Oh Boy, another can of worms to deal with after all the preliminary
results are sent out. :(
Just to remind everyone about something you all already know. :)
the problem with scaler beam would not cause "loss of luminosity" so long
as unpol data are used for benchmark and efficiency folded back in. The
loss is in detection efficiency. If beam current is recored too large as
it is now with scalers, one gets a rate too low, when compared to unpol
with known target density, one gets a number lower than the real
efficiency.But again, two wrongs make a right, ABS intensity is backed
out right, because all it matters is the ratio between event rates from
ABS and unpol, the charge factor cancels out, just like this problem would
not affect asymmetry measurements by too much.
Since we are on this, the 50% figure I reported this morning is for sure
an under estimation. One factor is ofcourse this scaler current
mis-calib, seems beam current is measured 10% too high in scaler stream.
an other is that the ed-elastic cuts excludes all events in the radiative
tail, which can easily be 10-20% (number borrowed from ep elastics) of all
ed-elastic events, so when compare to Monte Carlo projection which does
not radiate, target density is under estimated. the 3rd factor is the very
strict ed-elastic cuts, the priority has been establishing a very pure
sample rather than getting all the counts, this might reject another 10%
of ed events. This missing events in e-right low Q2 is a 5% loss too.
So all in all, I would guess, the efficiency figure would go up to
well above 60%, probably even approaching or exceeding 70%. However, the
actual luminosity with ABS remains 4e31 after "corrected for efficiency".
As a matter of fact, it is the density remains unchanged, lumi will drop
after the over counting in charge is corrected.
Chi
P.S. Is there a way to clean BlastTalk a little bit, the archive is now
filled with titles like "lowest rates in 45 years" which is a bit scary,
making one think the ABS blew all up.
On Wed, 6 Apr 2005, Chris Crawford wrote:
>
> One possible reason for loss of luminosity, is that our scaler current is miscalibrated. See the attached figure. The top line is EPICS beam current, the line just below is the scalers current, and the fuzz is the beam-gated scalers current. I think happened is that it was calibrated at low currents, before we were so optimistic about our beam operating parameters. There was a small nonlinearity in the fit with to quadratic, and now we are extrapolating.
> --Chris
>
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