Final analysis

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87.41 +/- 0.12 (stat) +/- 1.51 (sys) %


88.92 +/- 0.04 (stat) +/- 1.12 (sys) %


Mindy's HAPLOG post:

Final result: 87.41 +/- 0.12 (stat) +/- 1.51 (sys) % (??)

chi^2/NDoF = 1.09 for 13 points over the experiment

Systematic Error

Rel Uncer. (%)
Laser Pol. 0.07
Gain shift 1.3 email from Megan Friend, see below
Collimator Pos. 0.02
Nonlinearity 0.3

Systematic Error Notes

  • From Megan over gain shift:
I don't think I'm going to be able to get a number for the gain shift
during PREX, although it should scale linearly with signal-background
size. Using the 1% gain shift we saw during HAPPEX (signal+bkg to
background = 122e6 to 53e6), that would give us a 0.8% gain shift for
PREX (signal+bkg to background = 48e6 to 30e6 at 1kHz * 3.3 to convert
to 30Hz). This would give us a change in the final polarization number
of 1.3%, if it's folded into the background subtraction ( which comes
from comparing a background subtraction of
(48e6-30e6*(1.008))/(48e6-30e6) ). So I'd say the possible gain shift
gives you a 1.3% systematic error.
  • From Megan over radiative correction:
I just ran through the PREX data to add in a radiative correction, and I
get that this increases the analyzing power by 0.3% (from 0.01828903 to
0.01834393). This is consistent with what I've seen, so I think it's
safe to add that in there.

There is an increase in analyzing power of 0.3%, which corresponds to a decrease in beam polarization by 0.3%. Therefore, the final analyzing power should multiply by (1+0.003) due to A_real = A_exp * (1+0.3%)

w/o the radiative correction:

analyzing power = 0.01824526 +- 0.00002583 (sta.) +- 0.000279099 (sys.)

w/ the radiative correction:

analyzing power = 0.018299996 +- 0.00002583 (sta.) +- 0.000279099 (sys.)


Final result weighted average: 88.92 +/- 0.04 (stat) +/- 1.12 (sys) %

(not sure if the systematic is relative or absolute)

The first measurement of the set is much lower than the others

Systematic Error

Sasha's report at the Jan 2011 collaboration meeting

Uncer. (%)
Fe Pol. 0.25
Targ Discrep. 0.5
Targ Saturation 0.3
Analyzing power 0.3
Levchuk 0.5
Targ temp 0.02
Dead time 0.3
Background 0.3
Others 0.5
Current diff 0.3
TOTAL 1.12

Finite Acceptance

The radiative tail leads to an effective drop in the measured Q^2 by 0.68%



See Kiad's presentation

The acceptance for the first excited state of Pb is <0.1%. It's expected to have an asymmetry ~1.3 of the elastic asymmetry so it is totally negligable.

The first excited state of carbon is outside our acceptance and does not contribute


In progress


In progress