Acceptance

In order to produce a cross section measurement effects of the detector acceptance have to be corrected. For the proper acceptance determination, parameters like detector efficiencies and resolution are very important, because they can influence the track reconstruction efficiency. For the simulation of the Innner Tracker performance, the efficiencies obtained in the ITR performance studies and described in the Section 5 are used.

For the study of the acceptance of HERA-B 1,000,000 inelastic events for each target wire, used during data taking in 2002, were generated with the HERA-B Monte-Carlo chain, including the full detector simulation.

The overall acceptance can be subdivided into the geometrical acceptance and the reconstruction efficiency. A $V_0$ is considered to be in the geometrical acceptance of the detector if its decay products pass through enough layers of tracking stations in the PC area. In additional, the decay's vertex has to be measured, which means the tracks have to be measured by the Vertex Detector System. These requirements are checked with the Monte Carlo Impact Points (MIMPs) and the decay position ($z$ coordinate) of the $V_0$ candidate. A requirement on the number of crossed layers in the PC area is needed, because a track with less than 5 MIMPs can not be reconstructed by the pattern recognition program. The cut at the $z$ position of the secondary vertex is based on the plot shown in Fig. 6.6

Figure 6.6: Reconstructed secondary vertex $Z$ coordinate distribution for run 20678 taken with Below I wire (Carbon). The cut applied in the analyzed events is indicated.

Finally, the requirements used for this study were:

1. tracks have to pass through at least one superlayer and produce at least 5 MIMPs each in Main Tracker (Inner + Outer tracker),
2. the $V_0$ has to decay before 155 cm from the target ( $z_{\rm {target}}-z_{\rm {vert. second.}}$).

The geometrical description used for this acceptance study is valid for the detector configuration used during data taking in 2002/2003 (both Inner Tracker and Outer Tracker were fully installed and equipped). The resulting geometrical acceptance functions are shown in Fig. 6.7.

Figure 6.7: Geometrical acceptances for $V_0$s.

For the cross-section calculation a full acceptance (geometrical acceptance plus reconstruction efficiencies) function is needed. In order to obtain the full acceptance function, the generated MC sample was reconstructed and distributions of the kinematical variables ($x_F$, $p_t^2$) are produced. Such variables are chosen because they are of special interest for the fragmentation processes. The kinematical variables are plotted for reconstructed MC and the original MC truth. The ratios of these distributions:

$$A= \frac { X_{\rm {MC~reco}} } { X_{\rm {MC~truth}} },$$ (6.5)

are the final acceptance functions (see Fig. 6.8).

Figure 6.8: Acceptances for $V_0$s, including geometrical acceptances and reconstruction efficiencies. For the acceptance determination the standard MC sample for Below I wire (Carbon) was used.

The HERA-B spectrometer covers the central production region in the overall proton-nucleon center of mass system. This can be seen from the acceptance plots as a function of $x_F$. The range -0.03 $< x_F <$ 0.01 is covered with an acceptance larger than 0.1 and with lower acceptance it extends up to -0.12 and 0.04 for $K_S^0$. Acceptances of $\Lambda$s and $\bar \Lambda$s are approximately covering the same region with lower values $\approx$ 6% and the center of gravity of the distributions are shifted more to the negative region compared to the acceptance of the $K_S^0$.