Selection Cuts

To provide a fast background rejection the following cuts are used:

• Tracks which do not have VDS or Main Tracker segments are rejected by the cut on the number of hits used in tracks from the tracking detectors. The distribution of hits used for reconstruction of track segments in the VDS and Main Tracker obtained on data are shown in Fig. 6.2.

  Figure 6.2: On the left, number of hits used in the Silicon detector for the track segment reconstruction. On the right, number of hits used in the Main tracker (ITR+OTR) for the track segment reconstruction. The chosen cut values are indicated by the vertical arrows.

  

• The next cut is based on the Distance of Closest Approach (DCA) between two $V_0$ tracks. The idea of the DCA cut is to reject tracks which are far from each other to speed up the selection algorithm.

• Cut on the vertex impact parameter. This cut selects the best primary vertex.

These three cuts can be understood as a soft preselection in order to reduce the amount of data to be further processed.

The main cuts for extraction of $V_0$ signals are DCA, impact parameter and flight path cuts. In order to optimize these cuts studies on MC and data are performed. Efficiencies versus applied cut distributions are shown in Fig. 6.3 for $K^0_S$. The plots at the top show the behavior of signal and background versus the applied cut. Signals are taken from generated MC and the background is obtained from data. The lower plots show the significances ( $S/\sqrt{S+B}$) for these cuts.

To cut on the flight path of the $V_0$ in the rest frame of the $V_0$ candidate is the most efficient possibility to get rid of shortlived background, coming from the target region.

A similar dependence of signal and background on the applied cuts was observed for $\Lambda$ and $\bar \Lambda$, but as $\Lambda$s are longer lived particles compared to $K^0_S$ (the flight path cut for $K^0_S$ is 0.13 cm), the cut for them was increased to 0.39 cm, this corresponds to 5$\%$ of the flight path of $\Lambda$s.

Figure 6.3: On the left, the dependence of signal and background on the cut on the applied distance between two tracks is shown. In the middle plot, the dependence of signal and background on the applied impact parameter cut. On the right, the dependence of signal and background on the applied flight path cut. The chosen cut values are indicated by the vertical arrows.

All cuts are selected such that they are placed in the flat regions of the efficiency curves. This allows us to be sure that if the $V_0$ MC signals behave not exactly like in data, additional systematic uncertainties are small.