craig roberts physics division. search for exotic hadrons –discovery would force dramatic...

The Future of Hadron Physics

Craig Roberts

Physics Division

Craig Roberts: Future of Hadron Physics (24p)

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Science Challenges for the coming decade: 2013-2022

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 structureHadron Town Meeting at DNP2012


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Science Challenges for the coming decade: 2013-2022

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


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Overarching Science Challenges for the

coming decade: 2013-2022


Discover meaning of confinement, and its relationship to DCSB – the origin of visible mass


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What is Confinement?


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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)



Folklore “The color field lines between a quark and an anti-quark form flux tubes.

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Problem: 16 tonnes of force makes a lot of pions.



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Problem: 16 tonnes of force makes a lot of pions.



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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



G. Bali et al., PoS LAT2005 (2006) 308

http://inspirebeta.net/record/694488?ln=en



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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

complex-P2 complex-P2

o Real-axis mass-pole splits, moving into pair(s) of complex conjugate singularitieso State described by rapidly damped wave & hence state cannot exist in observable spectrum

Normal particle Confined particle


timelike axis: P2<0

s ≈ 1/Im(m) ≈ 1/2ΛQCD ≈ ½fm

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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?




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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.Hadron Town Meeting at DNP2012


DCSB

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Mass from nothing!


C.D. Roberts, Prog. Part. Nucl. Phys. 61 (2008) 50M. 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 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.






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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) limitNo 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



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Meson SpectroscopyAnomalies:

– 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



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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.



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Structure of HadronsNucleon 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 eliminatednumerous proposals for explainingRoper resonance



CLAS12 projected

CLAS N (20p 09)

CLAS p+p-p (2011)

CLAS p+p-p (2012)

LF QM with M(p2) DSE – M=constant DSE – M(p2)

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Structure of HadronsDuring 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



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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



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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 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 calculatedSuccess of programme demands very close collaboration between experiment and theory



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Need for calculation is emphasised by Saga of pion’s valence-quark distribution:o 1989: uv

π ~ (1-x)1 – inferred from LO-Drell-Yan & disagrees with QCD;

o 2001: DSE predicts uv

π ~ (1-x)2 Argues that distribution inferred from data can’t be correct;



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Need for calculation is emphasised by Saga of pion’s valence-quark distribution:o 1989: uv

π ~ (1-x)1 – inferred from LO-Drell-Yan & disagrees with QCD;

o 2001: DSE predicts uv

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



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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



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Future Clay Mathematics Institute

Prove confinement in pure-gauge QCDPrize: $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




25Hadron Town Meeting at DNP2012

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 - Latifa Elouadhriri8:20 QCD and nuclei - Larry Weinstein8:40 The future of hadronic physics at RHIC - Elke Aschenauer9:00 Hadronic physics at other facilities - Jen-Chieh Peng9:20 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

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Meson Spectroscopy

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 ~ f2

π0, 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



fπ0, 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.

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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



craig roberts physics division. search for exotic hadrons –discovery would force dramatic...

Documents

future of hadron physics

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