Quark color charge, gluon flux tubes, and string breaking
Color confinement is the phenomenon that quarks are never observed in isolation — they are always bound into color-neutral hadrons. Unlike electromagnetism where field lines spread out, QCD gluon fields form a narrow flux tube between quarks, creating a linear potential V(r) ≈ σr where σ ≈ 1 GeV/fm is the string tension. When quarks are pulled far enough apart (~1 fm), the stored energy creates a new quark-antiquark pair — string breaking — rather than allowing free quarks.