It will also be possible to use the FPD to determine the luminosity
on a run-by-run basis by counting elastic events and using previously
measured values for 
, 
, and 
, or values measured
by DØ in the special run described above. The accuracy will be much
better in the latter case.
For |t|> 0.5 GeV
the effective elastic cross section is
0.8 
b (applying a |t| acceptance factor of 0.0002 and a 
acceptance of 20%). For a typical luminosity of 
cm
,
the single interaction elastic
rate will be about 10 Hz.
A special elastic scattering data stream which writes out selected
detector information at about 1 Hz will yield 10,000 elastic events per run.
This will give a 1% statistical uncertainty, so a few
percent measurement on a run-by-run basis will likely be obtainable.
These events would also be used for FPD calibration.
The background to elastics will be the overlap of
one p and one 
halo event, which will have a rate of
Hz where 1.7 Mhz is the crossing rate and 0.0008
is the halo suppression factor from Sec. 5.3.2. A cut on the
timing of the
trigger scintillators in the two arms will reduce this background to a
few percent of the elastic rate.