Z disk location accuracy

Propagation of the Z hit location inaccuracy  on to the error of predicted Rxy location of the Endcap EM shower.

Slide 1.  Every FGT disk is mounted only at  only 2 points to the Alu rail r=6mm, disks have R=38 cm, so the leverage arm is 1:63.
The tension from individual (stiff) cables located also at 90 deg vs. Alu rods  could cause a tilt of disks ( during cables assembly or sliding the 6-disk unit to its final position at STAR).
Slide 2.  An example of FGT-only reconstructed track. The inset shows there is an intrinsic fluctuation of primary ionization over 3mm in Z, we can't compensate off-line. This sets the scale for accuracy Z-alignment of the disks.
Slide 3  shows  systematical displacement of predicted EM shower radial distance at the Endcap due to extreme ionization spread in 2 FGT disks . In reconstruction the mean Z location of FGT disks is used. The predicted radial distance of the reco tracks at Z=SMD  can be off by up to 1.2cm.
This is not that small  but since we will impose isolation cut in EEMC, there should be no other significant energy deposit in calorimeter at much larger radii. Note, the EM shower radii is of 3 cm, but we claim its center will be reco with 1mm accuracy.