Tracking Parameters

The parameter set for HKS DC tracking comprises geometrical constants, tracking criterion and other wire chamber constants. They are needed in the tracking routine to convert raw wire hit into coordinates and fitting the tracks.

Each wire plane of the chamber is described by the following constants:

1. Plane number: From 1 to 12.

2. hdc_zpos: The coordinate in the particle traveling direction.

3. hdc_beta, hdc_gamma and hdc_alpha: These are the three rotation angles of wire plane about the Y axis, X axis and Z axis.

4. hdc_nrwire: number of wires in plane.

5. hdc_central_wire: The fictitious wire number of the middle wire. It is used in calculation of the wire center position:

   " hdc_wire_center(goodhit) = hdc_pitch(plane)
        &                  * ( (hdc_nrwire(plane) + (1 - wire))
        &                  - hdc_central_wire(plane) ) - hdc_center(plane)"
   

6. hdc_wire_counting: A flag that indicates whether wire is counted along positive (-1) or negative (+1) x direction. For HKS DC, all wires are counted from the positive X side. So it is +1 for all the planes.

7. hdc_center: offsets of the planes along local coordinates.

8. hdc_length_x and hdc_length_y: The dimensions of the DC active area in x and y directions.

9. sigma: the Gaussian width of the tracking fitting residues

Currently, the parameter values used in the tracking are:

Table 3: Tracking parameters

Plane	No	Zpos	$\alpha$	nrwire	Central	sigma
		(cm)	(degree)		Wire	(cm)
1u1	1	-1.905	30	87	43.75	0.02
1u2	2	-1.270	30	87	44.25	0.02
1x1	3	-0.635	90	122	61.75	0.02
1x2	4	+0.3175	90	122	61.25	0.02
1v1	5	+0.9525	150	87	43.75	0.02
1v2	6	+1.5875	150	87	44.25	0.02

The $\beta$, $\gamma$ angles are 0 degree for all planes. hdc_center are 0 now. The wire plane Z positions in the table are relative positions within one chamber. Assuming a 1 meter seperation between the 2 chambers after they are mounted on the spectrometer, the actual Z positions for DC1 wire planes will be $Zpos-48.095$ cm from the HKS reconstruction plane, for DC2, it will be $Zpos+51.905$ cm.

An important tracking criterion constant is space_point_criterion, the minimum squared distance to combine two hit pairs into one space point. hpace_point_criterion is 1.2 cm currently.

These tracking parameters are set in files according to standard Hall C CTP file structure.

Figure 2: File structure of DC tracking parameters