Nuclear forces and ab initio nuclei from QCD

6 problems

• Simultaneous binding energies and charge radii of medium-mass nuclei

  Partially resolved

  First-principles calculations with many common nuclear forces either bind medium-mass nuclei correctly but make them too small, or get the size right and the binding wrong. The cause is thought to lie in the three-nucleon force (a force that acts only when three nucleons are close together).

• A renormalizable power counting for chiral nuclear forces

  Contested

  Chiral effective field theory builds nuclear forces as a series ordered by powers of small momenta divided by roughly 600 MeV. The standard ordering gives results that depend on an arbitrary cutoff in some channels, and no consistent alternative has been agreed upon.

• Neutron skins of 208Pb and 48Ca from parity-violating scattering

  Contested

  In heavy nuclei the extra neutrons form a thin outer skin whose thickness tells how the pressure of neutron-rich matter rises with density. Two Jefferson Lab experiments give values for lead and calcium that most nuclear models cannot fit at the same time.

• Two-nucleon binding from lattice QCD at physical quark masses

  Contested

  The deuteron (a bound proton and neutron) is held together by only 2.2 MeV, while two neutrons do not bind at all. Lattice QCD calculations at heavier-than-real quark masses disagree with each other on whether two nucleons bind.

• Isospin-asymmetry dependence of single-nucleon removal strength

  Contested

  When one nucleon is knocked out of a nucleus, experiments find fewer nucleons in each orbit than the simple shell model predicts, a reduction called quenching. In knockout reactions with fast radioactive beams the reduction grows strongly for the more tightly bound kind of nucleon in very neutron- or proton-rich nuclei, while other reaction types show little such trend.

• Does a four-neutron resonance exist?

  Contested

  Four neutrons alone are not expected to hold together, yet a 2022 experiment saw a short-lived bump in the energy of four outgoing neutrons. Theory disagrees on whether a true resonance (a quasi-bound state that falls apart quickly) can exist.