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1、Introduction to High Energy Physics,Chapter 6: Quark Interactions and QCD,6.1 The Color Quamtum Number,The color quantum number has been introduced as an extra degree of freedom in the quark model of hadrons. (explain the e+e- hadrons cross section in experiment) Quantum Chromodynamics (QCD) is the
2、formal theory of the strong color interactions between quarks. The color charge of a quark has 3 possible values: Red, Green or Blue (RGB) The bosons mediating the quark-quark interactions are called “gluons”, each carrying a color and an anticolor and postulated to belong to an octet of states.,2,6
3、.1 The Color Quamtum Number,3,6.2 The QCD Potential at Short Distances,4,6.2 The QCD Potential at Short Distances,5,6.2 The QCD Potential at Short Distances,The two-jet events are interpreted in terms of the elastic scattering of a parton (quark or gluon) in the proton from one in the antiproton, ea
4、ch scattered parton giving rise to a hadron jet in the manner familiar from the process e+e- hadrons. The kinematics of the collision are shown in the figure: p3, p4 are the 4-momenta of the observed jets and p1, p2 are the momenta of the partons before the collision.,6,6.2 The QCD Potential at Shor
5、t Distances,7,6.2 The QCD Potential at Short Distances,The expected cross section of proton-antiproton scattering to two jets will be,8,6.2 The QCD Potential at Short Distances,Scattering may proceed via momentum transfer in t-channel,9,6.2 The QCD Potential at Short Distances,10,6.2 The QCD Potenti
6、al at Short Distances,11,6.3 The QCD Potential at Large Distances,In QCD, a characteristic feature of the gluon mediators of the color force is their strong self-interaction, because the gluons themselves are postulated to carry color charges. In analogy with the electric lines of force between two
7、electric charge (Fig 6.7a), we can imagine that quarks are held together by color lines of forces (Fig 6.7b), but gluon-gluon interaction pulls these together into the form of a tube or string.,12,6.3 The QCD Potential at Large Distances,13,6.3 The QCD Potential at Large Distances,The observed linea
8、r dependence of J on M2 is evidence for the linear potential.,14,A typical hadron mass is 1 GeV and its radius is about 1 fm, so the linear energy density Will be k 1 GeV fm-1,6.4 Gluon jets in the e+e- annihilation,Dramatic demonstration of quark substructure are obtained in e+e- annihilation to ha
9、drons at very high energy. At cms of 30 GeV, about 10 hadrons (mostly pions) are produced. e+e- fragmentation of the quarks to hadrons The average hadron momentum along the original quark direction is large compared with its transverse momentum pT (0.5 GeV). Hence, the hadron appear in the form of t
10、wo jets collimated around the -axis.,15,6.4 Gluon jets in the e+e- annihilation,The angular distribution in the three-jet events allows a determination of the gluon spin (1 - vector).,16,6.5 Running Couplings in QED and QCD,The magnetic momenta of the electron and muon are correctly predicted in QED
11、. The anomaly is expressed in terms of the departure of the gyromagnetic ratio g from 2 and appears in the form of a perturbation expansion in powers of a by summing the relevant Feynman graphs.,17,6.5 Running Couplings in QED and QCD,Even in a vacuum, a test charge is continually emitting and reabs
12、orbing virtual photons that can temporarily produce e+e- pairs, producing a shielding effect, the so-called “vacuum polarisation”.,18,6.5 Running Couplings in QED and QCD,The QED coupling parameter as a function of q2, the effective coupling increases with momentum transfer q2. For QCD, the coupling
13、 parameter is:,19,6.5 Running Couplings in QED and QCD,The Eq.6.22 shows that at large q2, the coupling as0, the quarks behave as if free, a phenomenon known as asymptotic freedom. At low q2 such that q L, as becomes very large and perturbation expansion becomes meaningless. Such large interquark co
14、upling is presumably connected with the confinement of quarks at large distances. The main point is that, like QED, QCD has the form of a perturbation theory and applies only for q2 L2.,20,6.6 Gluonium and the quark-gluon plasma,QGP is a phase of QCD Which exists at extremely High tempertaure and/or Density, it consists of asymptotically free quarks and gluons. CERN SPS first tried to create The QGP in 1980s and 1990s. CERN announced indirect evidence for a “new state of matter” in 2000. 2011, BNLs RHIC and LHC ALICE continuing this effort.,21,
