Report to the TWIST collaboration

10 August 2000

Alignment Study Group

This brief report will focus on the November 2000 run.

Goals:
-----

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.  This will include both the temporal alignment of
the drift chambers and the spatial alignment of all available wire chambers.

A secondary goal will be to test the alignment of the detector stack
with respect to 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.  It is further assumed that these wire positions will be entered
based on wire position measured with the traveling microscope, assuming
that the central wire is centered in the detector.  It is from these
nominal positions that the wire offsets will be measured.  The relative
z-positions will be based on the cital spacer sizes and used in the
database.  Measurements of the deviations from these values will be a
challenge for the alignment procedure.

In this context, the alignment study group has had several discussions
regarding alignment determinations, especially regarding the measurement
of the z-position of the wire planes.  A vexing question remains as to
how best to detect a systematic mis-alignment in the z-position of the
chambers.  The matter is not settled.

Needs:
-----

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

o A working straight-line and helix tracking tracking code.  

-- STATUS: The development and testing of the Kalman filtering algorithm
is proceeding and it is expected that a tested version of this code will
be ready for the November run for use with magnetic field-off,
straight-line tracks.  The availability of a helix tracking code based
on the Kalman filter will probably not be available before the November
run.  An alternative helix tracking code may be available from
Valparaiso University by the November run.

o A working alignment code.

  -- STATUS: Maher has said that he has an alignment code which
was used to analyze the data taken in August 1997.  This code needs to
be updated to F90 and to be appropriately linked to the present data
structures, and then tested extensively with MC  Paul Nord has offered
to do the update.  These codes have been used to provide the temporal
alignment of the drift chambers as well as the determination of the
spatial alignments from fitting straight tracks.  A copy of the report
(excluding the plots) can be found at http://stoney.phys.ualberta.ca/~e614//Projects/E614TN/00046/index.html.

o Monte Carlo testing.

  -- STATUS: We have the GEANT Monte Carlo which will need to have the
geometry file edited to represent the minimal detector configuration for
the November run.  This Monte Carlo will need to be run and the "data"
generated  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, we will need to estimate how many tracks will be
needed  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. (Some of this may be available at Valparaiso University.)

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:

The CCD alignment system is intended to check the mechanical alignment of
the detector cradle with respect to the solenoid at regular intervals. The
locations of 5 "targets" mounted on the cradle (two at the upstream end
to give X-Y, two at the downstream end to give X-Y and one to give Z) will
be determined by CCD cameras through holes in the magnet yoke. The X-Y
cameras will be mounted outside the upstream yoke endplate, and the Z
camera will view the end of the cradle through a hole in  the yoke
side-plate. The optimum type of "target" (LED, laser illuminated optical
fiber or white paint spot locally illuminated) is currently being
studied. The CCD cameras will be equipped with telephoto lenses to give an
appropriate magnification. Since the holes in the yoke will not be drilled
this year, this system will not be available for the Nov/Dec run.

---------------------------------------------
donald.koetke@valpo.edu
http://www.physics.valpo.edu/faculty/dkoetke/

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