Search for $\eta $-mesic Nuclei with WASA-at-COSY

Abstract

We search for an evidence of η-mesic He with the WASA detector. Two dedicated experiments were performed at the Cooler Synchrotron COSY-Jülich. The experimental method is based on the measurement of the excitation functions for the two reaction channels: dd → 3 He pπ − and dd → 3 He nπ 0 , where the outgoing N -π pairs originate from the conversion of the η meson on a nucleon inside the He nucleus. In this contribution, the experimental method is shortly described and the current status of the analysis is presented.

$\eta$ η -nucleus interaction from the d + d reaction around the $\eta$ η

“…(27), and the parameter p 0 appeared in Eq. (29) to determine the property of the function F which physical meanings are explained in Eqs. (1) and (2).…”

“…In the experiments of this reaction at WASAat-COSY [29,30,31,32,33,34], the total energy of the system is varied by changing the deuteron beam momentum around the η + α threshold energy, which corresponds to the beam momentum p = 2.3 GeV/c, and the η + α bound state production signal is expected to be observed as peak structures of the cross section of the d + d → 3 He + p + π − and d + d → 3 He + n + π 0 reactions in the energy region below the η-α production threshold energy region.…”

“…Recently, the new experiments of the d+d → (η+α) → X reaction have been proposed and performed at WASAat-COSY [29,30,31,32,33,34,35]. In the experiments, the formation cross section of the η mesic nucleus in the α particle in the d + d fusion reaction is planned to be measured by observing the emitted particles from the decay of the η mesic nucleus below the η production threshold.…”

Momentum distribution of N* in nuclei

“…The present work is a first attempt to evaluate the N * (1535) resonance momentum distribution in nuclei. This distribution, as mentioned in the beginning, is necessary to establish the detector system acceptance for the registration of the d d → ( 3 He-N * ) → 3 He Nπ reaction and to determine the data selection criteria [6]. A calculation of the momentum distribution of N * -d in 3 He would be necessary for the analysis of the p d → (d-N * ) → d Nπ reaction aimed at searching η-mesic 3 He whose prospects seem higher due to the fact that we have already seen a strong enhancement in the p d → 3 He η reaction near threshold.…”

“…2: Elementary potential as given in Eq. (6). Note that whereas π + η exchange contributes to the n N * → n N * potential, only π contributes to the p N * → p N * potential in N * -3 He.…”