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The high luminosity running of CESR Phase 3 and the superb vertexing,
particle identification, and calorimetry of the proposed CLEO III
detector provide a unique opportunity for the study of two photon reactions.
Studies of collisions at very high energies will only be possible at LEP or
a linear collider, but the two photon physics program of CLEO will be more
than competitive in the medium energy regime. In addition, this program will
complement future investigations at low energy electron-positron colliders
and studies of low energy proton-antiprotron annihilation.
The current two photon physics program at CLEO may be divided into four main
categories, all of which will benefit from the improved luminosity.
Over the past two years CLEO has obtained results for the two photon production
of proton-antiproton pairs[74]. Measurements of this reaction,
when extended to
sufficiently high two photon center of mass energies 
, provide a test of
perturbative QCD. The recent CLEO results extend previous measurements to
values of well beyond 3 GeV. The results show a measured cross section that
is larger than the perturbative QCD prediction by approximately a factor
of five at low .
The data approach the QCD prediction at large where the perturbative
calculation is expected to yield more accurate predictions. A calculation
assuming
the excitation of ``diquark'' intermediate states, on the other hand,
is much more successful at predicting the reaction rate at lower .
More recently CLEO has identified charged pion and kaon pairs at values of
up to 5 GeV[75]. These
results also provide a test of QCD, but because of the smaller number of quarks
involved and the larger values of , the prediction is less model dependent.
With more luminosity the range of these QCD tests will be extended even further
so that the predictions may be challenged more accurately.
The ``diquark'' calculation
also predicts the rates for the production of strange baryon pairs. The large
data sets proposed should allow for successful searches for the production of
, 
, and
pairs. Measurements of the rates for these
reactions should yield considerable information on the ``diquark'' production
mechanism.
The two photon widths of charmonium states provide tests of the charmonium
model and, when measured precisely, also test the QCD correction to the two
photon widths. In the search for charmonium we have found evidence for the two
photon production of the 
through its decay into a photon and a
, and have measured its two photon width[76].
The two photon production of the
has also been detected through its decay into four charged pions.
Although these processes have been observed by earlier experiments,
the two photon width of the 
, nevertheless, remains somewhat
controversial
as all present studies are severely limited by the available statistics.
It will be possible with a much larger data set to settle the question of the
two photon width of the 
.
Two photon production of the 
has been measured through its decay into
, and 
.
The two photon production of the will prove to be an
important laboratory for the study of this resonance. In particular, the
relative branching ratios of the will be measured to a precision
of about 
, far better than at present.
The search for gluonium has started by looking for the production of two photon
produced all-neutral final states. In this search good use is made of the high
resolution CsI electromagnetic calorimeter.
The three tensor resonances 
, 
, and 
are observed at present in both neutral and charged final states.
With increased statistics it will be possible to
measure the resonance parameters to high precision, but more interestingly,
the mixing angle in the tensor nonet will be extracted with a
precision of a few percent.
The superb resolution of the calorimeter has enabled the reconstruction of
several ``all photon'' final states including
and 
.
With increasing luminosity, the unusual line shape of the 
will be resolved well through the decay 
. In
addition, searches for other scalar states will result in very restrictive
upper limits on the two photon widths.
Previous experiments have observed associated production of vector mesons in
two photon collisions. The production rates have evidenced many puzzling
features which have led to speculation that exotic intermediate states are
involved. However, except for the case of 
production, all of the
previous studies have been limited by the available statistics. CLEO III will
be able to measure the production rates and decay spectra very well and set
stringent upper limits on the previously unobserved final
states 
and 
.
The greatest challenge for the upgraded CLEO III
detector will be maintaining an
efficient two photon trigger. With the capabilities of the vertex detector for
rejecting cosmic ray, beam-wall and beam-gas backgrounds, we have the promise
of a high luminosity source of two photon events to probe QCD processes and
study rare or exotic states.
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