Observing Dynamical Chiral Symmetry Breaking

  • View
    36

  • Download
    0

Embed Size (px)

DESCRIPTION

Observing Dynamical Chiral Symmetry Breaking. Craig Roberts Physics Division. Q C D s Challenges. Understand emergent phenomena. Quark and Gluon Confinement No matter how hard one strikes the proton, one cannot liberate an individual quark or gluon. - PowerPoint PPT Presentation

Transcript

Slide 1

Observing Dynamical Chiral Symmetry Breaking

Craig Roberts

Physics Division

30+5 or thereabouts. This means 25-30 transparencies.Go to Insert (View) | Header and Footer" to add your organization, sponsor, meeting name here; then, click "Apply to All"1

QCDs Challenges

Dynamical Chiral Symmetry Breaking Very unnatural pattern of bound state masses; e.g., Lagrangian (pQCD) quark mass is small but . . . no degeneracy between JP=+ and JP= (parity partners)Neither of these phenomena is apparent in QCDs Lagrangian Yet they are the dominant determining characteristics of real-world QCD. Both will be important herein QCD Complex behaviour arises from apparently simple rules.Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs2 Quark and Gluon ConfinementNo matter how hard one strikes the proton, one cannot liberate an individual quark or gluon

Understand emergent phenomena2Universal TruthsSpectrum of hadrons (ground, excited and exotic states), and hadron elastic and transition form factors provide unique information about long-range interaction between light-quarks and distribution of hadron's characterising properties amongst its QCD constituents.Dynamical Chiral Symmetry Breaking (DCSB) is most important mass generating mechanism for visible matter in the Universe. Higgs mechanism is (almost) irrelevant to light-quarks.Running of quark mass entails that calculations at even modest Q2 require a Poincar-covariant approach. Covariance requires existence of quark orbital angular momentum in hadron's rest-frame wave function.Confinement is expressed through a violent change of the propagators for coloured particles & can almost be read from a plot of a states dressed-propagator. It is intimately connected with DCSB.

Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs3

3

Dyson-SchwingerEquationsWell suited to Relativistic Quantum Field TheorySimplest level: Generating Tool for Perturbation Theory . . . Materially Reduces Model-Dependence Statement about long-range behaviour of quark-quark interactionNonPerturbative, Continuum approach to QCDHadrons as Composites of Quarks and GluonsQualitative and Quantitative Importance of:Dynamical Chiral Symmetry Breaking Generation of fermion mass from nothingQuark & Gluon Confinement Coloured objects not detected, Not detectable?Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs4Approach yields Schwinger functions; i.e., propagators and verticesCross-Sections built from Schwinger FunctionsHence, method connects observables with long- range behaviour of the running couplingExperiment Theory comparison leads to an understanding of long- range behaviour of strong running-coupling

4ConfinementQuark and Gluon ConfinementNo matter how hard one strikes the proton, or any other hadron, one cannot liberate an individual quark or gluonEmpirical fact. HoweverThere is no agreed, theoretical definition of light-quark confinementStatic-quark confinement is irrelevant to real-world QCDThere are no long-lived, very-massive quarksConfinement entails quark-hadron duality; i.e., that all observable consequences of QCD can, in principle, be computed using an hadronic basis. Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs5

XConfinementConfinement is expressed through a violent change in the analytic structure of propagators for coloured particles & can almost be read from a plot of a states dressed-propagatorGribov (1978); Munczek (1983); Stingl (1984); Cahill (1989); Krein, Roberts & Williams (1992); Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs6complex-P2complex-P2 Real-axis mass-pole splits, moving into pair(s) of complex conjugate poles or branch points Spectral density no longer positive semidefinite & hence state cannot exist in observable spectrumNormal particleConfined particle

Dressed-gluon propagatorGluon propagator satisfies a Dyson-Schwinger EquationPlausible possibilities for the solutionDSE and lattice-QCDagree on the resultConfined gluonIR-massive but UV-masslessmG 2-4 QCD Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs7perturbative, massless gluonmassive , unconfined gluonIR-massive but UV-massless, confined gluonA.C. Aguilar et al., Phys.Rev. D80 (2009) 085018

S(p) Dressed-quark propagator- nominally, a 1-body problemGap equationD(k) dressed-gluon propagator(q,p) dressed-quark-gluon vertex

Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs8

With QCDs dressed-gluon propagatorWhat is the dressed-quarkmass function?Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs9

Frontiers of Nuclear Science:Theoretical AdvancesIn QCD a quark's effective mass depends on its momentum. The function describing this can be calculated and is depicted here. Numerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies.Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs10

(20)10

Frontiers of Nuclear Science:Theoretical AdvancesIn QCD a quark's effective mass depends on its momentum. The function describing this can be calculated and is depicted here. Numerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies.Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs11

DSE prediction of DCSB confirmedMass from nothing!

Frontiers of Nuclear Science:Theoretical AdvancesIn QCD a quark's effective mass depends on its momentum. The function describing this can be calculated and is depicted here. Numerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies.Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs12

Hint of lattice-QCD support for DSE prediction of violation of reflection positivity

12GeVThe Future of JLabNumerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies.Craig Roberts: Observing Dynamical Chiral Symmetry Breaking. JLab 16 May 2011 - Nucleon Resonance Structure with the CLAS12 Detector - 41pgs13

Jlab 12GeV: Scanned by 2