How Does Short Distance Behavior
Affect the Nucleus
Don Geesaman
12 January 2007
DNP QCD Town Meeting
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?
Don Geesaman
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We want to describe a nucleus
 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
Don Geesaman
 Pure QCD Description
– what are the clusters of
quarks in a nucleus?
– know the parton
distributions change
• EMC effect
• shadowing
• x>1
 The problem is always
whether our description of a
bare proton is good enough
and then how to actually
calculate many body effects?
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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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average spacing at ρnm ~ 1.8 fm
Radius of a nucleon
~ 0.8 fm
average spacing at 3ρnm ~ 1.3 fm
Our visual images
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
Don Geesaman
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,e’p)
• x>1 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 pm>240 MeV/c and
Em> 40 MeV
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Spectroscopic factors
Don Geesaman
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Distribution of spectroscopic strength (from
Dickhoff)
Note ab initio calculations do very good job in, for example 7Li – SRC+LRC.
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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
 (r ) 




1
1  exp[( r  R) / a]
R=1.2 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
Don Geesaman
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
1960’s: Shell Model and transfer
reactions assumed pure single
particle states.
1970’s: electron scattering
showed only 60% occupancy in
valence single particle states.
1980’s: Understood based on
correlations.
1990’s: Correlations viewed as
universal, approximately nucleus
independent.
2000’s: 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
Don Geesaman
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Another way to look at high momentum
components: x>1 data
 2 and 3 nucleon
correlations
CLAS Egiyan et al.
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
Modern lattice
QCD result
S.R. Beane et al,
PRL 97 (2006)
 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:
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Nucleon-Baryon Interactions
 Λ-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?
 Σ – N and Ξ – 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?
e-d elastic scattering
4
Alexa et al PRL 82, 1374 (99)
Don Geesaman
 
He(e , e' p)
Schiavilla et al PRL 94, 072303 (05)
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Quark Meson Coupling predictions
4
Don Geesaman
Short Distance Behavior in Nuclei
 
He(e , e' p)
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Parton Distributions in Nuclei
Don Geesaman
Short Distance Behavior in Nuclei
Alde et al (Fermilab E772) Phys. Rev. Lett. 64 2479 (1990)
 1984 – Parton distributions are different
EMC effect – nucleon carries smaller
fraction of momentum or changes
structure
Shadowing
 1990 – little change in sea quarks for x>0,1
 2007
– x >1 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
Smith and Miller
 Sufficient uncertainly that CTEQ is
worried about using neutrino data on
Fe to establish nucleon antiquark
distributions.
 MINERva – neutrino A dependence
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Advantages of 120 GeV Main Injector
The (very successful) past:
The future:
Fermilab E866/NuSea
Fermilab E906
 Data in 2009
 1H, 2H, and nuclear targets
 120 GeV proton Beam
 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
Tevatron
800 GeV
Main
Injector
120 GeV
50 x statistics!!
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Can we measure binding energy and spectator
momentum dependence?
 Test technical issue of how to include binding in calculation
SLAC fit to heavy nuclei
(scaled to 3He)
Calculations by Pandharipande
and Benhar for 3He and 4He
Approximate
uncertainties
for 12 GeV
coverage
 Do we see nuclear dependence change for high momentum spectators
which involve short distance interactions- Spectator tagging?
Don Geesaman
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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 β-equilibrated matter in neutron stars
 probe densities to 6 ρNM
 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?
Don Geesaman
Recent observation of high mass
neutron stars
2.1 ± 0.2 M
Nice et al. astro-ph/0508050
2.1 ± 0.28 M, R=13.8 ± 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 massradius 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
xtarget
xbeam
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
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