ACD Recon update to handling gaps in the ACD

This page describes how we handle gaps in the ACD

We greatly improved the way we handle gaps in the ACD with the new AcdRecon stuff.  Previously we had to rely on propagated intersections with the GEANT model, with no tolerance for near misses.  To find how close tracks came to gaps we:

1. Tried to see if a track hit any ribbons in the GEANT model, if it did we used that to calculate the gap, with the active distance defined as the ribbon active distance.  Otherwise,
2. Tried to see if a track hit any tiles in the GEANT model, if it did we used that to calculate the gap, with the active distance defined as the negative of the tile active distance.  Otherwise,
3. Used the idealized positions of the gaps in the sides. 

 Gaps are defined as the spaces between the tiles

Since the tiles are very efficient what we are really concerned with in the chance that a track missed all the tiles.  With the new pattern recognition we know all the tiles that a track comes close to.  Therefore we can use only the tiles.  This in nice because it puts everything on an equal footing.

 The size of the gap is important

 The orientation of the gap is important