Quality Cuts

A vertex cut is the primary means used to eliminate background roads; the $\chi^2$ cut is left open ( $\chi^2/\nu < 1000$, where $\nu$ is the number of degrees of freedom) at present. To apply the vertex cut, the position of the intersection point of the straight-line road trajectory with the vertex plane⁴ is determined, and the X and Y positions are checked for consistency with the position of the nominal interaction point at (0,0,0). A vertex cut of $\pm 180 cm$ is used (that is, the X and Y positions of the intersection point must both be less than 180 cm), being reasonably efficient for muons from the interaction point, while vastly reducing background.

It was originally hoped that a vertex cut would be unnecessary for roads containing hits in the non-seed planes: the requirement of collinear hits in the various MuID planes was expected to be a sufficient constraint. Detailed studies demonstrated that this was not the case: the rate of background hits in the non-seed planes was large enough so that a significant number of false roads were present. This problem was observed primarily in central HIJING Au+Au events in the South Arm, where that arm had no additional shielding; as a result, the rate of background hits was rather high (need occupancy numbers here!) Since current plans include effective shielding for the South Arm, it may be interesting in the future to study the possibility of making less-restrictive vertex cuts.