The tagged-photon energy comes from the E-counters of the CLAS photon tagger. There are known inaccuracies in the tagger energy found in the
focal plane. This effect can be studied using inclusive gp -> p pi+pi- events with kinematic fitting. The three charged
particles are detected and kinematically fit to the missing incoming photon. The events that pass a 10% confidence level cut were used to find the
difference between the measured and the kinematically fit photon energy.
The corrections are binned in tagger E-counters and from each bin a Gaussian mean is extracted. Here a generic correction is applied that does not depend on the run number to obtain the beam offset. For an event only the photon energy and E-counter number are required.
This correction has been previously derived for g11a with additional studies on the need for the correction due to the
sagging of the focal plane between its four support yokes provided not seen for g12 from the plots below. Much better than g11, g12 still sees a shift. The sag shifts the narrow
E-counters from the set locations, which can lead to detecting electrons with slightly distorted energies. The Gaussian mean over the full range is used to apply
a systematic correction for each E-counter. A correction can still be applied to these E-counters so no E-counters are excluded.