit certain details of the effective Lagrangian models of such a resonance [17], and shed some light, for example, on 1 chiral models which suggest a reduction of its partial width for decaying into the ηN channel [16], or assume S 11 to be a quasi-bound state of the K meson and a Σ hyperon [18]. The interesting suggestion of considering S 11 as a joint manifestation of threshold (casp) and resonance phenomena [19] could also be checked.In a sense, the problem of choosing the most adequa...

Search for quasi boundηmesons

“…Within this approach η mesic nuclei exist for 4 He. But for sufficiently large a ηN even the deuteron can bind the η [46]. The same method together with time delay analysis was applied in [47] to derive the position of η bound states in 3 He and in Ref.…”

Coherent photoproduction of η-mesons on three-nucleon systems

“…To the best of our knowledge, so far only the simplest of these systems, namely the η(2N) system, was considered [12][13][14][15] within the exact AGS theory [16].…”

“…It is simply assumed that all mesons should have somewhat similar properties and therefore the off-shell behavior of this amplitude could be approximated (like in the case of π mesons) by appropriate Yamaguchi form-factors (see, for example, Refs. [12][13][14][15]17,18]). However, if the available data are used to construct a potential via, for example, Fiedeldey's inverse scattering procedure [19], the resulting form factor of the separable potential is not that simple.…”

Photoproduction of mesons off nuclei

“…It runs all the way from a negative value of −0.15 fm to numbers close to and even above +1 fm [73]. However, most of the more recent analyses prefer large values above 0.5 fm, which has raised discussions about very light mesic nuclei and prompted theoretical studies of the η-interaction with 2 H, 3 H, 3 He, and 4 He systems [74,75,76,77,78,79,80,81,82,83] . Experimental evidence for light η-mesic nuclei has mostly been searched in the threshold behavior of η-production reactions.…”