How Does Short Distance Behavior Affect the Nucleus

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How Does Short Distance BehaviorAffect the
Nucleus

• Don Geesaman
• 12 January 2007
• DNP QCD Town Meeting

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Why

• We built JLab and did experiments at SLAC, FNAL,
  DESY... because the short-distance behavior of
  nuclei was not understood.
• the nucleus is more than mean-field and
  long-range correlations.
• High momentum transfer short distances
• short range components of N-N interaction
• High momentum transfer resolve the QCD
  structure
• where are the QCD effects in nuclei?

• We know at high temperature or density things
  must change.
• how high is high
• is transition continuous or abrupt?
• where do neutron stars lie?

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We want to describe a nucleus

• Pure QCD Description
• what are the clusters of quarks in a nucleus?
• know the parton distributions change
• EMC effect
• shadowing
• xgt1
• The problem is always whether our description of
  a bare proton is good enough and then how to
  actually calculate many body effects?

• Hadronic Description
• exemplified by ab initio calculations with
  potentials
• NN
• NNN NNNN
• Bare form factors
• Meson exchange currents
• Past two decades have shown this is remarkably
  successful

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Issues in Proton Structure new data has been
critical!

• Nucleon form factors
• spin carried by the quarks and gluons and angular
  momentum
• nature of the sea

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Our visual images
average spacing at ?nm 1.8 fm Radius of a
nucleon 0.8 fm average spacing at
3?nm 1.3 fm
OR
nucleons held apart by short range
repulsion but even in 208Pb, half the nucleons
are in the surface
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What do we know about short distance behavior in
nuclei?

• Strong N-N potential does have impacts

NN Interaction
NN Correlation Functions
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What do we know about short distance behavior in
nuclei?

• Impact of correlations on high momentum structure
  of wave functions
• direct observation
• high momentum components in (e,ep)
• xgt1 correlations
• indirect (quenching) effects
• reduction of single particle strength
  Spectroscopic factors
• apparent changes in bare form factors quenching
  of GA

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Direct measurement
JLab E97-006 Rohe et al. PRL 93, 182501
(04) 0.61/- .06 protons in pmgt240 MeV/c and
Emgt 40 MeV
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Spectroscopic factors
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Distribution of spectroscopic strength (from
Dickhoff)
Note ab initio calculations do very good job in,
for example 7Li SRCLRC.
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Basic facts of nuclear physics that may be
wrong in neutron-rich nuclei

• The radius and diffuseness of the neutron and
  proton distributions are similar
• R1.2 A1/3, a 0.55 fm
• The magic numbers of the shell model are fixed.
• The deformations of the neutrons and protons are
  similar
• The valence quasi-particles are renormalized by
  about 0.6 by short-range correlations.
• The charge-independence of the strong interaction
  makes isospin a good quantum number

This is only illustrative. There are a number of
other mechanisms that also lead to changes in the
shell structure as N/Z varies.
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Does the impact of correlations change
dramatically away from valley of stability?
From Gade and Tostevin, NSCL
History 1960s Shell Model and transfer
reactions assumed pure single particle
states. 1970s electron scattering showed only
60 occupancy in valence single particle
states. 1980s Understood based on
correlations. 1990s Correlations viewed as
universal, approximately nucleus
independent. 2000s In nuclei far from
stability, observed large changes in correlation
effects.
22O
34Ar
?S Sn-Sp for neutron knockout and Sp-Sn for
proton knockout
Note Rs is ratio to shell model not spectroscopic
factor
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Another way to look at high momentum components
xgt1 data
CLAS Egiyan et al.

• 2 and 3 nucleon correlations
• It appears that correlations dominate the deep
  inelastic structure functions at high x.
• Not likely to tell us about quark substructure!

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Towards better understanding of short range
behaviour

• NN, NNN Data
• a number of puzzles
• what is the key experiment?
• Lattice very long way to go
• Effective field theory
• Need at least N3LO Chi2 of order 1
• Still a fit to data, but about ½ the free
  parameters!
• 3NF still working on N3LO
• Other baryon-nucleon interactions

Modern lattice QCD result S.R. Beane et al, PRL
97 (2006)
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Nucleon-Baryon Interactions

• ?-N No one pion exchange
• small spin-orbit interaction
• perhaps more direct window on short range
  behavior
• Will low energy data (scattering length,
  hypernuclear spectroscopy) provide enough
  constraints?
• S N and ? N Important for neutron star
  matter. How to probe?
• P P Interactions
• G parity says short range part changes
• problem is absorption is so strong that little
  information seems to be obtainable

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Does structure of baryons change in nuclei?

• So far JLAB has taught us that hadron
  structure/interactions do not change much (to the
  precision we can determine today) at normal
  matter densities.

Perhaps the smoking gun?
Schiavilla et al PRL 94, 072303 (05)
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Quark Meson Coupling predictions
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Parton Distributions in Nuclei

• 1984 Parton distributions are different
• EMC effect nucleon carries smaller fraction of
  momentum or changes structure
• Shadowing
• 1990 little change in sea quarks for xgt0,1
• 2007
• x gt1 data dominated by correlations
• still need flavor separation and larger x range
  for antiquarks.
• Will we finally be able to tag parton
  distributions vs the momentum and binding energy
  of spectator particles?
• predicted large effects in spin structure

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Still only one high precision measurement of
antiquarks Where are the nuclear pions?

• The binding of nucleons in a nucleus modifies the
  x dependence.
• Most contemporary models still predict large
  effects to antiquark distributions as x
  increases.
• Models must explain both DIS-EMC effect and
  Drell-Yan
• Sufficient uncertainly that CTEQ is worried about
  using neutrino data on Fe to establish nucleon
  antiquark distributions.
• MINERva neutrino A dependence

Smith and Miller
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Advantages of 120 GeV Main Injector

• The future
• Fermilab E906
• Data in 2009
• 1H, 2H, and nuclear targets
• 120 GeV proton Beam

• The (very successful) past
• Fermilab E866/NuSea
• Data in 1996-1997
• 1H, 2H, and nuclear targets
• 800 GeV proton beam

• Cross section scales as 1/s
• 7 x that of 800 GeV beam
• Backgrounds, primarily from J/? decays scale as
  s
• 7 x Luminosity for same detector rate as 800 GeV
  beam
• 50 x statistics!!

Fixed Target Beam lines
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Can we measure binding energy and spectator
momentum dependence?

• Test technical issue of how to include binding in
  calculation
• Do we see nuclear dependence change for high
  momentum spectators which involve short distance
  interactions- Spectator tagging?

SLAC fit to heavy nuclei (scaled to 3He)
Calculations by Pandharipande and Benhar for 3He
and 4He
Approximate uncertainties for 12 GeV coverage
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Nuclear Effects in Spin Dependence

• Why its big?
• Quark-Meson Coupling model
• Lower Dirac component of confined light quark
  modified most by the scalar field

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Neutron Stars
Correlation between neutron skin thickness in
finite nuclei and pressure of ß-equilibrated
matter in neutron stars

• probe densities to 6 ?NM
• Is neutron matter superfluid?
• low density yes
• higher density ???
• Do we see transition to kaon-condensed, hyperson,
  or quark matter ?
• Nuclear Observables
• neutron skins
• N/Z dependence of giant resonances
• nuclear equation of state studies
• Astronomical observations
• What are the limits on mass and radii?
• cooling?

Recent observation of high mass neutron
stars 2.1 0.2 M? Nice et al.
astro-ph/0508050 2.1 0.28 M?, R13.8 1.8
km Ozel, Nature 441, 04858 (2006)
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Constraints on neutron star equations of state
Mass-Radius constraints from observations and
model predictions for the mass-radius of
nucleonic stars, hybrid stars and strange quark
stars. (From Jaikumar, Page and Reddy)
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What really happens at high density?
Stone, Guichon, Matevosyan and Thomas
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Summary

• Success
• Two body correlations mapped out
• Dickhoff unique for a correlated many body
  system
• beginning to get information on three body
  correlations
• correlations may be quite different in nuclei far
  from stability
• Still to do and a lot harder than we had hoped
• QCD description of short range N-N behavior
• definitive evidence for changes in proton
  structure in nuclei beyond easily understood (if
  hard to calculate) mean-field effects.
• Spin and binding/spectator momentum effects
• flavor dependence - extend nuclear anti-quark
  measurements to regions where effects may be much
  larger.
• a long way to go to be confident about what
  happens in neutron stars

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Drell-Yan scattering A laboratory for sea
quarks
E906 Spect. Monte Carlo

• Detector acceptance chooses xtarget and xbeam.
• Fixed target ? high xF xbeam xtarget
• Valence Beam quarks at high-x.
• Sea Target quarks at low/intermediate-x.

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Separating structure and dynamics