[Halld-cal] [EXTERNAL] FCAL calibration follow-up

[Shepherd, Matthew]

Igal,

 Because the pi0 fitting (which is of poor quality) is systematically biased by 1 to 2 MeV when he doing the energy dependence correction the pi0 mass is shifted to lower mass and falls within 2-3% around the pi0 PDG mass and stops there.

Please reference:

https://halldweb.jlab.org/DocDB/0043/004387/001/presentation.pdf

The left plot of slide 4 shows that if the average energy for photons used in making the pi0 peak varies, then the pi0 peak will vary by about +/- 1.5%.  The pi0 mass on the left of slide 2 clearly averages over energies in a way that is location dependent.

It happens even with perfect fitting and in the absence of systematic bias.  It is a pure byproduct of energy non-linearity and variation in average energy of photons from pi0 decays.

Matt

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