Impulse Approximation limitations to the e,ep reaction on 208Pb

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E06-007, March 3 March 25, 2007
Impulse Approximation limitations to the (e,ep)
reaction on 208Pb Identifying correlations and
relativistic effects in the nuclear medium
K. Aniol, A. Saha, J. M. Udías, G. Urciuoli
Spokepersons
Students Juan Carlos Cornejo, Joaquin Lopez, ?
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Outline

• What will be done?
• Physics Motivation
• Experimental Preparations
• Invitation for students and Post Docs
• Call for shift workers and run coordinators

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What will be done? 208Pb and 209Bi(Kin04)
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What will be done? 208Pb and 209Bi(Kin04)
Investigate states near the Fermi surface
207Tl Ex Jp config 0. 1/2
3s1/2 0.351 3/2 2d3/2 1.348 11/2-
1h11/2 1.683 5/2 2d5/2 3.474 7/2 1g7/2
209Bi Ex Jp config 0 9/2- 1h9/2
Based on s-FACTORs, the 4 lowest levels are
predominantly single-proton hole states 3s½,
2d3/2, 1h11/2, and 2d5/2, and the 3474 level is
predominantly the 1g7/2 single-proton hole state.
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Physics Motivation
How well do we understand nuclear structure
? The underlying physical picture Dense system
of fermions whose motions to first order can be
treated as independent particles moving in a mean
field Electromagnetic interactions Best
probes for investigating the validity of the
independent particle picture because they are
sensitive to a large fraction of the nuclear
volume Realistic microscopic calculations of the
spectroscopic factors are not possible for
medium-heavy (Agt20) nuclei. Approximations are
made that build on top of the shell model (mean
field or independent particle picture) that is
still the basic building block upon which most
calculations of complex nuclei rely
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Physics Motivation
Deviations from independent particle motion for
orbits near the Fermi surface are attributed to
effects beyond mean field (correlations) which
reveal their presence in two ways (i) Changes
in the occupation and spectroscopic factors with
respect to mean-field predictions (kin01-kin07, 0
to 300 MeV/c in pmiss ) (ii) Changes in the
momentum distribution of particles, especially at
high momentum and binding energies (kin08-kin11,
400 to 500 MeV/c in pmiss) V.R. Pandharipande,
I. Sick and P.K.A. deWitt Huberts,
Independentparticle motion and correlations in
fermions systems, Reviews of Modern Physics 69
(981) 1997
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Physics Motivation

• The (e,ep) reaction at quasielastic kinematics
  and in exclusive conditions, for the outermost
  shells, becomes one of the most powerful and
  cleanest test of the mean field and the
  correlations needed to supplement it
• 208Pb is the most suitable candidate to employ
  the mean field prediction, and thus it has been
  measured in the past in order to measure
  spectroscopic factors
• The reaction has been measured in the past,
  mainly in parallel kinematics and for moderate
  values of Q2. Spectroscopic factors have been
  measured

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Still open issues (i) possible dependence on Q2
of the spectroscopic factors?
data 4, theory 5.
12C(e,ep) data over wide range of Q2. There
appears to be a Q2 dependence to the
spectroscopic factors observed in this
reaction2. This interpretation has been
disputed and the Q2 dependence attributed to
analysis methods of SRC3 208Pb(e,e'p) has been
studied in the past at low momentum transfers and
spectroscopic factors in the range of 0.6 to 0.7
have been reliably extracted from parallel
kinematics 4,5,6 at low Q2 A measurement at
several high values of Q2 will directly address
the question of momentum transfer dependence of
the spectroscopic factors
Kin01, kin12, kin13
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• Shell model (mean field) calculations the shape
  of the cross-section is well described, but the
  measured spectroscopic factors are below the mean
  field prediction. How large/small must be the
  spectrocopic factors?

About 30 depletion is observed for states near
the Fermi level. This cannot be explained only
with short-range correlations Shape at moderate
pm and parallel kinematics is well
understood Long range correlations are predicted
to be visible at large pm
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Open issues (ii) Long range correlations and
cross-sections at high pm (gt 300 MeV/c)
Excess strength at high pmiss
xB 0.18
E. Quint, thesis, 1988, NIKHEF
I. Bobeldijk et al., PRL 73 (2684)1994
J. M. Udias et al. PRC 48(2731) 1994
J.M. Udias et al. PRC 51(3246) 1996
If long range correlations are the reason for
the small spectroscopic factors, then they should
produce a large effect at high missing momentum.
An experiment was performed at NIKHEF-K to
measure the large momentum region, but the
kinematics was far from XB1. Additional strength
was indeed found, but this can be explained
either via long-range correlations Bobeldijk,6
or by relativistic effects in the mean field
model Udias,7.
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open issues (iii) Are there any other signatures
of relativistic dynamics in nuclei besides the
spin orbit force?
It seems that TL-observables (that change sign
when the cross-section is measured when the
knocked out proton is at both sides of the
transferred momentum) are sensitive to dynamical
relativistic effects A measurement at both sides
of q will directly measure ATL and address the
possible dynamical relativisitc effects. Yet such
a measurement for 208Pb has never been attempted
kin01-kin07, for 0ltpmisslt300 MeV/c nucleon wave
functions dominated by the mean field
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Pb C spectra, Kin01, pmiss 0 MeV/c Ee
2.649 GeV
GEANT simulation
Experimental Preparations
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Emiss for Pb C, Pmiss 200 MeV/c kin04
GEANT simulation
Experimental Preparations
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Experimental Preparations
Emiss, PbC Pmiss -400 MeV/c kin09
GEANT simulation
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Experimental Preparations
Determine spectroscopic factors from the low pm
kinematics (kin01 to kin07). Expected data were
extracted with the peak fitting procedure after
averaging the theory over the acceptance with a
detailed GEANT simulation
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Experimental Preparations
Non relativistic dynamics
Relativistic dynamics
ATL for all the shells added up, compared with
the acceptance averaged simulations
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Look at large pmiss for correlations.
xB 1
With correlations
We compare to theoretical predictions after
acceptance averaging. We do not attempt to
separate the five shells in this region nor to
compute ATL. If cross-sections are higher than
the mean field prediction, the improved
statistics might eventually allow for shell
separation and ATL measurement
Without correlations
(II) Mahaux and Sartor (SF0,6) (I) Ma and
Wambach (SF0,7)
208Pb(e,ep)207Tl(all 5 shells)
Long range correlations are needed to explain the
30 depletion. If correlations are excluded,
neither Nonrel. or Rel. theory can produce large
strength at high pmiss. This is in contrast to
the ambiguous signature at xB ltlt 1 of former
experiment at NIKHEF-K
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Experimental Preparations
Target Issues
Cold Lead in diamond sandwich E03-011 has
demonstrated that a 0.5 mm lead foil sandwiched
between two 0.15 mm diamond foils at cryogenic
temperatures can withstand currents up to 80 uA
and calculations indicate that they can operate
at 100 µ A. Thinner (0.2mm) lead foil is not a
problem
Direct cooling of target block using cryogen flow
from loop2.
Target block contains 2 Pb targets and a Bi
target.
Will the diamond structure survive weeks of
bombardment? High thermal conductivity is due to
diamond structure.
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Invitation for students and Post Docs
At present we have two students, J. Cornejo
(CSULA) who will get a MSc thesis from this
experiment and Joaquin Lopez (U. Madrid) who will
include part of the experiment in a PhD thesis.
We invite collaboration members to suggest PhD
students for this experiment.
A post doc associated with the analysis is being
sought. Any volunteers?
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Call for shift workers, run coordinators
http//curriculum.calstatela.edu/faculty/djm/E0600
7.html
Requesting a 5 shift minimum or comparable
contribution to be listed as an author on
publications.
Cryotarget operators will be needed because
cryogen will be drawn from the ESR.