Report on the wN and fN Total Cross Sections

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Report on the wN and fN Total Cross Sections

• M. H. Wood
• University of Massachusetts, Amherst
• for the CLAS Collaboration

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Motivation
By VMD, vector mesons are studied through photo-
or electro-production. The target of choice is
1H or 2H to avoid final-state interactions of the
outgoing vector meson.

• Another approach 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)
w prod. threshold
Using an optical model
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Photoproduction of Vector Mesons off of Nuclei
(g7 Experiment)

• JLab Experiment E01-112 in Hall B with CLAS
  detector
• Search for Medium Modifications of the r Meson
• A(g, ee-)A' reactions by identifying the
  coincident electron pairs in the CLAS
• Targets LH2, LD2, C, Ti, Fe and Pb
• CLAS e/p separation
• Single lepton 10-3,Lepton Pair 10-6
• Leptonic decay
• Almost no final state interaction!
• Low branching ratio10-5.

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ee- Invariant Mass 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 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)
• Generates ee- decays of the ????? and ? Dalitz
  decays of the ?0, ?, ? and ? , and can be coupled
  with the CLAS detector simulation software (GSIM)
• Includes conventional medium effects such as
  Pauli blocking, shadowing for photon induced
  reactions, Fermi motion of nucleons, collisional
  broadening (targets other than proton).

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Extracting the w and f Yields
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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 analysis has matured to the point where the w
and f yields can be extracted reliably.
Preliminary fits of A-1/3 are consistent with VM
interacting throughout the volume of the
nucleus. What are the next steps?

• Evaluate and apply the proper systematic errors.
• Fit the distributions to a realistic model to
  extract sVN and fVN.
• Get a permanent job.