Alignment

Technical Definition

This section explains how the tracker alignment is defined.

• There are three places that alignment transforms are inserted
  • between the whole tracker and the nominal geometry
  • between each plane and the aligned tracker
  • between each panel and the aligned plane
• transforms
  • apply rotation before displacement
  • a rotation is defined as right-handed about the local x, then y, then z axes (rad)
  • add an offset in position (mm)
• tracker geometry object are in coordinate centered at the nominal tracker center

Here is how the geometry is conceptually built up, including the alignment

1. create straws within the panel
   • straw direction is +y
   • straws are placed around the +x axis with the longer (innermost) straw near x=0
   • the origin is the average position of the two innermost straws
   • straw z is plus or minus half the layer spacing
2. Apply the panel alignment of rotations and displacements. The coordinates are these current local coordinates. For example, to move a panel to a larger radius in the final detector, apply a +x alignment.
3. Place the panel in the plane. This involves rotating the panel to its nominal phi position around the z axis, and adding its nominal displacement of about x += 380*cos(phi) and y += 380*sin(phi)
4. Apply the plane alignment of rotations and displacements. The coordinates are the center of the plane. For example, to move a plane vertically up in the final detector, apply a +y alignment.
5. Place the plane within the tracker. This only involves moving it along the z axis.
6. Place the tracker by applying the tracker alignment. The coordinates are all the same as the final detector. To move it closer to the calorimeter, apply a +z alignment. To spin it about its axes apply a z roation.