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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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 precisionQCD as a probe for physics beyond the Standard
Model
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
5
Light quarks & Confinement
 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 (24p)
Hadron Town Meeting at DNP2012
6
Light quarks & Confinement
Problem:
16 tonnes of force
makes a lot of pions.
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
7
Light quarks & Confinement
Problem:
16 tonnes of force
makes a lot of pions.
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
8
G. Bali et al., PoS LAT2005 (2006) 308
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 lightquarks.
 Flux-tube is not the
correct paradigm for
confinement in
hadron physics
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
9
Confinement
 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-P2
complex-P2
timelike axis: P2<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 (24p)
Hadron Town Meeting at DNP2012
10
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?
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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 lightquarks.
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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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
Mass from nothing!
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.
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
13
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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
14
 Anomalies:
Meson Spectroscopy
– 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
• Quantitative understanding of η-η' mixing gives access
to strength of topological fluctuations in QCD
 Both are intimately & inextricably linked with DCSB
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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.
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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
LF QM with M(p2)
 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
DSE – M=constant
DSE – M(p2)
CLAS Np (2009)
CLAS p+p-p (2011)
CLAS p+p-p (2012)
CLAS12 projected
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
17
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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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Parton Structure of Hadrons
 Light front provides a link with quantum mechanics
– If a probability interpretation is ever valid, it’s in the lightfront 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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
20
Parton Structure of Hadrons
 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 (24p)
Hadron Town Meeting at DNP2012
21
Parton Structure of Hadrons
 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 (24p)
Hadron Town Meeting at DNP2012
22
 Lattice-QCD
Theory
– 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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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-offreedom 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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
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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
Craig Roberts: Future of Hadron Physics (24p)
Hadron Town Meeting at DNP2012
25
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
 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 ~ 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 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 (24p)
Hadron Town Meeting at DNP2012
26
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 crosssections
 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 (24p)
Hadron Town Meeting at DNP2012
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