Report to the TWIST collaboration

10 August 2000

Alignment Study Group

This brief report will focus on first on the November 2000 run.


For November 2000
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Goals:
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The principal alignment goal of the November run will be to acquire and
analyze sufficient data to permit us to estimate our ultimate likely
alignment precision. 

A secondary goal will be to test the alignment of the detector stack
with the magnetic field using tracking in the detector planes.

Methods:
-------

To achieve the principal alignment goal, we will require an undetermined
number of positron tracks from muon decays occurring in the target. 
Estimates of the number of tracks required will need to be made with
Monte Carlo prior to the run.  By fitting tracks to the observed DC hits
and allowing the location of the detector elements (position and
orientation of the wires in the planes) to vary, we can find an optimum
set of offset quantities to represent the displacement or rotation of
the individual wires from their nominal position.  These offsets will be
stored in the calibration files and be available to the analysis codes
through the MOFIA data structures.

It is assumed that there will be some nominal values for the wire
positions and orientations which will be entered into this calibration
file.  Exactly how these nominal values will be obtained is not clear
and needs to be articulated.

[JOB: We need an outline of how the nominal wire positions are obtained,
i.e., from what measurements and what assumptions are these nominal
values obtained?]

[JOB: We need an outline of the specific coordinate system(s) we will be
using and specifically what the corrections are that will be measured. 
Much of this is already assumed in the MOFIA data structures. However,
it might be good to have that stated here.]

Needs:
-----

To accomplish the above goals, we will need the following:

o A working helix tracking code.  

  -- STATUS: [JOB: We should have a status report here on what is done,
what is needed to be done and by what time, etc.]

o A working alignment code.

  -- STATUS: [JOB: Maher has said that he has an alignment code which
was used to analyze the data taken in the 1998 run.  This code needs to
be updated to F90 and to be appropriately linked to the present data
structures, and then tested extensively with MC.  Perhaps Maher can give
some status report here...  Paul Nord has offered to do the update.]

o Monte Carlo testing.

  -- STATUS: We have the GEANT Monte Carlo which will need to be
configured for the minimal configuration for the November run.  The
Monte Carlo will need to be run and these generated date will need to be
analyzed with the tracking code and with the alignment code. The Monte
Carlo will need to be run with non-ideal geometries and used to
determine whether the alignment codes can discover the mis-alignments
which are used to run the Monte Carlo.  

o Statistics estimation.

  -- STATUS: When the tracking codes and the alignment codes are ready
and they are tested initially, we will need to estimate how many tracks
we will need to estimate the sensitivity of the alignment process.  This
may mean generating rather large Monte Carlo runs and we will need the
horsepower to do that.

What we will NOT be doing in November:

It appears that given the realities of time and scheduling, we will not
anticipate doing the following before or during the November running period:

o Cosmic ray alignment studies.

  -- STATUS: It appears unlikely that the detector assembly will be in
place and fully configured prior to the November beam run to be able to
take advantage of the use of cosmic rays for alignment studies.  We do
not anticipate that a cosmic ray trigger will be established prior to
the November beam run.  We do, however, anticipate setting up a cosmic
ray trigger after the November run and using what time is available to
explore the use of cosmic rays for alignment studies.

o High energy, large incidence angle, no B-field beam tracks

  -- STATUS: This exercise, if and when it may be done, is intended to
explore absolute alignments along the z-direction with tracks which have
minimal multiple scattering and which make rather larger angles with the
normal to the detector planes.  This will not be possible in M13 with
the magnet in place.  Whether this is done in M11, for example, at a
later date is still an open question.

o CCD camera alignment studies

  STATUS: [I would appreciate it if someone could write a note here...]

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