Search for quasi boundηmesons

Abstract

The search for a quasi bound η meson in atomic nuclei is reviewed. This tentative state is studied theoretically as well as experimentally. The theory starts from elastic η nucleon scattering which is derived from production data within some models. From this interaction the η nucleus interaction is derived. Model calculations predict binding energies and widths of the quasi bound state. Another method is to derive the η nucleus interaction from excitation functions of η production experiments. The s wave interaction is extracted from such data via final state interaction theorem. We give the derivation of s wave amplitudes in partial wave expansion and in helicity amplitudes and their relation to observables. Different experiments extracting the final state interaction are discussed as are production experiments. So far only three experiments give evidence for the existence of the quasi bound state: a pion double charge exchange experiment, an effective mass measurement, and a transfer reaction at recoil free kinematics with observation of the decay of the state.

Production of η and η′ mesons on nucleons and nuclei

“…The data are in quite good agreement with the results of the model of Egorov and Fix [125], so that their predictions for the γ 4 He → 4 He ηπ 0 reaction (total cross sections up to 100 nb) are also probably realistic. With the additional degree of freedom from the π 0 meson kinematics, where events can be selected for which the η is slow with respect to the 4 He nucleus, it seems that the γ 4 He → 4 He ηπ 0 reaction may be a promising tool for the search for 4 η He. A corresponding experiment proposal has been accepted for CBall/TAPS at MAMI but the measurement has not yet been undertaken.…”

“…After taking into account the momentum distribution in the circulating deuteron beam and the smearing connected with the measuring apparatus, the best fit was achieved with [181] 4) where the first error is statistical and the second systematic. Note that these data cannot determine the sign of the imaginary parts of the p i , a point that is critical in the discussion of η-mesic nuclei in Sec.…”

“…By increasing the size of the real part of the scattering length Re(a ηN ) from 0.27 fm to 0.51 fm, the simplistic potential model predicts quasi-bound η-mesic nuclei down as far as 4 η He [283] but this at the expense of generating states with decay widths in the p-shell nuclei of the order of 10-15 MeV. Since an η would be expected to stick to an excited nuclear level with about the same binding energy as for the ground state [284], the search for such states is clearly made much more difficult when the decay width is larger than the nuclear level spacing.…”

“…The exact location of the pole in the complex Q plane is far more uncertain because the data are not very sensitive to the phase of Q 0 and its magnitude has also a comparatively large error, |Q| = 4.3 ± 1.3 MeV [275]. It was argued by Willis et al [273] that an η would be attracted more to 4 He than to 3 He because of the extra nucleon and the smaller nuclear radius. Hence, since |Q 0 | is bigger for 4 He, it would imply that η 4 He is quasi-bound.…”

“…Earlier articles have surveyed various aspects of the field [1,2,3,4,5], but possibly with not such a wide remit. Our primary thrust will be to discuss physics phenomena rather than the details of theoretical models.…”