Observation of Eta-Mesic Nuclei in Photoreactions: Results and Perspectives

Abstract

Recent results from the LPI experiment on searching for η-mesic nuclei in photoreactions are discussed and further perspectives are summarized.This talk concerns experimental investigations of new objects of the nuclear physics, η-mesic nuclei ( η A) which are bound systems of the η-meson and a nucleus. Discovery and learning properties of these objects are of fundamental significance for understanding interactions of the η-meson with nucleons and nucleon resonances, for understanding a behavior of hadrons in nuclear matter.A mechanism of creation of η-mesic nuclei in photoreactions and their decay into a πN pair is shown in Fig. 1. At the first stage of the reaction, an incoming photon produces a slow η-meson and a fast nucleon N 1 escaping the nuclear target. Then the η-meson propagates in the nuclear matter and undergoes multiple rescattering off nucleons. At last, η annihilates producing an πN 2 pair escaping the nucleus. The S 11 (1535) nucleon resonance plays a fundamental role in all that dynamics. It mediates creation and annihilation of η and it makes the η-meson bound in the nucleus due to an S 11 (1535)-induced ηN interaction (attraction). Knowing energies/momenta of the ingoing (γ) and outgoing (N 1 , πN 2 ) particles, one can determine energies/momenta of the η and S 11 (1535) propagating in the nuclear matter. The width and binding energy of η in the nucleus can be determined as well.Theoretical estimates given in Refs. 1,2 show that the binding effects lead to a full dominance of the reaction mechanism related with the multiple rescattering of η and a formation of the intermediate η-nucleus over a mechanism of nonresonance background production of the πN pairs in the subthreshold invariant-mass region √ s πN < m η + m N of the subprocess η + N → π + N . A peak in the mass distribution of πN is theoretically expected in this region. This is illustrated in Fig. 2 where the spectral function S(E) of the (kinetic) energy E of η in the nucleus is shown which is proportional to the number of ηN collisions which the η experiences when travels through the nucleus. Another spectral function, S(E, q), shows a distribution of the produced πN pairs over their total energy (E + m η + m N ) and momentum q. The presence

Status and perspectives of the search for Eta-Mesic nuclei

“…The first evidence for the existence of an η-mesic bound states was claimed by the LPI group [27,28] which carried out the measurement of photoproduction processes: (i) 11 B nuclei. The invariant mass distribution of the correlated π + n pairs for the first process shows a narrow peak structure below the η meson production threshold which is in agreement with the theoretical prediction supporting the η-mesic nuclei formation via N * (1535) resonance excitation and its decay into π-nucleon pair.…”

Non-mesonic decay of the η-mesic 3He via pd→(3Skurzok1, Hirenzaki2, Kinutani3 et al. 2020Nuclear Physics A9010View full textAdd to dashboardCite

“…However, none of the experiments performed till now have found a clear signature confirming their existence. They only provided signals which might be interpreted as indications of the η-mesic nuclei as well as allow the determination of the upper limits of the total cross section for the bound state formation [7,8,9,11,12,23,25,41,42,43,44,45,46,47,48,49,50,51,54,52,53].…”

Search for the η-mesic bound states with the WASA-at-COSY detector

“…Current investigations of hadron-and photon-induced production of the η meson deliver a wide range of ηN scattering length values (a ηN ) indicating the η-nucleon interaction to be strong enough to create light η-mesic nuclei [9][10][11][12][32][33][34]. However, despite many experiments performed, there is still no clear evidence of their existence [35][36][37][38][39][40][41][42]. The interested reader can find recent reviews on the η mesic bound state searches in Refs [6,7,16,18,30,[45][46][47][48][49].…”