Quarks are bound together into protons and neutrons by a force known as the strong interaction, mediated by the exchange of force carrier particles called gluons. The strong interaction is also responsible for binding together the protons and neutrons inside atomic nuclei.
Although much of the physics of strong interaction is, today, well understood, two very basic issues remain unresolved: the origin of confinement and the mechanism of the generation of mass. Both are thought to arise from the way the properties of the vacuum are modified by strong interaction.
No quark has ever been observed in isolation: the quarks, as well as the gluons, seem to be bound permanently together and confined inside composite particles, such as protons and neutrons. This is known as confinement. The exact mechanism that causes it remains unknown.
Generation of mass
Protons and neutrons are known to be made of three quarks, but by adding together the masses of the three quarks one gets... only about 1% of the proton or neutron mass. Where does the remaining 99% come from?
Is the mechanism that confines quarks inside protons and neutrons also responsible for the generation of most of the mass of ordinary matter?
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