How the Proton and Neutron Got Their Masses
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Nucleons (protons and neutrons) are complex structures, whose primordial ingredients are three "valence quarks": for the proton and for the neutron. The electric charges of the up quark and the down quark are and , respectively, which add up to a proton charge of and a neutron charge of 0. Each quark flavor exists in three possible charge states, commonly designated as red, green, and blue. The quark colors are continually exchanged by absorption and emission of gluons. This is the mechanism of the strong interaction, described by a theory known as quantum chromodynamics (QCD). Hadrons can exist only in color-neutral combinations: either three colored quarks for a baryon or a color-anticolor pair for a meson. This implies perpetual quark confinement, negating the possibility of observing free isolated quarks.
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Contributed by: S. M. Blinder (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
Snapshot 1: quarks interact very weakly when close together; this is called "asymptotic freedom", a feature of QCD
Snapshot 2: as their separation increases, the force between quarks becomes stronger, shown as exchange of gluons
Snapshot 3: gluons, which themselves carry color charge, can interact with one another...
Snapshot 4: ...and create virtual quarks and antiquarks
References:
A. S. Kronfeld, "Quantum Chromodynamics with Advanced Computing," Journal of Physics: Conference Series, 125, 2008 012067.
F. Wilczek, The Lightness of Being: Mass, Ether, and the Unification of Forces, New York: Basic Books, 2008.
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