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What is the Empirical Evidence for QCD?

What is the Empirical Evidence for QCD?. The Quark Model:. Baryon Decouplet: Delta ~ 1240 Y* ~ 1380 Cascade* ~ 1529 Omega ~ 1674. Baryon Octet: Nucleon ~ 940 Sigma ~ 1190 Lambda ~ 1115 Cascade ~ 1314. Pseudo-scalar mesons: Pion ~ 140 MeV Kaon ~ 500 MeV Eta ~ 550 MeV

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What is the Empirical Evidence for QCD?

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  1. What is the Empirical Evidence for QCD? The Quark Model: Baryon Decouplet: Delta ~ 1240 Y* ~ 1380 Cascade* ~ 1529 Omega ~ 1674 Baryon Octet: Nucleon ~ 940 Sigma ~ 1190 Lambda ~ 1115 Cascade ~ 1314 Pseudo-scalar mesons: Pion ~ 140 MeV Kaon ~ 500 MeV Eta ~ 550 MeV Eta’ ~ 960 MeV Spin ½ quarks describe quantum numbers of these states. Vector Mesons: Rho ~ 780 MeV K* ~ 900 MeV Omega ~ 780 Phi ~ 1020 MeV Constituent quark model: Mu ~ Md ~ 350 MeV, Ms ~ 500 MeV give a good description of the masses except for the pseudo scalar mesons (some spin splitting) Additive cross sections Additive magnetic moments

  2. Problems: No fractional charge states in nature Symmetry of wavefunction: Delta++ has spin 3/2 and made of three up quarks, Need an extra quantum number to not violate Fermi statistics => Color Why is octet of pseudo-scalar mesons so low in mass?: Chiral symmetry: Independent rotations of left and right handed quarks => Baryons are massless Or Massive states occur in parity doublets Or Chiral symmetry is broken and there are 9 Goldstone Bosons The last possibility is almost correct! Why is there no U(1) Goldstone mode? Some explicit breaking of chiral symmetry associated with quark masses.

  3. The Problem of Color: 2 below charm, 3 1/3 above charm 3 2/3 above upsilon Spin important

  4. QCD: Quarks with Color Action for QCD (written for massless quarks) Has chiral symmetry U(1) symmetry explicitly violated by the anomaly Solves the U(1) problem Topological transitions: instantons

  5. Asymptotic Freedom Suggests confinement might be possible

  6. Weak Coupling Calculations Sensible

  7. Hadron jets Have additional complication of fragmentation functions The data at short distances is described very well Unambiguous computation of evolution of structure functions

  8. Confinement “Understood” from Lattice Monte Carlo Computation QCD has a linear potential (sans quarks) Reasonable description of mass spectrum from lattice Can this provide precision tests of QCD?

  9. Potential reasonably well tested in heavy quark systems

  10. What About Glue? Inferred by measuring assuming DGLAP, Measuring quark distributions First seen in Petra, inferred from asymmetry in

  11. Glueballs? Particle Data Book Must understand mixing of glue balls with quark antiquark states Hybrid states, Naked glue? Where are they?

  12. How does glue control the high energy limit of QCD? What is the high energy limit of cross sections? What are the gluon and quark distribution at small x? How is matter distributed spatially inside of hadrons? Diffraction, Compton scattering

  13. Glue makes the matter seen in hadronic collision. Source of CGC and Glasma, important for QGP How is matter produced in such collisions?

  14. Do gluons carry the spin in nucleons?

  15. What is the ultimate nature of strongly interacting gluonic matter at high energy density? QGP CGC Glasma

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