E852 Documentation: The MPS Alignment Program

Introduction

The MPS Alignment program, 'mpsAlign' produces MPS wire chamber alignment and calibration parameters and various chamber diagnostic information.

Wire chamber parameters

In addition to parameters store in the 'space.db' file in the 'db-files.Map' map manager file, derived from the 'je852.par' master parameter file, each chamber has additional parameters stored in the 'space.Map' map manager file.

offset - additional offset (it is added to the offset in the 'space.db' file)
exclude001, exclude002, ... - array of 'bad' wires. Positive numbers in this array correspond to wire numbers that have to be ignored. The 'event' program will display such wires as small crosses. Number zero is ignored. Number (-1) is the end of the exclude list.
d0 - see discussion below.
t0 - see discussion below.
driftv - see discussion below.
tdc_min - see discussion below.
tdc_max - see discussion below.
efficiency - chamber efficiency

MPS PWC (beam and TPX) parameters

The TPX chambers have only the 'offset' and 'exclude' entries.

MPS Drift Chamber parameters

These chambers have the 'offset', 'exclude', 'd0', 't0', 'driftv' and 'tdc_min' parameters.

tdc_min is the array of lower TDC thresholds for each wire. The hits with TDC less than the thresholds are ignored. The wires that have a non-zero threshold are displayed as small 'v' (half-crosses) by the 'event' program. This threshold is used to suppress noise on noisy wires without completely turning them off.
t0, d0 and driftv are the drift chamber calibration parameters used for drift distance calculation. The formula used to calculate the drift distance dd given the drift time tdc is:
```
          dd = d0 + driftv * (tdc - t0)
```
The MPS alignment program gives the following meaning to these parameters: 'driftv' is the drift velocity in 'cm' per 'drift chamber clock', the 't0' is the average drift time (calculated from the alignment data), 'd0' is either the average drift distance calculated from alignment data or wire spacing divided by 4, depending on how the calibration was done.

TDX4 parameters

TDX4 uses the 'offset', 'exclude', 'd0', 't0', 'driftv', 'tdc_min' and 'tdc_max' parameters.

tdc_min and tdc_max define the TDC time window used to suppress 'out-of-time' hits, which are seen because of the large dynamic range of the LeCroy 1876 TDCs used for the chamber readout.
t0, d0 and driftv have same meaning as for the MPS Drift Chambers. Also, depending on how the alignment was done, these parameters might be defined as: 'd0' defined to be zero, 't0' is the minimum drift time (the front edge of the TDC distribution). Also note that the drift velocity for TDX4 is negative because the LeCroy 1876 TDCs run in the 'common stop' mode.

Alignment Modes

The MPS Alignment program can operate in different modes:

-F1 (external alignment) - the chambers are aligned using the beam extrapolation from the A5 and A6 beam chambers. This alignment requires working beam chambers and one should be able to see the beam profile in the chambers to be aligned. Before running the alignment program, the chambers must either have to be aligned to about 1 cm by hand by looking at the events in the 'event' program or the -w10 switch should be used. In this mode, the offsets of the chambers can be determined with 0.0100 cm accuracy, the 't0', 'd0' and 'driftv' parameters are also calculated.
The (-F1) mode is usually used to check the alignment of TDX4 with respect to the beam line before running the (-F2) mode.
-F2 (external alignment with TDX4) - the chambers are aligned using the beam track reconstructed from the A5 and A6 beam chambers and TDX4.
Before proceeding with this alignment, the correct MPS angle (angle between the beam line and the MPS magnet center line) has to be entered in the 'je852.par' and 'space.db' files. If an incorrect angle is used, the MPS chambers will be 'scewed' (shifted) and will be inconsistent with the MPS magnetic field map.
Otherwise, The (-F2) mode is identical to the (-F1) mode.
-F3, -F4 (internal alignment) - uses the reconstructed tracks to calculate offsets, t0, d0 and drift velocities. The (-F3) and (-F4) modes provide the final alignment and calibration.
Usually, after the (-F1) and (-F2) alignment is done, the track reconstruction software becomes operational and starts reconstructing tracks. At this point, the alignment process becomes iterative:
1. the analyzer is used to reconstruct tracks, using the current drift parameters
2. the alignment program uses these tracks to calculate improved drift parameters
3. the analyzer reconstructs tracks, using the improved parameters
4. the procedure repeats starting from step 2 unless the person doing alignments is satisfied with the obtained results.
-F5,...,-F8 modes: check the alignment program 'usage' and source code to learn about parameters calculated in these modes.

Usage of the alignment program

See the 'mpsAlign' usage by running 'mpsAlign -h'.

In case of inconsistencies between this document and 'mpsAlign -h', the output of 'mpsAlign -h' should be assumed to be more correct.

Switches are:

-h - print usage
-H - produce a bunch of histograms in the E852_HIST directory. These histograms can be looked at with the 'Hist' program.
-Fm - set the alignment mode (see discussion above)
-rNNNN - load parameters for run NNNN. This switch *must* be set. The alignment program does *not* look at data to determine run numbers.
-Mxxx - process only NNNN events from the input file.
-wNNN - consider only hits within NNN wires from the track.
-Wxxx - ignore all wires except wire xxx.
-pN1,N2,... or -pN1-N2 - align planes number N1,N2,... or planes N1 through N2.
-pX,Y,U,V - align all planes X, Y, U and V. Any combination of planes is allowed. The plane numbers are same as in the 'space' printout.
-f0, -fh - turn magnet off or half field. Default is 'magnet on'.
-fsXXX - set the field scaling factor.
-xNNN - consider only tracks with chi-square less than NNN. By default, no cut is made.
-3 - use beam chambers A3 and A4 instdead of A5 and A6 for beam reconstruction.
-4 - use beam chambers A3, A4, A5 and A6 for beam reconstruction. Will produce incorrect results for data where the Quadrupole magnets were on.

A typical command would be:

     > mpsAlign -r7027 -f0 -p25,40 -F1 run7072.dat -H

Output of the Alignment Program

The alignment program produces the following output:

alignment results in human readable form - on standard output, see also the discussion below.
new offsets, t0, d0 and drift velocities - in the './auto_align.db' file. To use the new parameters, one has to load them into the map manager using the storeMM program. See below on how to do it.
histograms in E852_HIST - these are shared memory histograms produced by the E852 'hist' package (see the 'hist.h' include file for more details). These histograms can be viewed and printed using the 'Hist' program.

Stdout output

This section is supposed to explain the output printed by 'mpsAlign'

storeMM

The alignment program writes the alignment results in the auto.align.db. Tho store these results into the Map Manager, run the following command:

      > storeMM auto.align.db

The typical output of this program looks like this:

loki % storeMM auto.align.db
param.loadDatabase: Loading database 'align-setup'      for run 7060...from the Map Manager
Alignment file auto.align.db, run 7060 would be stored as run 7060
Processing chambers: TDX4A.t.d.v.o.TDX4B.t.d.v.o. done.
loki %

Usually, the 'storeMM' program uses the run number written into the 'auto.align.db' file by the 'mpsAlign' program which takes the run number from it's -rRRRR argument. However, one can save the alignment results into an arbitrary run number by putting the run number as second argument on the 'storeMM' command line, for example:

      > storeMM auto.align.db 7060

histograms

This section is supposed to explain the meaning of various histograms produced by the 'mpsAlign' program.

//CO 09 May 1994
//CO 18 Dec 1994