CEBAF is now a working accelerator. Design goals for energy and average current
have been met and exceeded. An average multiplicity of slightly less than 2
halls receives beam when the accelerator is running. The energy of the machine
is now typically 4.0 GeV beams and 5.0 GeV beams have been run. New
developments in polarized sources have made polarized beams commonplace. This
summer, experiments using a high polarization strained GaAs source are underway.
A few accelerator cavities have received helium processing and the average
increase in gain is significant. Standard running at 6.0 GeV is planned for
1999.
Thus, it is clear the lab is ready for realistic discussions about an upgrade in
energy to 8 GeV or higher. This workshop was meant to provide a venue for these
discussions and these proceedings summarize those presentations and
deliberations. The existing halls have each gone through design exercises and
have started to plot a path to future facilities compatible with the higher
energy. In addition, a collaboration has been formed to build a new Hall D with
a focus on meson photoproduction. A number of talks on new detector, target,
and beam technologies presented new possibilities for the upgrade. Finally, the
physics justification for a higher energy accelerator was reexamined. To keep a
focus for the workshop, most of the plenary talks were aimed at experiments to
be done with up to about 12 GeV beam energy.
In broad view, the physics justification is very similar to what is contained in
the 1994 workshop proceedings. However, the issues are better defined. This is
particularly true for the meson spectrum. While exotics were anticipated
previously, papers have now been published by Brookhaven experiment E852 in
Physical Review Letters that present partial wave solutions that show stronger
evidence for hybrid mesons. In the plenary sessions, review talks on the
overall status of the meson spectrum (Curtis Meyer) and the status of searches
for exotics (Eric Swanson) were presented. Based on new results from LEAR and
elsewhere, significant additions have been made to the spectrum of scaler and
tensor states, but much work remains. This provides a basis for the
justification for a new hall with a new detector to study the meson spectrum.
In addition, many new results from experiments at higher energy have been
obtained at CERN, DESY, and SLAC, giving a clearer picture of the successes
achieved and the challenges raised in trying to understand the perturbative
picture of the proton. Recent results from Hermes for inclusive and
semi-exclusive processes were given by Naomi Makins. The role of the sea quarks
in the proton is much more clear from results at low x. In another talk,
Jianwei Qui gave a theoretical basis for what can be learned about parton
correlation functions at high x using a higher energy CEBAF. This promises to
be an exciting direction for the lab.
Much excitement was generated by a talk by Tony Thomas on the nuclear medium
effects on bound nucleons and on mesons -such as Rho, psi, and phi produced in
the medium. It is possible to formulate the problem using the quark model for
the nucleons and using mesons to provide a mean field for the particles of
interest. Significant effects are predicted at CEBAF energies.
The fourth physics example in this short overview is exclusive reactions.
Plenary talks by Carl Carlson, Jean-Marc Laget, and Anatoly Radyushkin covered
this subject from various directions. Experiments with roughly 10 GeV beams
study the region where constituent quark models are invalid; yet, the
applicability of perturbative QCD for these processes has not yet been
established. New structure functions (called Off-Forward Parton Distributions
or OFPD's) for describing the OFPD's and their application. These structure
functions allow a better formulation of the issues and give a definite
prediction for scaling behavior.
Four plenary speakers provided summaries of detector technologies available for
use today and glimpses of what we might imagine using in upgraded or new
detector packages. Peter Weilhammer presented a "State of the Art" summary,
setting the stage for in depth reports from other speakers. His own follow-up
report on solid-state detectors described not just precision tracking techniques
but a highly segmented silicon-based photon sensor with the potential for
simplifying ring-imaging particle identification. Within his presentation on
particle ID, Jerry Va'Vra provided a detailed look at experiences at SLAC
building and using hte "Cerenkov Ring Imaging Detector" (CRID), and lessons fro
construction of the "Detector of Internally Reflected Cerenkov light" (DIRC).
David Winn overwhelmed us with a broad spectrum of possibilities for calorimetry
at high rates, (unfortunately, no manuscript is available at the time of
printing). Howard Wieman provided a thorough survey of exciting developments in
the field of gas chambers, and a close look at what one can achieve by using
integrated silicon detectors and electronics to sense the ionization in a time
projection chamber. Clearly ther are many exciting detector developments, some
available now and some needing more effort, which can be used to coax the best
physics from a future CEBAF.
Each existing hall provided an overview plenary talk and conducted parallel
sessions on specific projects. Hall A will continue to use the existing
spectrometers, but add new detectors to use their high resolution in a broader
class of reactions. They suggested scintillator hodoscopes for hadron knockout
studies, a photon calorimeter for real and virtual Compton scattering, and a
short high resolution spectrometer for hypernuclei studies. Hall B would extend
the already large solid angle of CLAS to generate higher acceptance for the more
complicated final states that will be seen with higher energy beams. New
calorimter blocks would cover the 8 degree gaps in phi acceptance where to
toroid and its cryostat sit. They also suggest that the inner tracking chamber
be replaced by a thinner tracking device and particle identification detectors.
These would extend the acceptance at lower cost than the placement of detectors
at the perphery. The main Hall C recommendation is for a new super-HMS that
would become the high momentum spectrometer at higher energies. This
spectrometer would achieve a minimum scattering angle of 6 degrees with slim
quadrupoles. It would have moderate resolution and large momentum acceptance to
match the present HMS and enable extension of the present class of experiments
to hgher beam energies.
A new hall is presently envisaged for new hgih mas (1-3 GeV) meson studies. The
present design for the hall uses it solely for real photon beams and contains a
'hermetic' spectrometer with complete coverage for both neutrals and charged
particles. A straw man design for the spectrometer was presented by Alex
Dzierba and discussed in detail in a parallel session at the workshop. A
collaboration has been formed and the design process is well underway.
Thus, the reader is encouraged to examine the papers in this volume. We hope it
will be one of the stepping-stones on the way to the next generation of
Jefferson Lab.
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