Craig Roberts Physics Division

 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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 2

 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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 3

Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 4

Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 5

Light quarks & Confinement 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) Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 6  Folklore “The color field lines between a quark and an anti-quark form flux tubes.

Light quarks & Confinement  Problem: 16 tonnes of force makes a lot of pions. Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 7

Light quarks & Confinement  Problem: 16 tonnes of force makes a lot of pions. Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 8

Light quarks & Confinement  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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 9 G. Bali et al., PoS LAT2005 (2006) 308PoS LAT2005 (2006) 308

 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 Confinement Craig Roberts: Future of Hadron Physics (24p) 10 complex-P 2 o Real-axis mass-pole splits, moving into pair(s) of complex conjugate singularities o State described by rapidly damped wave & hence state cannot exist in observable spectrum Normal particle Confined particle Hadron Town Meeting at DNP2012 timelike axis: P 2 <0 s ≈ 1/Im(m) ≈ 1/2Λ QCD ≈ ½fm

Light quarks & Confinement  In the study of hadrons, attention should turn from potential models 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? Is it possible that two phenomena, so critical in the Standard Model and tied to the dynamical generation of a mass-scale in QCD, can have different origins and fates? Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 11

Dynamical Chiral Symmetry Breaking  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. Craig Roberts: Future of Hadron Physics (24p) 12 Hadron Town Meeting at DNP2012

DCSB Craig Roberts: Future of Hadron Physics (24p) 13 Mass from nothing! Hadron Town Meeting at DNP2012 C.D. Roberts, Prog. Part. Nucl. Phys. 61 (2008) 50Prog. Part. Nucl. Phys. 61 (2008) 50 M. Bhagwat & P.C. Tandy, AIP Conf.Proc. 842 (2006) AIP Conf.Proc. 842 (2006)  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 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.

Meson Spectroscopy  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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 14

Meson Spectroscopy  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 m s /(m u +m d ) –non-Abelian anomaly via η-η' mixing Quantitative understanding of η-η' mixing gives access to strength of topological fluctuations in QCD  Both are intimately & inextricably linked with DCSB Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 15

Structure of Hadrons  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 nonpointlike diquark correlations –scale at which p-QCD effects become evident –strangeness content –meson-cloud effects –etc. Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 16

Structure of Hadrons  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→ P 11 (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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 17 CLAS12 projected CLAS N  09) CLAS     p (20 11) CLAS     p (20 12) LF QM with M(p 2 ) DSE – M=constant DSE – M(p 2 )

Structure of Hadrons  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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 18

Parton Structure of Hadrons  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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 19

Parton Structure of Hadrons  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 –Generalised parton 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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 20

Parton Structure of Hadrons  Need for calculation is emphasised by Saga of pion’s valence-quark distribution: o 1989: u v π ~ (1-x) 1 – inferred from LO-Drell-Yan & disagrees with QCD; o 2001: DSE predicts u v π ~ (1-x) 2 Argues that distribution inferred from data can’t be correct; Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 21

Parton Structure of Hadrons  Need for calculation is emphasised by Saga of pion’s valence-quark distribution: o 1989: u v π ~ (1-x) 1 – inferred from LO-Drell-Yan & disagrees with QCD; o 2001: DSE predicts u v π ~ (1-x) 2 Argues that distribution inferred from data can’t be correct; o 2010: NLO reanalysis, including soft-gluon resummation. Inferred distribution agrees with DSE-QCD Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 22

Theory  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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 23

Future  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 experimental and theoretical 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 Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 24

Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 25 6:00 JLab Users Satellite Meeting - Sebastian Kuhn 6:20 JLab 12 GeV upgrade status - TBA 6:40 View from JLab Management - Bob McKeown 7:00 Medium Energy Physics Overview - Roy Holt 7:40 The future of hadron physics - Craig Roberts 8:00 Nucleon structure with Jefferson Lab at 12 GeV - Latifa Elouadhriri 8:20 QCD and nuclei - Larry Weinstein 8:40 The future of hadronic physics at RHIC - Elke Aschenauer 9:00 Hadronic physics at other facilities - Jen-Chieh Peng 9:20 Open Mic - opportunity to present 1-3 slides, < 5 min 9:40 Discussion/Summary; what to present at DNP town meeting, further actions 10:00 Closeout/Adjourn

Meson Spectroscopy  Strength of matrix element for π 0, η, η' → γγ is inversely proportional to the mesons’ weak decay constant: M ~ 1/f π0, η, η' On the other hand, for “normal” systems, M ~ f 2 π0, η, η' /m π0, η, η' ; i.e., pattern completely reversed!  non-Abelian anomaly connects DCSB rigorously with essentially topological features of QCD: –Quantitative understanding of η-η' mixing gives access to strength of topological fluctuations in QCD Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 26 f π0, η, η' are order parameters for DCSB! Vacuum polarisation, measuring overlap of topological charge with matter sector 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.

Beyond the Standard Model  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 muon g-2 and dark matter puzzles –plausible masses are accessible to nonp-QCD machines Hadron Town Meeting at DNP2012 Craig Roberts: Future of Hadron Physics (24p) 27