Quarks and the Baryons A Review of Chapter 15 of Particles and Nuclei by Povh Evan Phelps University of South Carolina Department of Physics and Astronomy phelps@physics.sc.edu March 18, 2009 Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 1 / 21
Introduction: Hadronic Bound States Quarkonia heavy q q pairs Light mesons light q q pairs Baryons qqq u-, d-, and s-quarks Our study of deep inelastic scattering has taught us that [baryons] are composed of three valence quarks, gluons and a "sea" of quark-antiquark pairs. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 2 / 21
Outline of Topics 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 3 / 21
Outline of Topics From Resonance to Quarks 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 4 / 21
From Resonance to Quarks Arriving at the Quark Description of Nucleons The Statement that Distracted Me Our study of deep inelastic scattering has taught us that [baryons] are composed of three valence quarks, gluons and a "sea" of quark-antiquark pairs. Logical Steps to Show Nucleons are composite particles. The constituents are point-like call them quarks. The quarks are spin- 1 2. Three quarks fully explain the quantum numbers of the nucleons. A sea of antiquarks and quarks is also present. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 5 / 21
Resonance From Resonance to Quarks Excited baryon states exist, so baryons are composite particles. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 6 / 21
From Resonance to Quarks Electron (or Muon) Scattering Baryon structure functions are independent of Q 2, so their constituents are point-like. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 7 / 21
From Resonance to Quarks Electron (or Muon) Scattering The Callan-Gross relation holds, so the quarks have spin-1/2. Callan-Gross Relation For spin- 1 2 Dirac particles, 2xF 1 (x) = F 2 (x). Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 8 / 21
From Resonance to Quarks Quantum Number Hints At least two quarks must be present in order to have a bound state. The quantum numbers of the quarks should fully constitute the quantum numbers of the baryon. Since each quark is spin- 1 2, at least three quarks are required in order to preserve the spin of the nucleons. The highest charged baryon has +2e, and the lowest has 1e, suggesting that the quark charges are +2/3e and 1/3e. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 9 / 21
Neutrino Scattering From Resonance to Quarks W-boson exchange in Neutrino and Antineutrino scattering distinguishes quarks from antiquarks. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 10 / 21
Neutrino Scattering From Resonance to Quarks The results are consistent with the interpretation of a q q sea. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 11 / 21
Outline of Topics Nomenclature 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 12 / 21
Nomenclature Nomenclature Valence quarks are the quarks responsible for the quantum numbers of the nucleons. Sea quarks and antiquarks are produced and annihilated in the field of the strong interaction. Constituent quarks are the "effective valence quarks," which include the valence quarks and their dressings. Current quark masses are the masses of the bare valence quarks. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 13 / 21
Outline of Topics Production and Detection of Baryons 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 14 / 21
Production and Detection of Baryons Formation and Production Experiments Formation Experiments The baryon that is formed is detected as a resonance. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 15 / 21
Production and Detection of Baryons Formation and Production Experiments Production Experiments The excited baryons states are studied via their decay products. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 16 / 21
Production and Detection of Baryons Production Experiments Example of the Invariant Mass Method of Reconstructions Reconstruction shows that a Ξ 0 state decayed into a Ξ and a π +. Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 17 / 21
Outline of Topics Baryon Multiplets 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 18 / 21
Baryon Multiplets Baryon Multiplets The total baryonic wavefunction Quarks are fermions, so the baryonic wavefunction must be antisymmetric. ψ total = ξ spatial ζ flavor χ spin φ color Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 19 / 21
Outline of Topics Baryon Masses 1 From Resonance to Quarks 2 Nomenclature 3 Production and Detection of Baryons 4 Baryon Multiplets 5 Baryon Masses Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 20 / 21
Baryon Masses Baryon Masses Baryon Mass Spectrum Strangeness, isospin, and chromomagnetic spin-spin interaction help explain the baryon mass spectrum. Decuplet and Octet Masses s-quarks have larger mass Higher isospin -> higher mass Mass varies with spin-spin interaction Evan Phelps (Univ. of South Carolina) The Baryons March 18, 2009 21 / 21