
\section{Possibility of hadron beams}

The physics program planned for \hd centers around photoproduction
as a source of both conventional and exotic mesons, and emphasizes
the interest of these studies as follow-up to the experimental progress
that has been achieved over the last five years using pion and antiproton
beams at hadron facilities.  The possibility of using high energy
electrons from CEBAF to generate secondary meson beams at some time
in the future has arisen in \hd discussions, however, mainly for 
two reasons.  The first is that the detector that is being planned for
\hd is of a general-purpose design.  At present very little is known
concerning high energy meson photoproduction, aside from the fact that
it is a prolific source of vector mesons and possibly of vector hybrids.
The detector package described in this report is capable within the
first two years of operation of collecting as much data on
photoproduction as presently exists for any production mode, including
pion production.  Thus the possibility is very real that after a
relatively short running period in \hd , there might be reason to
turn again to pion or kaon production methods to examine states that
are suppressed or missing in photoproduction.  The second reason is 
that one of the most successful programs ever carried out in meson
spectroscopy was the LASS experiment, which used a secondary kaon beam
generated using 20 GeV electrons from SLAC to study kaon production.
The fact that the same detector could be used for both kinds of beams,
and that some mention has already appeared of energies even higher than
12 GeV at Jefferson lab, warrants a brief summary of these discussions
in this report.

The LASS experiment took 3-4 kaons per pulse and about 120 pulses
per second during normal operation, for a total of 300-500 kaons/s.
Averaged over the duty cycle of the accelerator, this beam rate was
achieved with an average electron beam current of $ 7 \mu A$ .  They used
rf separation to obtain a nearly pure kaon beam, which required a
beam line of 150m, rather long compared to the mean decay distance
of about 40m for a 10 GeV $ K^-$ .  A beam line of a similar design
but without rf separation would fit into the \hd design, with the
production target just before the electron beam dump and the secondary
beam being bent by a chicane onto the same axis as the photon beam.
The 80m space between the tagger building and the experimental hall
would allow room for one or more gas Cerenkov counters that could
be used in the place of rf separation to tag the different species in
the beam.  Using the same apertures as were used for LASS would give
in excess of 1000 kaons/s and about a factor of 60 more rate in pions
for a $ 10 \mu A $ electron beam.  The secondary particles have a broad
energy spectrum that peaks around half of the electron beam energy.
While these rates are not high compared with what is available at a
hadron machine, the availability of a high-quality detector designed
for this kind of physics makes it worthwhile to carry out the
design of the experiment and beam lines keeping this possibility
in mind.

