Multiplicity of self-adjoint realisations of the (2+1)-fermionic model of Ter-Martirosyan-Skornyakov type

We reconstruct the whole family of self-adjoint Hamiltonians of Ter-Martirosyan-Skornyakov type for a system of two identical fermions coupled with a third particle of different nature through an interaction of zero range. We proceed through an operator-theoretic approach based on the self-adjoint extension theory of Krein, Višik, and Birman. We identify the explicit 'Krein-Višik-Birman extension parameter' as an operator on the 'space of charges' for this model (the 'Krein space') and we come to formulate a sharp conjecture on the dimensionality of its kernel. Based on our conjecture, for which we also discuss an amount of evidence, we explain the emergence of a multiplicity of extensions in a suitable regime of masses and we reproduce for the first time the previous partial constructions obtained by means of an alternative quadratic form approach.