Three-nucleon clusters in nuclear matter

The effect of three-body clusters on the binding energy of nuclear matter is studied. An attempt is made to treat all such clusters of the third order as self-energy diagrams and to include their effect into the single-particle energies, thereby improving the convergence of the Goldstone expansion considerably. For this purpose, an expression for off-diagonal reaction matrix elements is derived. One of the consequences of this procedure is that, to a good approximation, the contribution of all the third-order diagrams together is equal to that arising from just the even "l" states of the simplest of these diagrams. Many of the arguments and approximations used are based on the previous paper of this issue by Bethe et al