We formulate the Gamow shell model (GSM) in coupled-channel (CC) representation for the description of proton/neutron radiative capture reactions and present the first application of this new formalism for the calculation of cross-sections in mirror reactions 7 Be(p, γ) 8 B and 7 Li(n, γ) 8 Li. The GSM-CC formalism is applied to a translationally-invariant Hamiltonian with an effective finiterange two-body interaction. Reactions channels are built by GSM wave functions for the ground state 3 2 − and the first excited state 1 2 − of 7 Be/ 7 Li and the proton/neutron wave function expanded in different partial waves.PACS numbers: 03.65. Nk, 33.15.Ry

The search for a resonant four-neutron system has been revived thanks to the recent experimental hints reported in Phys. Rev. Lett. 116, 052501 (2016) [1]. The existence of such a system would deeply impact our understanding of nuclear matter and requires a critical investigation. In this work, we study the existence of a four-neutron resonance in the quasi-stationary formalism using ab initio techniques with various two-body chiral interactions. We employ the No-Core Gamow Shell Model and the Density Matrix Renormalization Group method, both supplemented by the use of natural orbitals and a new identification technique for broad resonances. We demonstrate that while the energy of the four-neutron system may be compatible with the experimental value, its width must be larger than the reported upper limit, supporting the interpretation of the experimental observation as a reaction process too short to form a nucleus.