We smear the hit in local coordinates, that means we take the global coordinates (X,Y,Z in Star Frame) and get the local coordinates in the wafer (xL,yL).
Then we smear them with a gaussian to obtain x'L and y'L (zL' remains the same).
So we have a new point in local frame that we translate into global again.
It appears that for 4 ladders, X = x'L or Y = y'L(according to the localization of the ladder) , ie the global coordinates is the same as the local coordinates.
The transformation (roatation) gives the same coordinates for those hits

These ladders are ladder 1 ,6 ,11 and 16 , exactly located at the top, right, bottom and left part in the ssd barrel.
Figure 1 : with ladders 1,6,11,16 Figure 2 : without ladders 1,6,11,16
So we see clearly that the peak in the previous distribution comes from those ladders, then we have a perfect gaussian distribution.

Jonathan Bouchet
Last modified: Thu Aug 23 17:36:25 CEST 2007