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That's it! If you got to this point, either you already find a perfect alignment or you are ready to restart the full procedure from point 1 of the list at the beginning of this section, for an uncountable number of iterations.

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Note on beam spot integration in Millepede

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• include in the steering lcsim file the information for the beam spot (which you will modify in the following iterations by hand) providing the following parameters in GblOutputDriver: 
  
  <beamspotScatAngle>0.005</beamspotScatAngle>
  <beamspotWidthZ>0.05</beamspotWidthZ>
  <beamspotWidthY>0.2</beamspotWidthY>
  <beamspotTiltZOverY>0.26</beamspotTiltZOverY>
  <beamspotPosition>0.0 -0.11 -0.05</beamspotPosition>
  
  where you will need to modify the coordinates of the beam spot position (and widths)in subsequent iterations. These are the default values introduced for alignment version 4.4 optimized   on 2015 curved tracks. Moreover, you need to activate the inclusion of the beamspot adding the line 
  <addBeamspot>true</addBeamspot>
  By default, the beam spot coordinates are not added and the reconstruction assumes it to be located in (0,0,0). With this add-on the beam spot coordinates and track parameters at the additional 0-layers are added in the out.gbl ascii file, for further processing by gbltst-hps.py.
• include the --beamspot flag upon running the gbltst-hps.py script to extract the "layer 0" information from the gbl ascii file
• activate the implementation of the beam spot when running Millepede: this is done using the flag --BSC (which imposes constraints in pairs between the top and bottom degrees of freedom for the additional zero layers). You can also introduce the --SC flag to constrain parameters within a 50 microns precision (this can be done even without introducing the beam spot). In the floatoptions.py file, or in the list of parameters to be floated you can provide by hand, you can introduce L0t and L0b layers (axial and stereo), with the desired directions to be floated. The Millepede id's for each of the 0-layers are the following:
  • top:
    • u translations: 11198: L0t stereo, 11199: L0t axial
    • v translations: 11298: L0t stereo, 11299: L0t axial
    • w translations: 11398: L0t stereo, 11399: L0t axial
  • bottom:
    • u translations: 21198: L0b axial, 21199: L0b stereo
    • v translations: 21298: L0b axial, 21299: L0b stereo
    • w translations: 21398: L0b axial, 21399: L0b stereo
  Rotations for 0-layers can be dropped as not meaningful.
• Millepede writes in the .res file the offsets found for these new layers, but you cannot produce a new geometry out of them: that is, they cannot be read by the buildCompact.py procedure that produces the new geometry including the MP offsets, which can include information about the physical layers only. Therefore you need to translate the obtained information, which is in the 0-sensor reference system, to the tracking reference system, in which the beam spot coordinates introduced in the .lcsim file are defined. To do this, you should beforehand perform the proper reference system transformation from 0-layers reference system to the tracking one. As an approximation, however, the information from the impact parameters can be exploited. To do this:
  • prepare a new geometry with the modified parameters (all floated layers BUT layer-0)
  • re-run reconstruction with previous beamspot, get the mean value and sigmas of the d₀ and z₀ distributions produced after running the gbltst-hps.py script, and use the mean value between top and bottom (half sum with signs) as new input beam spot coordinates in the lcsim steering file. Remember that in the tracking system <d0> corresponds approximately to beamspotPosition_y, and <z0> to beamspotPosition_z (while beamspotPosition_x=0).
  • introduce the new beam spot coordinates in the .lcsim file and repeat the procedure until a reasonable convergence is found (it may take very long).

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