Three Mysteries of QCD

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Three Mysteries of QCD
Diabolical Strange quarks in the nucleon
Diabolical
Quark correlations and color non-singlet
spectroscopy
(QCD without experimental facilities)
Reluctant symmetry Parity doubling among the
hadrons -- a challenge for spectroscopy
R. L. Jaffe Workshop on Hadron Structure at
J-Parc November - December 2o05
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Why QCD?
The perfect (eg. string) theory
No parameters All interactions dictated by
symmetries Highly non-trivial vacuum
A warm up for the Theory of Everything
Emergent phenomena Confinement, chiral
symmetry breaking, hadrons!
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Whats to be done?
Basic QCD dynamics at the scale where hadrons
form is not understood. We dont understand the
mechanism of confinement or of chiral symmetry
breaking in quantitative terms. We dont
understand why quarks are the quasiparticles of
QCD, even after running through regions of such
strong renormalization. All the more reason to
look for other quasiparticles pseudoscalar
bosons, diquarks(!) We dont have a
satisfactory description of the confined
relativistic bound state to enable us to predict
and/or interpret information like magnetic
moments, excited state spectra, and flavor mixing
via OZI rule violation. We dont know how to
connect the precise information obtained with
short distance probes at high energies to the
properties we would like to understand at
hadronic scales --- sum rules, duality,
inclusive-exclusive connection...
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QCD is very difficult at the confinement scale
J-PARC
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I. Strangeness
Intelligent Design
Diabolical Design !
The strange quark content of important hadrons
is therefore remarkably difficult to model and
difficult to measure.
Important to remember Symmetry alone tells us
nothing about strange quark content of nucleon
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Compare our world to one where
What do baryon magnetic moments tell us about the
u,d, s quark magnetization in the nucleon?
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Brief remark about mechanisms...
How do pairs mix into the nucleon state?
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Mapping the strangeness in the nucleon
Ten years progress...
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Probing the strangeness content of the nucleon
with Z exchange
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Strange quark contribution to the nucleon spin...
Model independent measurement possible in
elastic neutrino proton scattering at J-Parc.
Direct measurement (analogous to magnetic
moment in electron scattering) A difficult, but
important experiment Free from assumptions
about SU(3) violation and low-x
extrapolation Had been an objective of LSND,
but never achieved! J-Parc...
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II. Quark correlations Color Non-Singlet
Spectroscopy
Message? Lattice QCD ? Phenomenological models
Quark correlations
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Evidence for the good diquark
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Only those baryons allowed by symmetry to contain
pure good diquark are anomalously abundant.
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Lattice QCD --- Experiment --- Model building
Given development of lattice QCD, why not just
dispense with phenomenological models? Because
whole areas of hadron physics will never (?) be
amenable to lattice methods. Scattering,
production, structure functions, fragmentation,
diffractive phenomena (the Pomeron), polarization
phenomena, resonance widths... Use lattice to
gain insight into QCD dynamics, even in
alternative realities, not accessible to
experiment. Vary Nc Vary quark
masses Construct hadrons that will perhaps never
be observed experimentally.
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Color non-singlet spectroscopy on the lattice
Neutralize with spectator Wilson line (
infinitely heavy quark) Compare with bottom
hadron spectroscopy (in principle) And with
phenomenological models
Even color sextet light quark states
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Baryons
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And more from AdFL
Stimulates further consideration of quark
correlations
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Proposed correlations?
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Internal correlation structure
Create states then study correlations in an
lattice calculation
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III. Parity Doubling in the Hadron Spectrum
Message? A weak, but nevertheless significant
symmetry between baryons the same I and J and of
opposite parity. Identify and classify baryon
and meson resonances above 1.5 GeV, especially
Strangeness -1 and -2.
RLJ, Dan Pirjol, Antonello Scardicchio
hep-ph/0511081 and II in preparation
Semi-Quantitative study of significance of
symmetry in baryon sector
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Trying to do better
Typical
Include widths and reliabilities (stars
assigned by PDG, , , , )
Compare Natures correlations with a control
set obtained by shuffling parities over the
existing resonances.
F. Iachello, 1989
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Nucleons and Deltas
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Sigmas and Lambdas
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Comments on data
Nucleon and Delta parity doubling correlation
is significant. Sigma and Lambda is not
convincing. More data needed. For example, many
Sigmas in 1.5 -- 2.5 GeV region lack spin-parity
assignments. Insufficient data on Cascades for
meaningful analysis Meson data are even less
complete and in many cases controversial. Could
be part of a program to re-examine light hadron
spectroscopy including many beams and processes
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Origins of parity doubling?
RLJ, Pirjol, Scardicchio
Cohen Glozman Jido, Hatsuda, Kunihiro, Oka,
Hosaka
Not chiral symmetry restoration
Attempt to define representation with
nontrivial parity transformation yields manifold
which is simply connected to the standard
non-linear representation with trivial parity
structure.
If one attempts to realize chiral symmetry in a
linear way on a subset of states in a world with
spontaneous symmetry breaking and massless pions,
the chiral symmetry in fact gives no relations
among the properties of these states, such as
masses and couplings. Such predictions, that are
typical of a symmetry realized in the Wigner-Weyl
mode, would hold only if certain chirally
invariant operators are dynamically suppressed.
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What, then, could be the origin of parity
doubling?
The task becomes to see if the matrix elements
of FF-dual are suppressed at moderate excitation
in the hadron spectrum. A project for lattice
QCD?
Like any explicitly broken continuous symmetry,
it can be restored if the matrix elements of the
divergence of the associated Noether current are
suppressed.
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What, then, could be the origin of parity
doubling?
Conformational degeneracy?
Any rigid body with tri-axial structure and
tunneling amplitude between conformations will
have parity doubling. A quark-diquark model of
baryons?
Would suggest parity doubling on leading Regge
trajectory But its a mystery why such an
effect should dominate for J1/2, 3/2, etc.
Like any explicitly broken continuous symmetry,
it can be restored if the matrix elements of the
divergence of the associated Noether current are
suppressed.
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Summary, reflections...
Despite years of theoretical and experimental
work, QCD still has not yielded the secrets of
confinement dynamics.
Some aspects may never be understood from first
principles (two body scattering at fixed angle,
for example)
But some problems are crisply defined and beg
for a deeper explanation
The role of the strange quark in the
nucleon The role of gluons in the
nucleon The structure of quark correlations
and the problem of exotics Parity
doubling And others too vector dominance, the
absence of large higher twist effects, the
transformation from current to constituent
quarks,...