NUCLEON-NUCLEON INTERACTION?

The earliest efforts to understand the interaction potential between nucleons was in terms of meson exchange. In other words the various mesons were considered the bosons of the strong interaction.  The nucleon-nucleon interaction displays obvious physical analogies to the atom-atom interaction!


The big problem with this approach to the nucleon-nucleon interaction is that the potential energy turns out to depend on absolutely everything it could possibly depend upon!

The tensor term breaks rotational symmetry!

Left: symmetric: TE, S = 1, T = 0 (d) and SE, S = 0, T = 1. Right: antisymmetric: SO, S = 0, T = 0, and TO, S = 1, T = 1.

Parametrized nucleon-nucleon potential, and state functions of the deuteron. [u(r) for s-state and w(r) for d-state].  Note most of the state is well outside the range of the potential!

Quarks come in three colors and gluons come in eight color-anticolor combinations (the green-antigreen plus red-anti-red plus blue-antiblue combination is symmetrically colorless, in other words neutral).  You won't be surprised to learn that Quantum Chromodynamics introduces color charge, color hypercharge, and color isospin!


Glueballs, predicted "bound" states consisting only of gluons, have been predicted using approximations to the Standard Model, but have never been observed. If they existed they would be very massive and have an extremely short lifetime.





CHIRAL PERTURBATION THEORY


ASYMPTOTIC FREEDOM

Note that it is essentially impossible to have a "free gluon." Gluons only exist as virtual particles, never real. Attempts to knock out quarks or gluons from a proton using very high energy electrons result in "jets" of hadrons... the quarks and gluons cease to exist as such, as they leave the proton interior.


Bag Models of Baryons

Largely ignored by textbooks, there have been a huge number of different so-called “bag models,” of the nucleon and its excited states, proposed since the 1970s. All the models confine three free valence quarks inside a surface  or spherical box of some kind, with various different boundary conditions at, and couplings to, the surface. People are still working on various complex bag systems, but it is hard to see how any real basic physical insight can result, although various basic symmetry principles can be applied in creating a bag system.  All bag models stem from a very simple model originally suggested by Bogoliubov in 1967.  Research using bag models continues even today (2022), with the latest wrinkles being chiral bag models, and bag models for mesons. 





THE ENERGY DESERT???

The so-called Standard Model consists of Quantum Chromodynamics, the theory of the strong interaction, and the Electroweak Theory, the theory of electromagnetic and weak processes. No flaw in the model has yet shown up; experimental results support the Standard Model to high precision. However, one obvious problem is that it contains 26 parameters that must be put in "by hand," based on experimental results, with no justification from some deeper theory.  Parameters of the Standard Model.


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