<P> Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons (NN, NNN) or protons (PP, PPP). Nuclei which have a single neutron halo include Be and C. A two - neutron halo is exhibited by He, Li, B, B and C. Two - neutron halo nuclei break into three fragments, never two, and are called Borromean nuclei because of this behavior (referring to a system of three interlocked rings in which breaking any ring frees both of the others). He and Be both exhibit a four - neutron halo . Nuclei which have a proton halo include B and P. A two - proton halo is exhibited by Ne and S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive electromagnetic forces of the excess proton (s). </P> <P> Although the standard model of physics is widely believed to completely describe the composition and behavior of the nucleus, generating predictions from theory is much more difficult than for most other areas of particle physics . This is due to two reasons: </P> <Ul> <Li> In principle, the physics within a nucleus can be derived entirely from quantum chromodynamics (QCD). In practice however, current computational and mathematical approaches for solving QCD in low - energy systems such as the nuclei are extremely limited . This is due to the phase transition that occurs between high - energy quark matter and low - energy hadronic matter, which renders perturbative techniques unusable, making it difficult to construct an accurate QCD - derived model of the forces between nucleons . Current approaches are limited to either phenomenological models such as the Argonne v18 potential or chiral effective field theory . </Li> <Li> Even if the nuclear force is well constrained, a significant amount of computational power is required to accurately compute the properties of nuclei ab initio . Developments in many - body theory have made this possible for many low mass and relatively stable nuclei, but further improvements in both computational power and mathematical approaches are required before heavy nuclei or highly unstable nuclei can be tackled . </Li> </Ul> <Li> In principle, the physics within a nucleus can be derived entirely from quantum chromodynamics (QCD). In practice however, current computational and mathematical approaches for solving QCD in low - energy systems such as the nuclei are extremely limited . This is due to the phase transition that occurs between high - energy quark matter and low - energy hadronic matter, which renders perturbative techniques unusable, making it difficult to construct an accurate QCD - derived model of the forces between nucleons . Current approaches are limited to either phenomenological models such as the Argonne v18 potential or chiral effective field theory . </Li>

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