Quantum chromodynamics binding energy (QCD binding energy),gluon binding energy orchromodynamic binding energy is theenergy bindingquarks together intohadrons. It is the energy of thefield of thestrong force, which is mediated bygluons.Motion-energy andinteraction-energy contribute most of the hadron's mass.[1]
Most of themass of hadrons is actuallyQCD binding energy, throughmass–energy equivalence. This phenomenon is related tochiral symmetry breaking. In the case ofnucleons —protons andneutrons— QCD binding energy forms about 99% of the nucleon's mass.
Thekinetic energy of the hadron's constituents, moving at near thespeed of light, contributes greatly to the hadron mass;[1] otherwise most of the rest is actual QCD binding energy, which emerges in a complex way from the potential-like terms in the QCDLagrangian.
For protons, the sum of therest masses of the threevalence quarks (twoup quarks and onedown quark) is approximately9.4 MeV/c2, while the proton's total mass is about938.3 MeV/c2. In the standard model, this "quark current mass" can nominally be attributed to theHiggs interaction. For neutrons, the sum of the rest masses of the three valence quarks (two down quarks and one up quark) is approximately11.9 MeV/c2, while the neutron's total mass is about939.6 MeV/c2. Considering that nearly all of theatom's mass is concentrated in the nucleons, this means that about 99% of the mass of everyday matter (baryonic matter) is, in fact, chromodynamic binding energy.
While gluons aremassless, they still possess energy — chromodynamic binding energy. In this way, they are similar tophotons, which are also massless particles carrying energy —photon energy. The amount of energy per single gluon, or "gluon energy", cannot be directly measured, though a distribution can by inferred fromdeep inelastic scattering (DIS) experiments (see ref [4] for an old but still valid introduction.) Unlike photon energy, which is quantifiable, described by thePlanck–Einstein relation and depends on a single variable (the photon'sfrequency), no simpleformula exists for the quantity of energy carried by each gluon. While the effects of a single photon can be observed, single gluons have not been observed outside of a hadron. A hadron is in totality[2] composed of gluons, valence quarks,sea quarks and othervirtual particles.
The gluon content of a hadron can be inferred from DIS measurements. Again, not all of the QCD binding energy is gluon interaction energy, but rather, some of it comes from the kinetic energy of the hadron's constituents.[3] Currently, the total QCD binding energy per hadron can be estimated through a combination of the factors mentioned. In the future, studies intoquark–gluon plasma will better complement the DIS studies and improve our understanding of the situation.
° Halzen, Francis and Martin, John, "Quarks and Leptons:An Introductory Course in Modem Particle Physics", John Wiley & Sons (1984).