Reconstruction of Tracks: Tracking Cut

The algorithms used for reconstructing individual tracks were those developed for the RMC experiment (26), and were applied for the present analysis essentially without any modification. Particle tracks in the drift chamber were fit to circles in the $x - y$ plane and to a straight line in the $z - \phi$ plane. A minimum number of DC and IWC hits in each plane was required for each track. The $x - y$ fit required nine hits in the DC, while the $z - \phi$ fit required two hits in the DC stereo layer and at least one hit in the IWC. In addition, a cut on the quality of the fit was applied based on the variance of the hits from the fitted track. The number of trigger events cut by $\chi ^2$ requirements on tracking was however negligible ($\sim$ 0.1%). The details of the tracking cut parameters are listed in Table 5.1. In the table, the wrap photons refer to the photons that traverse along the length detector following a helical path due to the axial magnetic field, while the non-wrap photons refer to the photons that escape along the radial direction.

A typical two-photon event is shown in Figure 5.3 ($xy$ plane) and Figure 5.4 ($r$-$z$ plane). The photons that originate from the target, convert into charged $e^+e^-$ pairs in the lead sheet and these charged tracks are fitted in a plane perpendicular to the beam axis in Figure 5.3. The trigger pattern of zero hits in the A-counter ring, two hits in the C-counter ring, and four hits in the D-counter ring is also displayed. Figure 5.4 shows the $z$ projection of the signal event shown in Figure 5.3. Only the points corresponding to hits in the stereo layer (Layer 3 of the drift chamber) and the IWC, which were used to reconstruct the $z$ coordinates of the tracks are shown.

Figure 5.3: $xy$ projection of a typical $\pi ^-p\rightarrow \gamma \gamma n$ event. The plot shows the fit in the plane perpendicular to the beam axis. For scale, the radius of the D ring is about 60 cm.

Figure 5.4: The $z$ projection of the signal event shown in Figure 5.3. Hits in the stereo layer (DC layer 3) and the IWC are shown in the $r-z$ plane. These points are fitted with a straight line.