Exploring the Origin of Mass using Resonance Electroproduction

1. Exploring the Origin of Mass using Resonance Electroproduction Craig Roberts

2. Students,Postdocs,Profs. Collaborators: 2017 – Now • Xiao-Yun Chen (Jinling Inst. Tech., Nanjing) • Feliciano C. De Soto Borrero (UPO); • Tanja Horn (Catholic U. America) • GastãoKrein (UNESP – São Paulo) • Yu-Xin Liu (PKU); • JoannisPapavassiliou (U.Valencia) • Jia-Lun Ping (Nanjing Normal U.) • Si-xue Qin (Chongqing U.); • Jose Rodriguez Quintero (U. Huelva) ; • Elena Santopinto (Genova) • Jorge Segovia (U. Pablo de Olavide = UPO); • Sebastian Schmidt (IAS-FZJ & JARA); • Shaolong Wan (USTC) ; • Qing-Wu Wang (Sichuan U) • Shu-Sheng XU (NJUPT, Nanjing U.) • Pei-Lin Yin (NJUPT) • Hong-Shi Zong (Nanjing U) • Zhao-Qian YAO (Nanjing U.) • Yin-Zhen XU (Nanjing U.) • Marco BEDOLLA (Genova, U Michoácan) • Chen CHEN (Giessen, UNESP - São Paulo, USTC & IIT); • Muyang CHEN (NKU, PKU) • Zhu-Fang CUI (Nanjing U.) ; • Minghui DING (ECT*, ANL, Nankai U.) ; • Fei GAO (Heidelberg, Valencia, Peking U.) ; • Bo-Lin LI (Nanjing U.) • Ya LU (Nanjing U.) • Cédric MEZRAG (INFN-Roma, ANL, IRFU-Saclay) ; • Khépani RAYA (Nankai U., U Michoácan); • Adnan Bashir (U Michoácan); • Daniele Binosi (ECT*) • Volker Burkert (JLab) • Lei Chang (Nankai U. ) ; Craig Roberts. Origin of Mass via Resonance Electroproduction

3. Strong Interactions in the Standard Model Craig Roberts. Origin of Mass via Resonance Electroproduction • Only apparent scale in chromodynamics is mass of the quark field • Quark mass is said to be generated by Higgs boson. • In connection with everyday matter, that mass is 1/250th of the natural (empirical) scale for strong interactions, viz. more-than two orders-of-magnitude smaller • Plainly, the Higgs-generated mass is very far removed from the natural scale for strongly-interacting matter • Nuclear physics mass-scale – 1 GeV – is an emergent feature of the Standard Model • No amount of staring at LQCD can reveal that scale • Contrast with quantum electrodynamics, e.g. spectrum of hydrogen levels measured in units of me, which appears in LQED

4. Whence Mass? Craig Roberts. Origin of Mass via Resonance Electroproduction • Classical chromodynamics … non-Abelian local gauge theory • Remove the current mass … there’s no energy scale left • No dynamics in a scale-invariant theory; only kinematics … the theory looks the same at all length-scales … there can be no clumps of anything … hence bound-states are impossible. • Our Universe can’t exist • Higgs boson doesn’t solve this problem … • normal matter is constituted from light-quarks • the mass of protons and neutrons, the kernels of all visible matter, are 100-times larger than anything the Higgs can produce • Where did it all begin? … becomes … Where did it all come from?

5. Trace Anomaly Trace anomaly QCD β function Craig Roberts. Origin of Mass via Resonance Electroproduction • In a scale invariant theory the energy-momentum tensor must be traceless: Tμμ ≡ 0 • Regularisation and renormalisation of (ultraviolet) divergences in Quantum Chromodynamics introduces a mass-scale … dimensional transmutation: Lagrangian’sconstants (couplings and masses) become dependent on a mass-scale, ζ • α → α(ζ) in QCD’s (massless) Lagrangian density, L(m=0) ⇒ ∂μDμ = δL/δσ = αβ(α)dL/dα = β(α)¼GμνGμν = Tρρ =: Θ0 Quantisation of renormalisable four-dimensional theory forces nonzero value for trace of energy-momentum tensor

6. Where is the mass? Craig Roberts. Origin of Mass via Resonance Electroproduction

7. Trace Anomaly Craig Roberts. Origin of Mass via Resonance Electroproduction Knowing that a trace anomaly exists does not deliver a great deal … Indicates only that a mass-scale must exist Can one compute and/or understand the magnitude of that scale? One can certainly measurethe magnitude … consider proton: In the chiral limit the entirety of the proton’s mass is produced by the trace anomaly, Θ0 … In QCD, Θ0measures the strength of gluon self-interactions … so, from one perspective, mp is (somehow) completely generated by glue.

8. Confinement & Origin of Mass Craig Roberts. Origin of Mass via Resonance Electroproduction The vast majority of mass comes from the energy needed to hold quarks together inside nuclei

9. On the other hand … Craig Roberts. Origin of Mass via Resonance Electroproduction

10. Trace Anomaly Craig Roberts. Origin of Mass via Resonance Electroproduction • In the chiral limit ⇒ • Does this mean that the scale anomaly vanishes trivially in the pion state, i.e. gluons contribute nothing to the pion mass? • Difficult way to obtain “zero”! • Easier to imagine that “zero” owes to cancellations between different operator contributions to the expectation value of Θ0. • Of course, such precise cancellation should not be an accident. It could only arise naturally because of some symmetry and/or symmetry-breaking pattern.

11. Whence “1” and yet “0” ? Craig Roberts. Origin of Mass via Resonance Electroproduction • No statement of the question “How does the mass of the proton arise?” is complete without the additional clause “How does the pion remain massless?” • Natural visible-matter mass-scale must emerge simultaneously with apparent preservation of scale invariance in related systems • Expectation value of Θ0 in pion is always zero, irrespective of the size of the natural mass-scale for strong interactions = mp

12. IR Behaviour of QCD Craig Roberts. Origin of Mass via Resonance Electroproduction • Gluons are supposed to be massless • This is true in perturbation theory • Not preserved non-perturbatively! No symmetry protects four-transverse gluon modes

13. IR Behaviour of QCD μg ≈ ½ mp Expression of trace anomaly: Massless glue becomes massive gluon mass-squared function Power-law suppressed in ultraviolet, so invisible in perturbation theory Dynamical mass generation in continuum quantum chromodynamics J.M. Cornwall, Phys. Rev. D 26 (1981) 1453 The Gluon Mass Generation Mechanism: A Concise Primer A.C. Aguilar, D. Binosi, J. Papavassiliou, Front. Phys. 11 (2016) 111203 Interaction model for the gap equation, S.-x.Qinet al., arXiv:1108.0603 [nucl-th], Phys. Rev. C 84 (2011) 042202(R) [5 pages] Combining DSE, lQCD and pQCD analyses of QCD’s gauge sector Craig Roberts. Origin of Mass via Resonance Electroproduction Running gluon mass Gluons are cannibals – a particle species whose members become massive by eating each other!

14. This is where we live ⇐ What’s happening out here?! QCD’s Running Coupling Craig Roberts. Origin of Mass via Resonance Electroproduction

15. Process independent strong running coupling Binosi, Mezrag, Papavassiliou, Roberts, Rodriguez-Quintero arXiv:1612.04835 [nucl-th], Phys. Rev. D 96 (2017) 054026/1-7 The QCD Running Coupling, A. Deur, S. J. Brodsky and G. F. de Teramond, Prog. Part. Nucl. Phys. 90 (2016) 1-74 Process-independent effective coupling. From QCD Green functions to phenomenology, Jose Rodríguez-Quintero et al., arXiv:1801.10164 [nucl-th]. Few Body Syst. 59 (2018) 121/1-9 Process-independent effective-charge in QCD Craig Roberts. Origin of Mass via Resonance Electroproduction • Modern continuum & lattice methods for analysing gauge sector enable “Gell-Mann – Low” running charge to be defined in QCD • Combined continuum and lattice analysis of QCD’s gauge sector yields a parameter-free prediction • N.B. Qualitative change in α̂PI(k) at k ≈ ½ mp

16. Process independent strong running coupling Binosi, Mezrag, Papavassiliou, Roberts, Rodriguez-Quintero arXiv:1612.04835 [nucl-th], Phys. Rev. D 96 (2017) 054026/1-7 The QCD Running Coupling, A. Deur, S. J. Brodsky and G. F. de Teramond, Prog. Part. Nucl. Phys. 90 (2016) 1-74 Process-independent effective coupling. From QCD Green functions to phenomenology, Jose Rodríguez-Quintero et al., arXiv:1801.10164 [nucl-th]. Few Body Syst. 59 (2018) 121/1-9 Process-independent effective-charge in QCD Craig Roberts. Origin of Mass via Resonance Electroproduction • α̂PI is a new type of effective charge • direct analogue of the Gell-Mann–Low effective coupling in QED, i.e. completely determined by the gauge-boson two-point function. • α̂PI is • process-independent • known to unify a vast array of observables • α̂PI possesses an infrared-stable fixed-point • Nonperturbative analysis demonstrating absence of a Landau pole in QCD • QCD is IR finite, owing to dynamical generation of gluon mass-scale, which also serves to eliminate the Gribov ambiguity • Asymptotic freedom ⇒ QCD is well-defined at UV momenta • QCD is therefore unique amongst known 4D quantum field theories • Potentially, defined & internally consistent at all momenta

17. What is Confinement? Confinement is dynamical! Craig Roberts. Origin of Mass via Resonance Electroproduction

18. Quark Fragmentation meson meson meson meson σ Confinement is a dynamical phenomenon! Craig Roberts. Origin of Mass via Resonance Electroproduction A quark begins to propagate But after each “step” of length σ≈ 1/mg, on average, an interaction occurs, so that the quark loses its identity, sharing it with other partons Finally, a cloud of partons is produced, which coalesces into colour-singlet final states

19. Enigma of Mass Craig Roberts. Origin of Mass via Resonance Electroproduction

20. Maris, Roberts and Tandy nucl-th/9707003, Phys.Lett. B420 (1998) 267-273  Pion’s Goldberger-Treiman relation B(k2) Miracle: two body problem solved, almost completely, once solution of one body problem is known Owing to DCSB & Exact in Chiral QCD Craig Roberts. Origin of Mass via Resonance Electroproduction Pion’s Bethe-Salpeter amplitude Solution of the Bethe-Salpeter equation Dressed-quark propagator Axial-vector Ward-Takahashi identity entails

21. Rudimentary version of this relation is apparent in Nambu’s Nobel Prize work Model independent Gauge independent Scheme independent fπ Eπ(p2) = B(p2) The most fundamental expression of Goldstone’s Theorem and DCSB Craig Roberts. Origin of Mass via Resonance Electroproduction

22. Rudimentary version of this relation is apparent in Nambu’s Nobel Prize work Model independent Gauge independent Scheme independent fπ Eπ(p2) = B(p2) Pion exists if, and only if, mass is dynamically generated Craig Roberts. Origin of Mass via Resonance Electroproduction

23. Rudimentary version of this relation is apparent in Nambu’s Nobel Prize work Model independent Gauge independent Scheme independent fπ Eπ(p2) = B(p2) π-nucleon coupling is strong if, and only if, DCSB effect is large Craig Roberts. Origin of Mass via Resonance Electroproduction

24. Nucleon mass from a covariant three-quark Faddeev equation G. Eichmann et al., Phys. Rev. Lett. 104 (2010) 201601 Faddeev Equation for Baryons Craig Roberts. Origin of Mass via Resonance Electroproduction

25. Baryon spectrum • Symmetry-preserving truncation of the strong-interaction bound-state equations = gap- and Faddeev-equations • No diquark approximation • Calculate spectrum of • ground-state J=1/2+ , 3/2+ (qq′q′′) -baryons, q,q′,q′′∈{u,d,s,c,b} , • their first positive-parity excitations • parity partners. • Using two parameters (RL), description of the known spectrum of 39 such states is obtained • with a mean-absolute-relative-difference between calculation and experiment of 3.6(2.7)%. • The framework is subsequently used to predict the masses of 90 states not yet seen empirically. Craig Roberts. Origin of Mass via Resonance Electroproduction Spectrum of light- and heavy-baryons, Si-Xue Qin, C. D. Roberts and S.M. Schmidt, arXiv:1902.00026 [nucl-th], Few Body Syst. 60 (2019) 26/1-18 (Contribution to a Special Issue dedicated to Ludwig Faddeev.)

26. Poincaré-covariant analysis of heavy-quark baryons, Si-Xue Qin et al., arXiv:1801.09697 [nucl-th], Phys.Rev. D 97 (2018) 114017/1-13 Baryons … J=3/2+ • Analysed internal structure of the ground and first positive-parity excited states of qqq, QQQ • Each system has a complicated angular momentum structure, e.g. • Ground states • primarily S-wave • but each possesses P-, D- and F-wave components • P-wave fraction is large in the u and s-quark states; • First positive-parity excitation • large D-wave component, • grows with increasing current-quark mass • but state also exhibits features consistent with a radial excitation. Craig Roberts. Origin of Mass via Resonance Electroproduction

27. Baryon Structure and QCD R.T. Cahill, C. D. Roberts, J. Praschifka Austral. J. Phys. 42 (1989) 129-145 Structure of Baryons - diquark correlations Craig Roberts. Origin of Mass via Resonance Electroproduction

28. Structure of Baryons Craig Roberts. Origin of Mass via Resonance Electroproduction Poincaré covariant Faddeev equation sums all possible exchanges and interactions that can take place between three dressed-quarks Direct solution of Faddeev equation using rainbow-ladder truncation is now possible, but numerical challenges remain Prediction: owing to DCSB in QCD, strong diquark correlations exist within baryons

29. Structure of Baryons Craig Roberts. Origin of Mass via Resonance Electroproduction • Poincaré covariant Faddeev equation sums all possible exchanges and interactions that can take place between three dressed-quarks • Direct solution of Faddeev equation using rainbow-ladder truncation is now possible, but numerical challenges remain • Prediction: owing to DCSB in QCD, strong diquark correlations exist within baryons • For many/most applications, diquark approximation to quark+quark scattering kernel is used • Confinement and DCSB are readily expressed • Diquark correlations are not pointlike • Typically, r0+ ~ rπ & r1+ ~ rρ(actually 10% larger) • They have soft form factors

30. Roper resonance: Toward a solution to the fifty year puzzle, Volker D. Burkert and Craig D. Roberts, arXiv:1710.02549 [nucl-ex], Rev. Mod. Phys. 91 (2019) 011003/1-18 Structure of the nucleon's low-lying excitations, Chen Chen et al., arXiv:1711.03142 [nucl-th], Phys. Rev. D 97 (2018) 034016/1-13 Structure of the nucleon’s low-lying excitations Craig Roberts. Origin of Mass via Resonance Electroproduction • Unified understanding of the four lightest (I = ½; J = ½±) baryon isospin-doublets • Proton/neutron • (I = ½; J = ½+) ground state • u + u + d valence-quarks • Roper resonance • Nucleon’s first positive-parity excitation • well-defined dressed-quark core • augmented by a meson cloud, which both reduces Roper's core mass by ≈ 20% and contributes materially to electroproduction transition form factors at low-Q2

31. Structure of the nucleon's low-lying excitations, Chen Chen et al., arXiv:1711.03142 [nucl-th], Phys. Rev. D 97 (2018) 034016/1-13 Structure of the nucleon's low-lying excitations Craig Roberts. Origin of Mass via Resonance Electroproduction • Regarding N(1535) ½ – and N(1650) ½ –, an analogous picture ought to be correct. However, new questions arise. • In constituent-quark models, typical to describe these states as P-wave baryons, i.e. quantum mechanical systems with one unit of constituent-quark orbital angular momentum, L • Classify them as members of the (70; 11–) supermultiplet of SU(3) ⊗ O(3): • lighter state: L = 1, constituent-quark total spin S = ½ … coupled to J = L+S = ½ • heavier state: L = 1, S = 3/2 and J = L + S = ½ • But in relativistic quantum field theory, L and S are not good quantum numbers. • Moreover, even if they were, owing to the loss of particle number conservation, it is not clear a priori just with which degrees-of-freedom L, S should be connected. • This issue is related to the fact that the constituent-quarks used in building quantum mechanical models have no known mathematical connection with the degrees-of-freedom featuring in QCD • Plainly, much still to learn about the nature of the nucleon's parity partner and its excitations.

32. Structure of the nucleon's low-lying excitations, Chen Chen et al., arXiv:1711.03142 [nucl-th], Phys. Rev. D 97 (2018) 034016/1-13 Structure of the nucleon's low-lying excitations Craig Roberts. Origin of Mass via Resonance Electroproduction • Faddeev amplitude … ψ± = ψ1± + ψ2± + ψ3± • Considering (I = ½; J = ½ ±) systems, the following five diquark correlations can play a role • Isoscalar-Scalar (0,0+) • Isovector-Pseudovector (1,1+) • Isoscalar-Pseudoscalar (0,0–) • Isoscalar-Vector (0,1–) • Isovector-Vector (1,1–) • Focus on ψ3± (others obtained by cyclic permutation) • Ignore (1,1–)… coupling into this channel is very small • Γ = diquark correlation amplitudes • Δ = propagators for the diquark correlations • u(P) = spinor for the on-shell bound-state fermion • φ = quark+diquark solution amplitudes

33. Structure of the nucleon's low-lying excitations, Chen Chen et al., arXiv:1711.03142 [nucl-th], Phys. Rev. D 97 (2018) 034016/1-13 Structure of the nucleon's low-lying excitations Craig Roberts. Origin of Mass via Resonance Electroproduction φ = quark+diquark solution amplitudes Reattach the quark and diquark propagators … one obtains the Faddeev wave function = Φ Φ has analogous expansion … with the following rest-frame L-decomposition

34. Structure of the nucleon's low-lying excitations • Intrinsic orbital angular momentum within the four lightest (I = ½; J = ½±) baryon isospin-doublets • Evidently … ground-states are S-wave Parity partners are primarily P-wave in character • Presence of (small) D-wave component means all these states possess intrinsic deformation = Q0 • However … Q = spectroscopic quadrupole moment & difference between Q0 and Q represents the averaging of the nonspherical charge distribution due to its rotational motion as seen in the laboratory/rest- frame. • Q = 0 for J = ½ … no measurable quadrupole moment of J = ½ particle in isolation Craig Roberts. Origin of Mass via Resonance Electroproduction

35. Observing Deformation Craig Roberts. Origin of Mass via Resonance Electroproduction

36. Transition form factors: γ∗ + p → Δ(1232), Δ(1600) Nucleon to Delta electromagnetic transition …, G. Eichmann, D. Nicmorus, Phys. Rev. D 85 (2012) 093004 Nucleon and Δ elastic and transition form factors, Jorge Segovia et al., arXiv:1408.2919 [nucl-th], Few Body Syst. 55 (2014) pp. 1185-1222 Transition form factors: γ∗ + p → Δ(1232), Δ(1600), Ya Lu et al., arXiv:1904.03205 [nucl-th] Craig Roberts. Origin of Mass via Resonance Electroproduction

37. Roper resonance: Toward a solution to the fifty year puzzle, Volker D. Burkert and Craig D. Roberts, arXiv:1710.02549 [nucl-ex], Rev. Mod. Phys. 91 (2019) 011003/1-18 Transition form factors: γ∗ + p → Δ(1232), Δ(1600), Ya Lu et al., arXiv:1904.03205 [nucl-th] Transition form factors: γ∗ + p → Δ(1232) Craig Roberts. Origin of Mass via Resonance Electroproduction • JLab … γ∗ + p → Δ(1232) are now available for 0 < Q2 < 8 GeV2 • These data have stimulated much theoretical analysis and speculation about, inter alia, hadron shape deformation & role that resonance electroproduction experiments can play in exposing nonperturbative aspects of QCD, such as the nature of confinement and dynamical chiral symmetry breaking (DCSB) • Roper resonance: • Nucleon’s first positive-parity excitation is lighter than first negative-parity excitation

38. Transition form factors: γ∗ + p → Δ(1232), Δ(1600), Ya Lu et al., arXiv:1904.03205 [nucl-th] Transition form factors: γ∗ + p → Δ(1600) Craig Roberts. Origin of Mass via Resonance Electroproduction • Similar pattern found with J=3/2 baryons • Namely, contradicting quark-model predictions, the 1st positive-parity excitation, Δ(1600) 3/2+ lies below the negative parity Δ(1700) 3/2– , with splitting approximately same as that in the nucleon sector. • This being the case, and given the Roper-resonance example, it is likely that elucidating the nature of the Δ(1600) 3/2+ will require both: • data on its electroproduction form factors which extends well beyond the meson-cloud domain • predictions for these form factors to compare with that data. • The data exist; and can be analysed with this aim understood. • Theoretical predictions are now also available

39. Transition form factors: γ∗ + p → Δ(1232) • Top panel: Magnetic dipole γ∗ + p → Δ(1232) form factor compared with contemporary data [84]. • Middle panel: Electric quadrupole transition form factor. • Bottom panel: Coulomb quadrupole transition form factor. • In all panels: • solid (black) curve, complete result; • long-dashed (blue) curve, result obtained when only those components of the Δ(1232) wave function are retained which correspond to S-waves in the rest frame; • dashed (blue) curve, obtained when both the proton and Δ(1232) are reduced to S-wave states. Craig Roberts. Origin of Mass via Resonance Electroproduction

40. Transition form factors: γ∗ + p → Δ(1232) • The role played by higher partial waves in the wave functions increases with momentum transfer (something also observed in meson form factors), here generating destructive interference • agreement with data on GM* is impossible without the higher partial waves • effect of these components is very large in GE* (The complete result for GE* exhibits a zero at x ≈ 4, which is absent in the S-wave-only result(s).) Craig Roberts. Origin of Mass via Resonance Electroproduction

41. Poincaré-covariant wave functionsΔ(1232) & Δ(1600) Δ(1232) Δ(1600) S P D Craig Roberts. Origin of Mass via Resonance Electroproduction Δ-Baryon ground-state and positive-parity excitation are primarily S-wave in character: the magnitudes of the curves in the top row are greater than those in the other rows. NB – A: ground-state mass is almost insensitive to non-S-wave components NB – B: Ground-state (elastic FF) quadrupole-moment is large in magnitude and negative ⇒ oblate deformation NB – C: 1st positive-parity excitation, P-wave components generate a little repulsion, some attraction is provided by D-waves, and F-waves have no impact. NB – D: Evidently, some S-wave strength is shifted into P- and D-wave contributions within 1st positive-parity excitation

42. Transition form factors: γ∗ + p → Δ(1600) • Top panel: Magnetic dipole γ∗ + p → Δ(1232) form factor compared with contemporary data [84]. • Middle panel: Electric quadrupole transition form factor. • Bottom panel: Coulomb quadrupole transition form factor. • In all panels: • solid (black) curve, complete result; • long-dashed (blue) curve, result obtained when Δ(1600) is reduced to S-wave state; • dashed (blue) curve, both the proton and Δ(1600) are reduced to S-wave states; • dotted (green) curve, obtained by enhancing proton's axial-vector diquark content; • shaded (grey) band, light-front relativistic Hamiltonian dynamics (LFRHD); • dot-dashed (brown) curve, light-front relativistic quark model (LFRQM) with unmixed wave functions; • and dot-dot-dashed (orange) curve, LFRQM with configuration mixing. Craig Roberts. Origin of Mass via Resonance Electroproduction

43. Transition form factors: γ∗ + p → Δ(1600) • Marked sensitivity to L ≠ 0 components • GE* changes sign with all components included • Recall D-wave component of 1st J = 3/2+ positive-parity excitation is larger than in ground-state • Dashed green curve … enhance 1+ diquark to mimic meson-baryon final-state-interactions … move low-x results in correct direction • Also … significant sensitivity to model details ⇒ highlights sensitivity to baryon wave function Craig Roberts. Origin of Mass via Resonance Electroproduction

44. Epilogue Craig Roberts. Origin of Mass via Resonance Electroproduction

45. Epilogue – particulars Craig Roberts. Origin of Mass via Resonance Electroproduction • Poincaré-covariance ⇒ all baryons possess intrinsic deformation • Wave function cannot be purely S-wave in character … all possible waves are present • Deformation can be measured in transitions: ground-state nucleon to excited baryons • Quantum field theory ⇒ magnitude and interpretation of angular momentum depends on frame and – importantly – on the scale at which the calculation is performed • Poincaré-invariant form factors are always the same; but their interpretation via subsystem degrees-of-freedom can/will change • Only light-front projections of a given quantity have a probability (quantum mechanical) interpretation • Unless protected by symmetry, even such projections are scale dependent

46. Epilogue – broader view Craig Roberts. Origin of Mass via Resonance Electroproduction • N* physics has made great progress in the past 20 years • Missing Resonances: Gaps being filled & spectrum follows pattern expected of 3-body system with correlations • Novel insights concerning momentum-dependence of running coupling and masses from of data on nucleon transition form factors • Critical contribution from electro-production on large-Q2 domain • Revealing internal structure of resonances • Suggesting solution to Roper puzzle = quark core plus a ∼ 20% meson cloud (like the Δ) • Future – resonance electro-production to Q2 ≈ 12 GeV2 • Flavour separation of transition form factors … revealing nature of correlated wave functions • Data on electro-excitation amplitudes for baryon parity partners • Reveal impacts of DCSB in the spectrum and structure of nucleon resonances • Resolve issues relating to chiral symmetry restoration in highly excited systems. • Charting running coupling and masses at length-scales associated with > 98% of measured hadron mass • Crucial steps toward understanding & answering SM’s most fundamental questions

47. Epilogue – broader view What is mass? What is Confinement? Are they identical twins? Craig Roberts. Origin of Mass via Resonance Electroproduction • N* physics has made great progress in the past 20 years • Missing Resonances: Gaps being filled & spectrum follows pattern expected of 3-body system with correlations • Novel insights concerning momentum-dependence of running coupling and masses from of data on nucleon transition form factors • Critical contribution from electro-production on large-Q2 domain • Revealing internal structure of resonances • Suggesting solution to Roper puzzle = quark core plus a ∼ 20% meson cloud (like the Δ) • Future – resonance electro-production to Q2 ≈ 12 GeV2 • Flavour separation of transition form factors … revealing nature of correlated wave functions • Data on electro-excitation amplitudes for baryon parity partners • Reveal impacts of DCSB in the spectrum and structure of nucleon resonances • Resolve issues relating to chiral symmetry restoration in highly excited systems. • Charting running coupling and masses at length-scales associated with > 98% of measured hadron mass • Crucial steps toward understanding & answering SM’s most fundamental questions

48. Craig Roberts. Origin of Mass via Resonance Electroproduction