"Universal aspects of the neutron-neutron and neutron-α scattering systems through the telescope of an effective field theory":
An effective field theory is a systematic description of a subset of the physical world of all observables. While those subsets may overlap, there is at least a trusted prescription of how to link their effective descriptions consistently. Although, effective field theories have been used in the realm of nuclei for decades, we still lack precise knowledge about the elements and boundaries of this peculiar subset, in particular, especially along the axis of increasing particle number. I will present two analysis in this border area. First, the membership of three-body observables to the area of applicability of the effective field theory without pions, EFT ( pionless), is shown to yield a model-independent constraint on the notoriously hard-to-measure neutron-neutron scattering length [1].
Second, 4- and 5-body scattering systems are investigated, where we conjecture the existence of an additional scale for their leading-order description in EFT(pionless). The cutoff dependence of S-matrix poles, i.e., resonances, in those systems is indicative to the conjecture of a correlation between 4- and 5-body data generalizing the 3- and 4-body Tjon-line analog [2].
The presentation of the results is preceded by an introduction to the effective field theory without pions (e.g. [3]) as an appropriate description of light nuclei which is consistent with the underlying theory of QCD. Furthermore, a refined version of the resonating group method [4] is explained as a versatile technique for the variational solution of a general few-body problem.