Craig Roberts Physics Division

New Challenges  Computation of spectrum of hybrid and exotic mesons  Equally pressing, some might say more so, is the three-body problem; viz., baryons in QCD DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 2 exotic mesons: quantum numbers not possible for quantum mechanical quark-antiquark systems hybrid mesons: normal quantum numbers but non- quark-model decay pattern BOTH suspected of having “constituent gluon” content ↔

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 3

Unification of Meson & Baryon Properties  Correlate the properties of meson and baryon ground- and excited- states within a single, symmetry-preserving framework  Symmetry-preserving means: Poincaré-covariant & satisfy relevant Ward-Takahashi identities  Constituent-quark model has hitherto been the most widely applied spectroscopic tool; whilst its weaknesses are emphasized by critics and acknowledged by proponents, it is of continuing value because there is nothing better that is yet providing a bigger picture.  Nevertheless,  no connection with quantum field theory & therefore not with QCD  not symmetry-preserving & therefore cannot veraciously connect meson and baryon properties Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 4 DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Unification of Meson & Baryon Spectra  Dyson-Schwinger Equations have been applied extensively to the spectrum and interactions of mesons with masses less than 1 GeV  On this domain the rainbow-ladder approximation, which is the leading-order in a systematic & symmetry-preserving truncation scheme – nucl-th/ , is a well-understood tool that is accurate for pseudoscalar and vector mesons:nucl-th/ e.g.,  Prediction of elastic pion and kaon form factors: nucl-th/ nucl-th/  Anomalous neutral pion processes – γπγ & BaBar anomaly: [nucl-th] [nucl-th]  Pion and kaon valence-quark distribution functions: [nucl-th] [nucl-th]  Unification of these and other observables – ππ scattering: hep-ph/ hep-ph/  It can readily be extended to explain properties of the light neutral pseudoscalar mesons: [nucl-th] [nucl-th] DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 5

Unification of Meson & Baryon Spectra  Some people have produced a spectrum of mesons with masses above 1GeV – but results have always been poor  For the bulk of such studies since 2004, this was a case of “Doing what can be done, not what needs to be done.”  Now understood why rainbow-ladder is not good for states with material angular momentum  know which channels are affected – scalar and axial-vector;  and the changes to expect in these channels  Task – Improve rainbow-ladder for mesons & build this knowledge into the study of baryon because, as we shall see, a formulation of the baryon bound-state problem rests upon knowledge of quark-antiquark scattering matrix DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 6

Nucleon Structure Probed in scattering experiments  Electron is a good probe because it is structureless Electron’s relativistic current is  Proton’s electromagnetic current DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 7 F 1 = Dirac form factorF 2 = Pauli form factor G E = Sachs Electric form factor If a nonrelativistic limit exists, this relates to the charge density G M = Sachs Magntic form factor If a nonrelativistic limit exists, this relates to the magnetisation density Lecture II

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 8 Which is correct? How is the difference to be explained?

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 9

Fully-covariant computation of nucleon form factors  First such calculations: –G. Hellstern et al., Nucl.Phys. A627 (1997) , restricted to Q 2 <2GeV 2Nucl.Phys. A627 (1997) –J.C.R. Bloch et al., Phys.Rev. C60 (1999) (R), restricted to Q 2 <3GeV 2Phys.Rev. C60 (1999) (R) Exploratory: –Included some correlations within the nucleon, but far from the most generally allowed –Used very simple photon-nucleon interaction current  Did not isolate and study G E p (Q 2 )/G M p (Q 2 )  How does one study baryons in QCD? DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 10

DSEs & Baryons  Dynamical chiral symmetry breaking (DCSB) – has enormous impact on meson properties.  Must be included in description and prediction of baryon properties.  DCSB is essentially a quantum field theoretical effect. In quantum field theory  Meson appears as pole in four-point quark-antiquark Green function → Bethe-Salpeter Equation  Nucleon appears as a pole in a six-point quark Green function → Faddeev Equation.  Poincaré covariant Faddeev equation sums all possible exchanges and interactions that can take place between three dressed-quarks  Tractable equation is based on the observation that an interaction which describes colour-singlet mesons also generates nonpointlike quark-quark (diquark) correlations in the colour-antitriplet channel Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 11 R.T. Cahill et al., Austral. J. Phys. 42 (1989) DSFdB2011, IRMA France, 27 June - 1 July. 58pgs SU c (3):

Faddeev Equation  Linear, Homogeneous Matrix equation  Yields wave function (Poincaré Covariant Faddeev Amplitude) that describes quark-diquark relative motion within the nucleon  Scalar and Axial-Vector Diquarks...  Both have “correct” parity and “right” masses  In Nucleon’s Rest Frame Amplitude has s−, p− & d−wave correlations Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 12 diquark quark quark exchange ensures Pauli statistics composed of strongly- dressed quarks bound by dressed-gluons DSFdB2011, IRMA France, 27 June - 1 July. 58pgs R.T. Cahill et al., Austral. J. Phys. 42 (1989)

 Why should a pole approximation produce reliable results? Faddeev Equation DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 13 quark-quark scattering matrix - a pole approximation is used to arrive at the Faddeev-equation

 Consider the rainbow-gap and ladder-Bethe-Salpeter equations  In this symmetry-preserving truncation, colour-antitriplet quark- quark correlations (diquarks) are described by a very similar homogeneous Bethe-Salpeter equation  Only difference is factor of ½  Hence, an interaction that describes mesons also generates diquark correlations in the colour-antitriplet channel Diquarks DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 14 Calculation of diquark masses in QCD R.T. Cahill, C.D. Roberts and J. Praschifka Phys.Rev. D36 (1987) 2804

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 15

 Vector-vector contact interaction m G is a gluon mass-scale – dynamically generated in QCD  Gap equation:  DCSB: M ≠ 0 is possible so long as m G <m G critical (Lecture II)  Studies of π & ρ static properties and π form factor establish that contact-interaction results are not realistically distinguishable from those of renormalisation-group- improved one-gluon exchange for processes with Q 2 <M 2 Consider contact-interaction Kernel DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 16 H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th] H.L.L. Roberts, A. Bashir, L.X. Gutiérrez-Guerrero, C.D. Roberts and D.J. Wilson, arXiv: [nucl-th], Phys. Rev. C in pressarXiv: [nucl-th]

 Studies of π & ρ static properties and π form factor establish that contact-interaction results are not realistically distinguishable from those of renormalisation-group- improved one-gluon exchange for Q 2 <M 2 Interaction Kernel DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 17 contact interaction QCD 1-loop RGI gluon M κπκπ 0.24 mπmπ 0.14 mρmρ fπfπ fρfρ M 2 /m ρ 2 cf. expt. rms rel.err.=13% One-loop RGI-improved QCD-based interaction

contact interaction M0.37 κπκπ 0.24 mπmπ 0.14 mρmρ 0.93 fπfπ 0.10 fρfρ 0.13  Contact interaction is not renormalisable  Must therefore introduce regularisation scheme  Use confining proper-time definition  Λ ir = 0.24GeV, τ ir = 1/Λ ir = 0.8fm a confinement radius, which is not varied  Two parameters: m G =0.13GeV, Λ uv =0.91GeV fitted to produce tabulated results Interaction Kernel - Regularisation Scheme DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 18 D. Ebert, T. Feldmann and H. Reinhardt, Phys. Lett. B 388 (1996) 154. No pole in propagator – DSE realisation of confinement

Regularisation & Symmetries  In studies of the hadron spectrum it’s critical that an approach satisfy the vector and axial-vector Ward-Takahashi identities.  Without this it is impossible to preserve the pattern of chiral symmetry breaking in QCD & hence a veracious understanding of hadron mass splittings is not achievable.  Contact interaction should & can be regularised appropriately  Example: dressed-quark-photon vertex  Contact interaction plus rainbow-ladder entails general form  Vector Ward-Takahashi identity  With symmetry-preserving regularisation of contact interaction, Ward Takahashi identity requires DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 19 P L (Q 2 )=1 & P T (Q 2 =0)=1 Interactions cannot generate an on-shell mass for the photon.

Regularisation & Symmetries  Solved Bethe-Salpeter equation for dressed- quark photon vertex, regularised using symmetry-preserving scheme DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V Ward-Takahashi identity ρ-meson pole generated dynamically - Foundation for Vector meson dominance (VMD) “Asymptotic freedom” Dressed-vertex → bare at large spacelike Q 2 Renormalisation-group-improved one-gluon exchange Maris & Tandy prediction of F π (Q 2 ) 20

Bethe-Salpeter Equations  Ladder BSE for ρ-meson ω(M 2,α,P 2 )= M 2 + α(1- α)P 2  Contact interaction, properly regularised, provides a practical simplicity & physical transparency  Ladder BSE for a 1 -meson  All BSEs are one- or –two dimensional eigenvalue problems, eigenvalue is P 2 = - (mass-bound-state) 2 DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 21

Meson Spectrum -Ground-states  Ground-state masses  Computed very often, always with same result  Namely, with rainbow-ladder truncation m a1 – m ρ = 0.15 GeV ≈ ⅟₃ x 0.45 experiment DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 22 ExperimentRainbow- ladder One-loop corrected Full vertex a ρ Mass splitting But, we know how to fix that (Lecture IV) viz., DCSB – beyond rainbow ladder  increases scalar and axial-vector masses  leaves π & ρ unchanged H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th]

Meson Spectrum -Ground-states  Ground-state masses  Correct for omission of DCSB-induced spin-orbit repulsion  Leave π- & ρ-meson BSEs unchanged but introduce repulsion parameter in scalar and axial-vector channels; viz.,  g SO =0.24 fitted to produce m a1 – m ρ = 0.45 experiment DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 23 m σ qq ≈ 1.2 GeV is location of quark core of σ-resonance: Pelaez & Rios (2006) Ruiz de Elvira, Pelaez, Pennington & Wilson (2010) First novel post-diction H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th]

Radial Excitations  Lecture III: Goldstone modes are the only pseudoscalar mesons to possess a nonzero leptonic decay constant, f π, in the chiral limit when chiral symmetry is dynamically broken.  The decay constants of their radial excitations vanish.  In quantum mechanics, decay constants suppressed by factor of roughly ⅟₃  But only a symmetry can ensure that something vanishes completely  Goldstone’s Theorem for non-ground-state pseudoscalars  These features and aspects of their impact on the meson spectrum were illustrated using a manifestly covariant and symmetry-preserving model of the kernels in the gap and Bethe-Salpeter equations. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 24 Pseudoscalar meson radial excitations A. Höll, A. Krassnigg & C.D. Roberts, Phys.Rev. C70 (2004) (R), nucl-th/ nucl-th/ see also A Chiral Lagrangian for excited pions M.K. Volkov & C. Weiss Phys. Rev. D56 (1997) 221, hep-ph/

Radial Excitations  These features and aspects of their impact on the meson spectrum were illustrated using a manifestly covariant and symmetry-preserving model of the kernels in the gap and Bethe-Salpeter equations. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 25 Pseudoscalar meson radial excitations A. Höll, A. Krassnigg & C.D. Roberts, Phys.Rev. C70 (2004) (R), nucl-th/ nucl-th/ In QFT, too, “wave functions” of radial excitations possess a zero Chiral limit ground state = 88MeV 1 st excited state: f π =0

Meson Spectrum - Radial Excitations  Key: in proceeding with study of baryons – 1 st radial excitation of a bound state possess a single zero in the relative-momentum dependence of the leading Tchebychev-moment in their bound-state amplitude  The existence of radial excitations is therefore obvious evidence against the possibility that the interaction between quarks is momentum- independent:  A bound-state amplitude that is independent of the relative momentum cannot exhibit a single zero  One may express this differently; viz.,  If the location of the zero is at k 0 2, then a momentum-independent interaction can only produce reliable results for phenomena that probe momentum scales k 2 < k 0 2.  In QCD, k 0 ≈ M. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 26

Meson Spectrum - Radial Excitations  Nevertheless, there exists an established expedient ; viz.,  Insert a zero by hand into the Bethe-Salpeter kernels  Plainly, the presence of this zero has the effect of reducing the coupling in the BSE & hence it increases the bound-state's mass.  Although this may not be as transparent with a more sophisticated interaction, a qualitatively equivalent mechanism is always responsible for the elevated values of the masses of radial excitations.  Location of zero fixed at “natural” location: k 2 =M 2 – not a parameter DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 27 ** (1-k 2 /M 2 ) A Chiral Lagrangian for excited pions M.K. Volkov & C. Weiss Phys. Rev. D56 (1997) 221, hep-ph/

Meson Spectrum Ground- & Excited-States  Complete the table …  Error estimate for radial excitations: Shift location of zero by ±20%  rms-relative-error/degree-of-freedom = 13%  No parameters  Realistic DSE estimates: m 0+ = , m 1+ =  Lattice-QCD estimate: m 0+ =0.78 ±0.15, m 1+ -m 0+ =0.14 DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 28 plus predicted diquark spectrum FIRST results for other qq quantum numbers They’re critical for excited states of N and Δ

 “Spectrum” of nonpointlike quark-quark correlations  Observed in –DSE studies in QCD –0 + & 1 + in Lattice-QCD  Scalar diquark form factor –r 0+ ≈ r π  Axial-vector diquarks –r 1+ ≈ r ρ Diquarks in QCD DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 29  Zero relation with old notion of pointlike constituent-like diquarks BOTH essential H.L.L. Roberts et al., [nucl-th] [nucl-th] Phys. Rev. C in press Masses of ground and excited-state hadrons Hannes L.L. Roberts, Lei Chang, Ian C. Cloët and Craig D. Roberts, arXiv: [nucl-th]arXiv: [nucl-th] Few Body Systems (2011) pp. 1-25

Spectrum of Baryons SStatic “approximation” IImplements analogue of contact interaction in Faddeev-equation IIn combination with contact-interaction diquark-correlations, generates Faddeev equation kernels which themselves are momentum-independent TThe merit of this truncation is the dramatic simplifications which it produces UUsed widely in hadron physics phenomenology; e.g., Bentz, Cloët, Thomas et al. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 30 Variant of: A. Buck, R. Alkofer & H. Reinhardt, Phys. Lett. B286 (1992) 29. H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th]

Spectrum of Baryons  Static “approximation”  Implements analogue of contact interaction in Faddeev-equation  From the referee’s report: In these calculations one could argue that the [static truncation] is the weakest [approximation]. From what I understand, it is not of relevance here since the aim is to understand the dynamics of the interactions between the [different] types of diquark correlations with the spectator quark and their different contributions to the baryon's masses … this study illustrates rather well what can be expected from more sophisticated models, whether within a Dyson- Schwinger or another approach. … I can recommend the publication of this paper without further changes. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 31 H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th]

Spectrum of Baryons DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 32  Faddeev equation for Δ-resonance  One-dimensional eigenvalue problem, to which only the axial-vector diquark contributes  Nucleon has scalar & axial-vector diquarks. It is a five-dimensional eigenvalue problem With the right glasses; i.e., those polished by experience with the DSEs, one can look at this equation and see that increasing the current-quark mass will boost the mass of the bound-state

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 33

Pion cloud  Kernels constructed in the rainbow-ladder truncation do not contain any long-range interactions –These kernels are built only from dressed-quarks and -gluons  But, QCD produces a very potent long-range interaction; namely that associated with the pion, without which no nuclei would be bound and we wouldn’t be here  The rainbow-ladder kernel produces what we describe as the hadron’s dressed-quark core  The contribution from pions is omitted, and can be added without “double counting”  The pion contributions must be thoughtfully considered before any comparison can be made with the real world DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 34 H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th]

Pion cloud  The body of results described hitherto suggest that whilst corrections to our truncated DSE kernels may have a material impact on m N and m Δ separately, the modification of each is approximately the same, so that the mass-difference, δm, is largely unaffected by such corrections.  This is consistent with analyses that considers the effect of pion loops, which are explicitly excluded in the rainbow-ladder truncation: whilst the individual masses are reduced by roughly 300MeV, the mass difference, δm, increases by only 50MeV.  With the educated use of appropriate formulae, one finds that pion-loops yields a shift of (−300MeV) in m N and (−270MeV) in m Δ, from which one may infer that the uncorrected Faddeev equations should produce m N = 1.24GeV and m Δ = 1.50GeV DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 35 H.L.L. Roberts, L. Chang, I.C. Cloët and C.D. Roberts, arXiv: [nucl-th], Few Body Syst. 51 (2011) pp arXiv: [nucl-th] M.B. Hecht, M. Oettel, C.D. Roberts, S.M. Schmidt, P.C.Tandy and A.W. Thomas, Phys.Rev. C65 (2002) Phys.Rev. C65 (2002)

Pion cloud  One can actually do better owing to the existence of the Excited Baryon Analysis Center (EBAC), which for five years has worked to understand and quantify the effects of a pseudoscalar meson cloud on baryons  For the Δ-resonance, EBAC’s calculations indicate that the dressed- quark core of this state should have m Δ qqq = 1.39GeV  These observations indicate that the dressed-quark core Faddeev equations should yield m N = 1.14GeV, m Δ = 1.39GeV, δm = 0.25GeV which requires g N = 1.18, g Δ = 1.56 DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 36 All three spectrum parameters now fixed (g SO =0.24)

Baryons & diquarks Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 37  From apparently simple material, one arrives at a powerful elucidative tool, which provides numerous insights into baryon structure; e.g.,  There is a causal connection between m Δ - m N & m 1+ - m 0+ m Δ - m N mNmN mΔmΔ Physical splitting grows rapidly with increasing diquark mass difference DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Masses of ground and excited-state hadrons Hannes L.L. Roberts, Lei Chang, Ian C. Cloët and Craig D. Roberts, arXiv: [nucl-th]arXiv: [nucl-th] Few Body Systems (2011) pp. 1-25

Baryons & diquarks Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 38  Provided numerous insights into baryon structure; e.g.,  m N ≈ 3 M & m Δ ≈ M+m 1+ DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Hadron Spectrum Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 39 Legend: Particle Data Group H.L.L. Roberts et al. EBAC Jülich o Symmetry-preserving unification of the computation of meson & baryon masses o rms-rel.err./deg-of-freedom = 13% o PDG values (almost) uniformly overestimated in both cases - room for the pseudoscalar meson cloud?! DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Masses of ground and excited-state hadrons H.L.L. Roberts et al., arXiv: [nucl-th]arXiv: [nucl-th] Few Body Systems (2011) pp. 1-25

Baryon Spectrum Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 40  In connection with EBAC's analysis, dressed-quark Faddeev-equation predictions for bare-masses agree within rms-relative-error of 14%.  Notably, EBAC finds a dressed-quark-core for the Roper resonance, at a mass which agrees with Faddeev Eq. prediction. DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

EBAC & the Roper resonance  EBAC examined the dynamical origins of the two poles associated with the Roper resonance are examined.  Both of them, together with the next higher resonance in the P 11 partial wave were found to have the same originating bare state  Coupling to the meson- baryon continuum induces multiple observed resonances from the same bare state.  All PDG identified resonances consist of a core state and meson-baryon components. Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 41 N. Suzuki et al., Phys.Rev.Lett. 104 (2010) Phys.Rev.Lett. 104 (2010) DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Hadron Spectrum Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 42 Legend: Particle Data Group H.L.L. Roberts et al. EBAC Jülich Now and for the foreseeable future, QCD-based theory will provide only dressed-quark core masses; EBAC or EBAC-like tools necessary for mesons and baryons DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 43

Nucleon form factors  For the nucleon & Δ-resonance, studies of the Faddeev equation exist that are based on the 1-loop renormalisation-group-improved interaction that was used efficaciously in the study of mesons –Toward unifying the description of meson and baryon properties G. Eichmann, I.C. Cloët, R. Alkofer, A. Krassnigg and C.D. Roberts arXiv: [nucl-th], Phys. Rev. C 79 (2009) (R) (5 pages) arXiv: [nucl-th] –Survey of nucleon electromagnetic form factors I.C. Cloët, G. Eichmann, B. El-Bennich, T. Klähn and C.D. Roberts arXiv: [nucl-th], Few Body Syst. 46 (2009) pp arXiv: [nucl-th] –Nucleon electromagnetic form factors from the Faddeev equation G. Eichmann, arXiv: [hep-ph]arXiv: [hep-ph]  These studies retain the scalar and axial-vector diquark correlations, which we know to be necessary and sufficient for a reliable description  In order to compute form factors, one needs a photon-nucleon current DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 44

Photon-nucleon current  Composite nucleon must interact with photon via nontrivial current constrained by Ward-Takahashi identities  DSE → BSE → Faddeev equation plus current → nucleon form factors Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 45 Vertex must contain the dressed-quark anomalous magnetic moment: Lecture IV Oettel, Pichowsky, Smekal Eur.Phys.J. A8 (2000) DSFdB2011, IRMA France, 27 June - 1 July. 58pgs L. Chang, Y. –X. Liu and C.D. Roberts arXiv: [nucl-th] arXiv: [nucl-th] Phys. Rev. Lett. 106 (2011)  In a realistic calculation, the last three diagrams represent 8-dimensional integrals, which can be evaluated using Monte-Carlo techniques

Nucleon Form Factors DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 46 Survey of nucleon electromagnetic form factors I.C. Cloët et al, arXiv: [nucl-th],arXiv: [nucl-th] Few Body Syst. 46 (2009) pp Unification of meson and nucleon form factors. Very good description. Quark’s momentum- dependent anomalous magnetic moment has observable impact & materially improves agreement in all cases.

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 47 I.C. Cloët, C.D. Roberts, et al. arXiv: [nucl-th]  Highlights again the critical importance of DCSB in explanation of real-world observables.  DSE result Dec 08 DDSE result – including the anomalous magnetic moment distribution I.C. Cloët, C.D. Roberts, et al. In progress DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

 DSE studies indicate that the proton has a very rich internal structure  The JLab data, obtained using the polarisaton transfer method, are an accurate indication of the behaviour of this ratio  The pre-1999 data (Rosenbluth) receive large corrections from so-called 2-photon exchange contributions DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 48 Proton plus proton-like resonances

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 49 I.C. Cloët, C.D. Roberts, et al. arXiv: [nucl-th] I.C. Cloët, C.D. Roberts, et al. In progress DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Linear fit to data ⇒ zero at 8GeV 2 [1,1] Padé fit ⇒ zero at 10GeV 2

What about the same ratio for the neutron? Quark anomalous magnetic moment has big impact on proton ratio But little impact on the neutron ratio DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 50 I.C. Cloët, C.D. Roberts, et al. In progress

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 51  New JLab data: S. Riordan et al., arXiv: [nucl-ex] arXiv: [nucl-ex]  DSE-prediction I.C. Cloët, C.D. Roberts, et al. arXiv: [nucl-th]  This evolution is very sensitive to momentum-dependence of dressed-quark propagator DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Phys.Rev.Lett. 105 (2010)

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 52  New JLab data: S. Riordan et al., arXiv: [nucl-ex] arXiv: [nucl-ex]  DSE-prediction I.C. Cloët, C.D. Roberts, et al. arXiv: [nucl-th]  Location of zero measures relative strength of scalar and axial-vector qq-correlations  Looks like we have it right Brooks, Bodek, Budd, Arrington fit to data: hep-ex/ DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 53 Deep inelastic scattering Neutron Structure Function at high x SU(6) symmetry pQCD, uncorrelated Ψ 0 + qq only Deep inelastic scattering – the Nobel-prize winning quark-discovery experiments Reviews:  S. Brodsky et al. NP B441 (1995)  W. Melnitchouk & A.W.Thomas PL B377 (1996) 11  N. Isgur, PRD 59 (1999)  R.J. Holt & C.D. Roberts RMP (2010) DSE: 0 + & 1 + qq I.C. Cloët, C.D. Roberts, et al. arXiv: [nucl-th] Distribution of neutron’s momentum amongst quarks on the valence-quark domain DSFdB2011, IRMA France, 27 June - 1 July. 58pgs

Baryon Spectrum DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 54 Masses of ground and excited-state hadrons Hannes L.L. Roberts, Lei Chang, Ian C. Cloët and Craig D. Roberts, arXiv: [nucl-th]arXiv: [nucl-th] Few Body Systems (2011) pp % %100% %43% %57%100% diquark content Fascinating suggestion: nucleon’s first excited state = almost entirely axial-vector diquark, despite the dominance of scalar diquark in ground state. In fact, because of it. Diquark content of bound state

Nucleon to Roper Transition Form Factors  Extensive JLab Programme has produced the first measurements of nucleon-to-resonance transition form factors  Theory challenge is to explain the measurements  Notable result is zero in F 2 p→N *, explanation of which is a real challenge to theory.  DSE study connects appearance of zero in F 2 p→N * with axial-vector-diquark dominance in Roper resonance and structure of form factors of J=1 state DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 55 I. Aznauryan et al., Results of the N* Program at JLab arXiv: [nucl-ex] I.C. Cloët, C.D. Roberts, et al. In progress Γ μ,αβ

Nucleon to Roper Transition Form Factors  Tiator and Vanderhaeghen – in progress –Empirically-inferred light-front-transverse charge density –Positive core plus negative annulus  Readily explained by dominance of axial-vector diquark configuration in Roper –Considering isospin and charge Negative d-quark twice as likely to be delocalised from the always-positive core than the positive u-quark DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 56 {uu} d 2 {ud} u 1+

DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 57

 QCD is the most interesting part of the Standard Model and Nature’s only example of an essentially nonperturbative fundamental theory  Dyson-Schwinger equations: o Exact results proved in QCD, amongst them: Quarks are not Dirac particles and gluons are nonperturbatively massive Dynamical chiral symmetry breaking is a fact. It’s responsible for 98% of the mass of visible matter in the Universe Goldstone’s theorem is fundamentally an expression of equivalence between the one-body problem and the two-body problem in the pseudoscalar channel Confinement is a dynamical phenomenon It cannot in principle be expressed via a potential The list goes on …  DSEs are a single framework, with IR model-input turned to advantage, “almost unique in providing an unambiguous path from a defined interaction → Confinement & DCSB → Masses → radii → form factors → distribution functions → etc.” DSFdB2011, IRMA France, 27 June - 1 July. 58pgs Craig Roberts: Dyson-Schwinger Equations and Continuum QCD, V 58 McLerran & Pisarski arXiv: [hep-ph]