Scattering Experiments Tease Out the Strong Force

Abstract

How does nature hold together protons and neutrons to form the wide variety of complex nuclei in the Universe? Describing many-nucleon systems from the fundamental theory of quantum chromodynamics has been the greatest challenge in answering this question. The chiral effective field theory description of the nuclear force now makes this possible but requires certain parameters that are not uniquely determined. Defining the nuclear force needs identification of observables sensitive to the different parametrizations. From a measurement of proton elastic scattering on 10 C at TRIUMF and ab initio nuclear reaction calculations, we show that the shape and magnitude of the measured differential cross section is strongly sensitive to the nuclear force prescription.Understanding the strong nuclear force is of fundamental importance to decipher nature's way of building visible matter in our Universe. Yet, more than a century after the discovery of the nucleus, our knowledge of the nuclear force is still incomplete. The formulation by Weinberg of chiral effective field theory [1] enabled a major breakthrough in arriving at a fundamental understanding of the low-energy nuclear interactions of protons and neutrons, by forging the missing link with quantum chromodynamics. However, the question of how to best implement the theory and constrain it with experimental data remains an active topic of research, and has already led to several parametrizations of the nuclear force [2][3][4][5][6]. It is, therefore, important to identify experimental observables that are sensitive to different parametrizations of the chiral forces in order to reach a definitive description of the nuclear force. The study of many-nucleon systems enables a more complete understanding of the nuclear force. In particular, proton-rich and neutron-rich nuclei located at the edges of nuclear stability (drip lines) can amplify less-constrained features of the nuclear force, such as its dependence on the proton-neutron asymmetry. However, there is a lack of experimental data on the properties of these systems.Among the properties of the drip-line nuclei, we hypothesize in this work that the nucleon-nucleus scattering differential cross section is highly sensitive to the details of the nuclear force and, hence, can be used for constraining it. Indeed, it should reveal both the spectroscopic properties of the reacting system, such as phase shifts and their interference, as well as the effect of exotic nucleon distributions. This confluence brings a greater selectivity in the elastic scattering differential cross section than is possible by independently investigating resonance energies, binding energies, or radii. The observations reported here show that the shape and magnitude of the elastic scattering angular distribution places stringent constraints on the chiral interactions, while a study of resonance energies alone could lead to incomplete and/or misleading conclusions. The study of elastic scattering for drip-line nuclei is, however, challenging because o...