Silicon Telescope Tracks

The DST stores hits from the telescope detectors, but not tracks, because the alignment might be unreliable when it was written. To access tracks you need:

An alignment file.
Some track finding code.
Code to intersect the tracks with the silicon plane.

If you run with the normal DST reading code the first two steps are performed for you using the routines STALIF and VIFTRK from the the strpge library and strpprog cards file respectively.

The tracks are stored in a common block STTR, ( This is defined in +KEEP,STTR in strpge.car ), which contains:

MAXTR - The maximum number of tracks allowed. Set to 4.
NTR - Integer, the number of tracks found.
TRX(MAXTR) - Real, the x at z=0.
TRY(MAXTR) - Real, the y at z=0.
TRGX(MAXTR) - Real, dx/dz The track gradient.
TRGY(MAXTR) - Real, dy/dz The track gradient.
ITRNH(MAXTR) - Integer, the number of hits on the track.
IHTR(8,MAXTR) - Integer, The number of each hit in the hit bank.
TRCHI2(MAXTR) - Real, the track chi2.

VIFTRK Does not attempt to resolve ambiguities, so if all planes but one have just one hit, and that one has two, two tracks might be found. A loose chi2 cut is used to accept combinations. You may wish to use only the 'best' track in the list, or alternatively require that exactly one track was found.

The alignment file may also contain coordinates for your detector, in which case use:

CALL STGLL2(INUM,ICOD,IXYF,X,Y,GX,GY,XLOC,YLOC)

This will return the local coordinates of the intersection of the track with your detector. INUM, ICOD and IXYF define your detector, and X, Y, GX, GY define the track. XLOC and YLOC are the position in centimetres of the track in your reference frame. This is defined by XLOC = PITCH * STRIP_NUMBER.

The precision should be better than 2 microns in each direction, but of course there may be some tails.

Comments to : Bill Murray