Three-body continuum states on a Lagrange mesh

Three-body continuum states are investigated with the hyperspherical method on a Lagrange mesh. The $R$-matrix theory is used to treat the asymptotic behaviour of scattering wave functions. The formalism is developed for neutral as well as for charged systems. We point out some specificities of continuum states in the hyperspherical method. The collision matrix can be determined with a good accuracy by using propagation techniques. The method is applied to the $^6$He (=$\alpha$+n+n) and $^6$Be (=$\alpha$+p+p) systems, as well as to $^{14}$Be (=$^{12}$Be+n+n). For $^6$He, we essentially recover results of the literature. Application to $^{14}$Be suggests the existence of an excited $2^+$ state below threshold. The calculated B(E2) value should make this state observable with Coulomb excitation experiments.