Craig Roberts Physics Division

1. The Future of Hadron Physics Craig Roberts Physics Division

2. Science Challenges for the coming decade: 2013-2022 Craig Roberts: Future of Hadron Physics (25p) • Search for exotic hadrons • Discovery would force dramatic reassessment of the distinction between the notions of matter fields and force fields • Exploit opportunities provided by new data on nucleon elastic and transition form factors • Chart infrared evolution of QCD’s coupling and dressed-masses • Reveal correlations that are key to nucleon structure • Expose the facts or fallacies in modern descriptions of nucleon structure

3. Science Challenges for the coming decade: 2013-2022 Craig Roberts: Future of Hadron Physics (25p) • Precision experimental study of valence region, and theoretical computation of distribution functions and distribution amplitudes • Computation is critical • Without it, no amount of data will reveal anything about the theory underlying the phenomena of strong interaction physics • Explore and exploit opportunities to use precision-QCD as a probe for physics beyond the Standard Model

4. Overarching Science Challenges for the coming decade: 2013-2022 Discover meaning of confinement, and its relationship to DCSB – the origin of visible mass Craig Roberts: Future of Hadron Physics (25p)

5. What is Confinement? Craig Roberts: Future of Hadron Physics (25p)

6. Light quarks & Confinement • Folklore “The color field lines between a quark and an anti-quark form flux tubes. Craig Roberts: Future of Hadron Physics (25p) A unit area placed midway between the quarks and perpendicular to the line connecting them intercepts a constant number of field lines, independent of the distance between the quarks. This leads to a constant force between the quarks – and a large force at that, equal to about 16 metric tons.” Hall-D CDR(5)

7. Light quarks & Confinement Craig Roberts: Future of Hadron Physics (25p) • Problem: 16 tonnes of force makes a lot of pions.

8. Light quarks & Confinement Craig Roberts: Future of Hadron Physics (25p) Problem: 16 tonnes of force makes a lot of pions.

9. G. Bali et al., PoS LAT2005 (2006) 308 Light quarks & Confinement Craig Roberts: Future of Hadron Physics (25p) In the presence of light quarks, pair creation seems to occur non-localized and instantaneously No flux tube in a theory with light-quarks. Flux-tube is not the correct paradigm for confinement in hadron physics

10. Confinement Confined particle Normal particle complex-P2 complex-P2 timelike axis: P2<0 s ≈ 1/Im(m) ≈ 1/2ΛQCD≈ ½fm • Real-axis mass-pole splits, moving into pair(s) of complex conjugate singularities • State described by rapidly damped wave & hence state cannot exist in observable spectrum Craig Roberts: Future of Hadron Physics (25p) • QFT Paradigm: • Confinement is expressed through a dramatic change in the analytic structure of propagators for coloured states • It can almost be read from a plot of the dressed-propagator for a coloured state

11. Light quarks & Confinement Craig Roberts: Future of Hadron Physics (25p) • In the study of hadrons, potential models must be replaced • Attention must turn toward the continuum bound-state problem in quantum field theory • Such approaches offer the possibility of posing simultaneously the questions • What is confinement? • What is dynamical chiral symmetry breaking? • How are they related? It is inconceivable that two phenomena, so critical in the Standard Model and tied to the dynamical generation of a mass-scale, can have different origins and fates.

12. Dynamical ChiralSymmetry Breaking Craig Roberts: Future of Hadron Physics (25p) • DCSB is a fact in QCD • Dynamical, not spontaneous • Add nothing to QCD , no Higgs field, nothing, • Effect achieved purely through the dynamics of gluons and quarks. • It’s the most important mass generating mechanism for visible matter in the Universe. • Responsible for approximately 98% of the proton’s mass. • Higgs mechanism is (almost) irrelevant to light-quarks.

13. DCSB C.D. Roberts, Prog. Part. Nucl. Phys. 61 (2008) 50 M. Bhagwat & P.C. Tandy, AIP Conf.Proc. 842 (2006) 225-227 • In QCD, all “constants” of quantum mechanics are actually strongly momentum dependent: couplings, number density, mass, etc. • So, a quark’s mass depends on its momentum. • Mass function can calculated and is depicted here. • Continuum- and Lattice-QCD Mass from nothing! • are in agreement: the vast bulk of the light-quark mass comes from a cloud of gluons, dragged along by the quark as it propagates. Craig Roberts: Future of Hadron Physics (25p)

14. Meson Spectroscopy Craig Roberts: Future of Hadron Physics (25p) • Exotics and hybrids are truly novel states • They’re not matter as we know it • In possessing valence glue, such states confound the distinction between matter fields and force carriers • But they’re only exotic in a quantum mechanics based on constituent-quark degrees-of-freedom • They’re natural in quantum field theory, far from the nonrelativistic (potential model) limit • No symmetry forbids them, QCD interaction promotes them, so they very probably exist! • Theory: • Expected mass domain predicted by models and lattice-QCD • However, need information on transition form factors, decay channels and widths

15. Meson Spectroscopy Craig Roberts: Future of Hadron Physics (25p) • Anomalies: • fascinating feature of quantum field theory • currents conserved classically, but whose conservation law is badly broken after second quantisation • Two anomalies in QCD are readily probed by experiment • Abelian anomaly, via γγ decays of light neutral pseudoscalars • Provides access to light-quark mass ratio 2 ms /(mu+md) • non-Abelian anomaly via η-η'mixing • Both are inextricably linked with DCSB

16. New Collaboration being built: JLab + MesonNet (Germany), to “mine” existing data, so as to improve our knowledge of meson decays and branching ratios. There is an obvious extension to 12GeV programme. Meson Spectroscopy • fπ0, η, η' are order parameters for DCSB! Vacuum polarisation, measuring overlap of topological charge with matter sector Craig Roberts: Future of Hadron Physics (25p) • Strength of matrix element for π0, η, η' →γγ is inversely proportional to the mesons’ weak decay constant: M ~ 1/fπ0, η, η' On the other hand, for “normal” systems, M ~ f2π0, η, η' /mπ0, η, η'; i.e., pattern completely reversed! • non-Abelian anomaly connects DCSB rigorously with essentially topological features of QCD: • Quantitative understanding ofη-η'mixing gives access to strength of topological fluctuations in QCD

17. Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • Elastic form factors • Provide vital information about the structure and composition of the most basic elements of nuclear physics. • They are a measurable and physical manifestation of the nature of the hadrons' constituents and the dynamics that binds them together. • Accurate form factor data are driving paradigmatic shifts in our pictures of hadrons and their structure; e.g., • role of orbital angular momentum and nonpointlikediquark correlations • scale at which p-QCD effects become evident • strangeness content • meson-cloud effects • etc.

18. Structure of Hadrons LF QM with M(p2) • DSE – M=constant DSE – M(p2) CLAS Np (2009) CLAS p+p-p (2011) CLASp+p-p (2012) CLAS12 projected Craig Roberts: Future of Hadron Physics (25p) • Nucleon to resonance transition form factors • Critical extension to elastic form factors and promising tool in probing for valence-glue in baryons • Meson excited states and nucleon resonances are more sensitive to long-range effects in QCD than are the properties of ground states … analogous to exotics and hybrids • N→ P11(1440) “Roper” • First zero crossing measured in any nucleon form factor or transition amplitude • Appearance of zero has eliminated numerous proposals for explaining Roper resonance

19. Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • During last five years, the Excited Baryon Analysis Center resolved a fifty-year puzzle by demonstrating conclusively that the Roper resonance is the proton's first radial excitation • its lower-than-expected mass owes to a dressed-quark core shielded by a dense cloud of pions and other mesons. (Decadal Report on Nuclear Physics: Exploring the Heart of Matter) • Breakthrough enabled by both new analysis tools and new high quality data. • This Experiment/Theory collaboration holds lessons for GlueX and future baryon analyses

20. Parton Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • Valence-quark structure of hadrons • Definitive of a hadron – it’s how we tell a proton from a neutron • Expresses charge; flavour; baryon number; and other Poincaré-invariant macroscopic quantum numbers • Via evolution, determines background at LHC • Sea-quark distributions • Flavour content and asymmetry • Former and any nontrivial structure in the latter are both essentially nonperturbative features of QCD

21. Parton Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • Light front provides a link with quantum mechanics • If a probability interpretation is ever valid, it’s in the light-front frame • Enormous amount of intuitively expressive information about hadrons & processes involving them is encoded in • Parton distribution functions • Generalisedparton distribution functions • Transverse-momentum-dependent parton distribution functions • Information will be revealed by the measurement of these functions – so long as they can be calculated Success of programme demands very close collaboration between experiment and theory

22. Parton Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • Need for calculation is emphasised by Saga of pion’s valence-quark distribution: • 1989: uvπ ~ (1-x)1 – inferred from LO-Drell-Yan & disagrees with QCD; • 2001: DSE predicts uvπ ~ (1-x)2 argues that distribution inferred from data can’t be correct;

23. Parton Structure of Hadrons Craig Roberts: Future of Hadron Physics (25p) • Need for calculation is emphasised by Saga of pion’s valence-quark distribution: • 1989: uvπ ~ (1-x)1 – inferred from LO-Drell-Yan & disagrees with QCD; • 2001: DSE predicts uvπ ~ (1-x)2 argues that distribution inferred from data can’t be correct; • 2010: NLO reanalysis including soft-gluon resummation, inferred distribution agrees with DSE and QCD

24. Theory Craig Roberts: Future of Hadron Physics (25p) • Lattice-QCD • Significant progress in the last five years • This must continue • Bound-state problem in continuum quantum field theory • Significant progress, too • Must also continue • Completed and planned experiments will deliver the pieces of the puzzle that is QCD. Theory must be developed to explain how they fit together

25. Future Craig Roberts: Future of Hadron Physics (25p) • Clay Mathematics Institute Prove confinement in pure-gauge QCD Prize: $1-million That’s about all this easy problem is worth • In the real world, all readily accessible matter is defined by light quarks Confinement in this world is certainly an immeasurably more complicated phenomenon • Hadron physics is unique: • Confronting a fundamental theory in which the elementary degrees-of-freedom are intangible and only composites reach detectors • Hadron physics must deploy a diverse array of probes and tools in order to define and solve the problems of confinement and its relationship with DCSB • These are two of the most important challenges in fundamental Science; and only we are equipped to solve them

26. Beyond the Standard Model Craig Roberts: Future of Hadron Physics (25p) • High precision electroweak measurements • Any observed and confirmed discrepancy with Standard Model reveals New Physics • Precise null results place hard lower bounds on the scale at which new physics might begin to have an impact • Experiment and theory bounds on nucleon strangeness content place tight limits on dark-matter – hadron cross-sections • Sensitive dark photon searches • dark photon is possible contributor to muong-2 and dark matter puzzles • plausible masses are accessible to nonp-QCD machines

27. 6:00 JLab Users Satellite Meeting - Sebastian Kuhn6:20 JLab 12 GeV upgrade status - TBA6:40 View from JLab Management - Bob McKeown7:00 Medium Energy Physics Overview - Roy Holt7:40 The future of hadron physics - Craig Roberts8:00 Nucleon structure with Jefferson Lab at 12 GeV - LatifaElouadhriri8:20 QCD and nuclei - Larry Weinstein8:40 The future of hadronic physics at RHIC  - ElkeAschenauer9:00 Hadronic physics at other facilities - Jen-ChiehPeng9:20 Open Mic - opportunity to present 1-3 slides, < 5 min9:40 Discussion/Summary; what to present at DNP town meeting, further actions10:00 Closeout/Adjourn Craig Roberts: Future of Hadron Physics (25p)