Vertex and mass resolutions

The vertex position of protons from $\gamma + d \rightarrow p + \pi^- + p$ was used in MC/DATA comparison. This events were identified from the missing mass distribution in $\gamma + d \rightarrow p + \pi^- + X$ (see Fig. 17), which has a relatively high cross section. The z-(longitudinal) and T-(transverse) vertexes from data are in reasonably good agreement with MC (RECSIS) reconstructed distributions (see Fig.7). It appeared, that the small shift between MC and DATA in Fig.7 depends on kinematics. This unexpected dependence in the DATA(Run 20111) of mean values, both for z-and T-vertexes on proton energy and angles is shown in Figs. 9, 11.

The MC reconstructed vertex compared to MC generated, for z and T vertexes, are plotted in Figs.8 and 10 respectively.

  

Figure 7: Comparison of MC and Data reconstructed vertexes (top plot for z-vertex, bottom T-vertex) for fast protons in the $\gamma + d \rightarrow p + \pi^- + p$.

  

Figure 8: Proton vertex reconstruction for fast protons in the $\gamma + d \rightarrow p + \pi^- + p$. The dashed line in the top plot is for the MC generated and solid line for MC (RECSIS) reconstructed. The bottom plot shows the difference between MC generated and reconstructed z-vertexes.

  

Figure 9: Mean Z-vertex as a function of proton energy, polar and azimuthal angle. The open circles are for all protons from $\gamma + d \rightarrow p + X$ and filled circles only for protons from $\gamma + d \rightarrow p + \pi^- + p$. The stars are from MC reconstructed data.

  

Figure 10: Transverse vertex (radial distance from the beam axis) distributions for fast protons in the $\gamma + d \rightarrow p + \pi^- + p$. The dashed line in the top plot is for the MC generated and solid line for MC (RECSIS) reconstructed. The bottom plot shows the difference between MC generated and reconstructed T-vertexes.

  

Figure 11: Mean T-vertex as a function of proton energy, polar and azimuthal angle. The open circles are for all protons from $\gamma + d \rightarrow p + X$ and filled circles only for protons from $\gamma + d \rightarrow p + \pi^- + p$. The stars are from MC reconstructed data.

The $\pi^0$ mass distributions as a function of $\pi^0$ energy, polar and azimuthal angles are shown in Fig.12. $\pi^0$'s are reconstructed from 2 photon clusters, in $\gamma + d \rightarrow p + \pi^0 +X$.

  

Figure 12: $\pi^0$ mass as a function of the azimuthal angle of detected $\pi^0$.