Craig Roberts Physics Division Search for exotic hadrons

Craig Roberts Physics Division

Ø Search for exotic hadrons – Discovery would force dramatic reassessment of the distinction between the notions of matter fields and force fields Ø 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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 2

Ø 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 theory underlying the phenomena of strong interaction physics Ø Explore and exploit opportunities to use precision. QCD as a probe for physics beyond the Standard Model Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 3

Discover meaning of confinement, and its relationship to DCSB – the origin of visible mass Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 4

Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 5

Light quarks & Confinement Ø Folklore “The color field lines between a quark and an anti-quark form flux tubes. 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) Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 6

Light quarks & Confinement ØProblem: 16 tonnes of force makes a lot of pions. Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 7

Light quarks & Confinement ØProblem: 16 tonnes of force makes a lot of pions. Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 8

G. Bali et al. , Po. S LAT 2005 (2006) 308 Light quarks & Confinement Ø In the presence of light quarks, pair creation seems to occur non-localized and instantaneously Ø No flux tube in a theory with lightquarks. Ø Flux-tube is not the correct paradigm for confinement in hadron physics Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 9

Confinement Ø 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 dressedpropagator for a coloured state Confined particle Normal particle complex-P 2 timelike axis: P 2<0 s ≈ 1/Im(m) ≈ 1/2ΛQCD ≈ ½fm 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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 10

Light quarks & Confinement Ø In the study of hadrons, attention should turn from potential models toward the continuum bound-state problem in quantum field theory Ø 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? Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 11

Dynamical Chiral Symmetry Breaking ØDCSB 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 lightquarks. Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 12

DCSB C. D. Roberts, Prog. Part. Nucl. Phys. 61 (2008) 50 M. Bhagwat & P. C. Tandy, AIP Conf. Proc. 842 (2006) 225 -227 Ø In QCD, all “constants” of quantum mechanics are actually strongly momentum Mass from nothing! dependent: couplings, number density, mass, etc. Ø So, a quark’s mass depends on its momentum. Ø Mass function calculated and is depicted here. Ø 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. Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 13

Meson Spectroscopy Ø 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 Ø 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 Ø No symmetry forbids them, QCD interaction promotes them, so they very probably exist! Ø Theory: – Expected mass domain predicted by models and lattice-QCD – However, need information on transition form factors, decay channels and widths Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 14

Ø Anomalies: Meson Spectroscopy – fascinating feature of quantum field theory – currents conserved classically, but whose conservation law is badly broken after second quantisation Ø 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 • Quantitative understanding of η-η' mixing gives access to strength of topological fluctuations in QCD Ø Both are intimately & inextricably linked with DCSB Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 15

Ø Elastic form factors Structure of Hadrons – 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. Ø 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. Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 16

Structure of Hadrons Ø 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 LF QM with M(p 2) Ø 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 DSE – M=constant DSE – M(p 2) CLAS Np (2009) CLAS p+p-p (2011) CLAS p+p-p (2012) CLAS 12 projected Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 17

Structure of Hadrons Ø 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) Ø Breakthrough enabled by both new analysis tools and new high quality data. Ø This Experiment/Theory collaboration holds lessons for Glue. X and future baryon analyses Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 18

Parton Structure of Hadrons Ø 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 Ø Sea-quark distributions – Flavour content and asymmetry Ø Former and any nontrivial structure in the latter are both essentially nonperturbative features of QCD Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 19

Parton Structure of Hadrons Ø Light front provides a link with quantum mechanics – If a probability interpretation is ever valid, it’s in the lightfront frame Ø 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 Ø 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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 20

Parton Structure of Hadrons Ø Need for calculation is emphasised by Saga of pion’s valence-quark distribution: o 1989: uvπ ~ (1 -x)1 – inferred from LO-Drell-Yan & disagrees with QCD; o 2001: DSE predicts uvπ ~ (1 -x)2 Argues that distribution inferred from data can’t be correct; Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 21

Parton Structure of Hadrons Ø Need for calculation is emphasised by Saga of pion’s valence-quark distribution: o 1989: uvπ ~ (1 -x)1 – inferred from LO-Drell-Yan & disagrees with QCD; o 2001: DSE predicts uvπ ~ (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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 22

Ø Lattice-QCD Theory – Significant progress in the last five years – This must continue Ø Bound-state problem in continuum quantum field theory – Significant progress, too – Must also continue Ø Completed and planned experiments will deliver the pieces of the puzzle that is QCD. Theory must be developed to explain how they fit together Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 23

Future Ø Clay Mathematics Institute Prove confinement in pure-gauge QCD Prize: $1 -million That’s about all this easy problem is worth Ø In the real world, all readily accessible matter is defined by light quarks Confinement in this world is certainly an immeasurably more complicated phenomenon Ø Hadron physics is unique: – Confronting a fundamental theory in which the elementary degrees-offreedom are intangible and only composites reach detectors Ø 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 Ø These are two of the most important challenges in fundamental Science; and only we are equipped to solve them Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 24

6: 00 JLab Users Satellite Meeting - Sebastian Kuhn 6: 20 JLab 12 Ge. V upgrade status - TBA 6: 40 View from JLab Management - Bob Mc. Keown 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 Ge. V - 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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 25

New Collaboration being built: JLab + Meson. Net (Germany), to “mine” existing data, so as to improve our knowledge of meson decays and branching ratios. There is an obvious extension to 12 Ge. V programme. Meson Spectroscopy Ø Strength of matrix element for π0, η, η' → γγ is inversely proportional to the mesons’ weak decay constant: fπ0, η, η' are order M ~ 1/fπ0, η, η' parameters for DCSB! On the other hand, for “normal” systems, M ~ f 2π0, η, η' /mπ0, η, η' ; i. e. , pattern completely reversed! Ø 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 Vacuum polarisation, measuring overlap of topological charge with matter sector Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 26

Beyond the Standard Model Ø 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 crosssections Ø 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 Craig Roberts: Future of Hadron Physics (24 p) Hadron Town Meeting at DNP 2012 27