Dissecting reaction calculations using Halo Effective Field Theory and ab initio input
Pierre Capel (Mainz)

Halo nuclei are among the most exotic quantum structures [1]. Unlike stable nuclei, which are tightly bound compact objects, halo nuclei exhibit an uncommonly large matter radius. This results from their unusual cluster structure. They can be seen as a core, which has all the features of a usual nucleus—viz. strongly bound and compact—to which one or two neutrons are loosely bound. Thanks to the quantum tunnel effect, the loose binding of these valence nucleons enables them to exhibit a high probability of presence at a large distance from the other nucleons. They thus form like a diffuse halo around a dense core [2].
Halo nuclei are among the most exotic quantal structures [1]. Unlike stable nuclei, which are tightly bound compact objects, halo nuclei exhibit an uncommonly large matter radius. This results from their unusual cluster structure. They can be seen as a core, which has all the features of a usual nucleus—viz. strongly bound and compact—to which one or two neutrons are loosely bound. Thanks to the quantum tunnel effect, the loose binding of these valence nucleons enables them to exhibit a high probability of presence at a large distance from the other nucleons. They thus form like a diffuse halo around a dense core [2].

Halo nuclei are mostly found close to or even at the neutron dripline, where additional neutrons can no longer be added to an isotopic line. Archetypical examples are 11Be or 15C, which exhibit a one-neutron halo, and 6He or 11Li, which have two neutrons in their halo [1]. Being located so far from stability, halo nuclei exhibit very short half lives and hence cannot be studied with usual spectroscopic techniques, like electron scattering. Their structure must be investigated through indirect techniques. In breakup reactions, the halo nucleus once produced is directly sent on a target. The different interactions its core and its halo neutrons experience with the target will lead to the dissociation of this fragile structure, hence revealing its internal structure.

To extract valuable information from experimental data, an accurate reaction model coupled to a realistic description of the projectile is needed [3]. In this seminar, after a summary on these exotic halo nuclei, I will explain how breakup reactions are modelled and how we can, thanks to Effective Field Theory, relate the most accurate ab initio calculations of halo nuclei to breakup cross sections and as such explore the structure of nuclei far from stability.
[1] I. Tanihata, J. Phys. G, 22, 157 (1996)
[2] P.G. Hansen and B. Jonson, Europhys. Lett. 4, 409 (1987)
[3] D. Baye and P. Capel, Lecture Notes in Physics 848, 121 (2012)