Cascade (and $\Omega^-$) reconstruction

Using the ``cascade'' reconstruction algorithms, we are able to reconstruct the decays $\xim \to \lb \pi^-$ and $\Omega^- \to \lb K^-$, which have branching ratios of 99.9% and 67.8% respectively. Because we are often able to reconstruct the in these decays, we are able to fully reconstruct many of these hyperon decays. This decay topology is illustrated in recon:casc_ill.

  

Figure 4.7: Illustration of the cascade topology for the decay $\Xi^- \to \lb \piminus$. The decay of $\Omega^- \to \lb \kminus$ is identical, except the replaces the first .

The cascade reconstruction considers two cases. In the first case, the or $\Omega^-$ decays in the target region, upstream of the SSD detectors. In this case, we require that the charged track (the or the ) forms a good vertex with a momentum vector and that the combination points back to another vertex.

In the second case, the or $\Omega^-$ decays downstream of the SSD detector. This is similar to the kink topology, but the neutral particle () is fully reconstructed. The algorithm begins by finding a vertex between a and an unlinked PWC track, both of which must be traced into the magnetic field of M1. When the best fit for this vertex is found, the unlinked SSD track (the or $\Omega^-$) is also traced into M1. If the two traced objects intersect, the entire decay is refit with the new decay vertex position.

  

Figure 4.8: Signals of $\xim \to \lb \pi^-$ and $\Omega^- \to \lb K^-$.

Both cascade topologies place additional requirements on the in order to reject backgrounds.