From Nuclear to Unnuclear Physics

Nuclear physics studies the stability, structure, and reactions of atomic nuclei, as well as the phase structure and properties of dense, strongly interacting matter. While nuclear reactions are exceedingly complicated, remarkably Hammer and Son (1) have discovered a low-energy regime—which they call “unnuclear physics”—where certain nuclear reactions obey simple scaling rules, related to a new, emergent scaling symmetry of the nuclear force. The term “unnuclear” is a reference to the work of Howard Georgi, who coined the term “unparticle physics” to describe a possible scale-invariant sector beyond the standard model of high-energy physics (2). To appreciate the Hammer and Son (1) result let us step back and recall some basics. Modern nuclear physics began with the discovery of the neutron in 1932, but ever since the 1970s we have known that the strong nuclear forces between neutrons and protons, collectively known as nucleons, are ultimately governed by quantum chromodynamics (QCD), the theory of quarks and gluons. Quarks carry color charges, a generalization of electrical charge, and forces between “colored” particles are mediated by gluons. Neutrons and protons are in fact composite particles, and the nuclear force is the residual interaction between “color-neutral” objects, similar to the van der Waals force between electrically neutral atoms and molecules. As a consequence, determining nuclear structure and reactions in detail requires numerical calculations based on complicated nuclear forces. Not only did Hammer and Son discover a simple tractable regime of nuclear reactions, even more remarkably, the dependence of the cross-sections on energy that appear in this regime can be related to a completely different observable, the ground-state energy of resonantly interacting ultracold atoms trapped in a harmonic potential. Nucleon as well as nuclear reactions at low incident energies can be described in terms of two parameters, the “scattering length,” a, and the effective … [↵][1]1To whom correspondence may be addressed. Email: gbaym{at}illinois.edu. [1]: #xref-corresp-1-1