Progress Report on the wN and fN Total Cross Sections

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Progress Report on the wN and fN Total Cross
Sections

• M. H. Wood
• University of Massachusetts, Amherst

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Motivation
A standard method of studying vector mesons is
through photoproduction (or electroproduction)
where the incident photon transforms into the
vector meson which in turn interacts with a
target nucleon. In most cases, the target is 1H
or 2H to avoid final-state interactions of the
outgoing vector meson.

• Another method is VM production off of heavy
  nuclei. Due to FSI, the VM may interact with the
  other nucleons and be absorbed.
• Bauer, Spital, Yennie, and Pipkin, Rev. Mod
  Phys., 50, 261 (1978)
• By plotting the nuclear forward production s vs
  A, one can determine
• Normalization gt single nucleon forward
  photoproduction cross sections.
• Shape or A-dependence gt the forward V-N
  scattering amplitude fVN

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Application to Medium Modifications
From fwN, the collisional medium effects can be
determined (A. Sibirtsev, Ch. Elster, and J.
Speth, nucl-th/0203044v1)
Using an optical model
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Photoproduction of Vector Mesons off of Nuclei
(g7 Experiment)

• JLab Experiment E01-112
• Spokespersons C. Djalali (USC), M. Kossov
  (ITEP),
• D. Weygand (Jlab)
• Photon beam (VDM, minimal disturbance to initial
  sate)
• Eg up to 3.8 GeV
• Targets LH2, LD2, C, Ti, Fe and Pb
• Leptonic decay
• Almost no final state interaction!
• Low branching ratio4.49 10-5
• Rare decay need to produce
• 100,000 rs to have 5 ee- decays.

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Mass Spectra by Target
Since the w and f mesons are narrow resonances,
they are expected to escape the nucleus before
decaying. Thus, observed ws and fs in g7a
should not show signs of medium
modifications. These plots due show a depletion
as the target A increases. The conclusion is
absorption in the nucleus. The VM yields will be
extracted by fitting the mass spectra.
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Simulations - BUU Transport Model

• Monte-Carlo simulations based on Giessen code
  using the BUU transport equations Mosel et al.
  Nucl. Phys. A671, 503 (2000)
• The code includes various decay channels and
  nuclear effects, and was coupled with the CLAS
  detector simulation software (GSIM)
• Generates 7 channels ee- decays of the ?????
  and ? Dalitz decays of the ?0, ?, ? and ?.
• Includes conventional medium effects such as
  Pauli blocking, shadowing for photon induced
  reactions, Fermi motion of nucleons, collisional
  broadening (targets other than proton).
• Mass shift added on demand according to the
  Hatsuda-Lee formula.

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Extracting the w and f Yields
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Yields vs. Target A
Below are preliminary plots of extracted yield
versus A, the number of nucleons in the target
nucleus.
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In the f-meson distribution, the 208Pb point
seems inconsistent. Since the f sits on the
tail of the r, a local fit was made using a
gaussian shape on top of an exponential
background. The yields were extracted and
compared with the results of the global fit.
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Comparison of Global to Local Fits for f Peak
This study will be used to estimate the
systematic error on the yield extraction.
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Ratio of Cross Sections
Yyield Nnumber of target nucleons eacceptance
A simple fit to a power of A RcAb for w,
b-0.33 for f, b-0.28
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Future Work
The g7a analysis has matured to the point where
the w and f yields can be extracted reliably.
What are the next steps?

• Evaluate and apply the proper systematic errors.
• Fit the distributions to a realistic model to
  extract fwN.
• Write a paper incorporating this with other g7a
  related work.

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Combinatorial Background
The combinatorial background is the random
combination of pairs ( ee-, e-e-, and ee) due
to the uncorrelated sources.
Assume N, and N- obey Poisson statistics
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The Extracted r Mass