Quantum Physics

996 Chapter 30 Nuclear Energy and Elementary ParticlesComputers at Fermilab create apictorial representation such asthis of the paths of particles aftera collision.Courtesy of Fermi National Accelerator LaboratoryTIP 30.3 Color is NotReally ColorWhen we use the word color todescribe a quark, it has nothing to dowith visual sensation from light. It issimply a convenient name for aproperty analgous to electric charge.q(a)(b)qMesonBaryonFigure 30.11 (a) A green quark isattracted to an anti-green quark toform a meson with quark structure( qq). (b) Three different-coloredquarks attract each other to form abaryon.30.13 COLORED QUARKSShortly after the theory of quarks was proposed, scientists recognized that certainparticles had quark compositions that were in violation of the Pauli exclusion principle.Because all quarks have spins of 1/2, they are expected to follow the exclusionprinciple. One example of a particle that violates the exclusion principle isthe (sss) baryon, which contains three s quarks having parallel spins, giving it atotal spin of 3/2. Other examples of baryons that have identical quarks with parallelspins are the (uuu) and the (ddd). To resolve this problem, Moo-Young Han and Yoichiro Nambu suggested in 1965 that quarks possess a new propertycalled color or color charge. This “charge” property is similar in many respectsto electric charge, except that it occurs in three varieties, labeled red, green, andblue! (The antiquarks are labeled anti-red, anti-green, and anti-blue.) To satisfy theexclusion principle, all three quarks in a baryon must have different colors. Just asa combination of actual colors of light can produce the neutral color white, a combinationof three quarks with different colors is also “white,” or colorless. A mesonconsists of a quark of one color and an antiquark of the corresponding anticolor.The result is that baryons and mesons are always colorless (or white).Although the concept of color in the quark model was originally conceived tosatisfy the exclusion principle, it also provided a better theory for explaining certainexperimental results. For example, the modified theory correctly predicts thelifetime of the 0 meson. The theory of how quarks interact with each other bymeans of color charge is called quantum chromodynamics, or QCD, to parallelquantum electrodynamics (the theory of interactions among electric charges). InQCD, the quark is said to carry a color charge, in analogy to electric charge. Thestrong force between quarks is often called the color force. The force is carried bymassless particles called gluons (which are analogous to photons for the electromagneticforce). According to QCD, there are eight gluons, all with color charge.When a quark emits or absorbs a gluon, its color changes. For example, a bluequark that emits a gluon may become a red quark, and a red quark that absorbsthis gluon becomes a blue quark. The color force between quarks is analogous tothe electric force between charges: Like colors repel and opposite colors attract.Therefore, two red quarks repel each other, but a red quark will be attracted to ananti-red quark. The attraction between quarks of opposite color to form a meson(qq) is indicated in Figure 30.11a.Different-colored quarks also attract each other, but with less intensity thanopposite colors of quark and antiquark. For example, a cluster of red, blue, andgreen quarks all attract each other to form baryons, as indicated in Figure 30.11b.Every baryon contains three quarks of three different colors.Although the color force between two color-neutral hadrons (such as a protonand a neutron) is negligible at large separations, the strong color force betweentheir constituent quarks does not exactly cancel at small separations of about 1 fm.This residual strong force is in fact the nuclear force that binds protons and