The track produced by charged particles is visible in the emulsion as track segments; a position and angle measurement for each emulsion layer that the particle passed through.
The particle trajectory is then reconstructed from segments in many consecutive plates. This image shows a typical situation with several hundred thousand tracks (also in .eps.gz). Each of the green lines represents one track, and each of the blue stubs represents one segment. The images shows the side view, the direction of the neutrino beam is up.
Finding a vertex in this haystack of tracks appears to be very difficult. Fortunately, most of the tracks turn out to be penetrating muons that can easily be rejected by requiring that a track start within the emulsion volume. After rejecting penetrating tracks (and also single segments that were not associated with any tracks), only a few hundred tracks remain (shown here, also in .eps.gz form).
The remaining tracks are then analyzed to find vertices. If the closest approach between the tracks is upstream of the first segment of both tracks and the distance of closest approach is smaller than a few tens of microns, the two tracks form a vertex. This image shows all reconstructed vertices |(in .eps.gz). Since there are not very many, you can find the neutrino interaction vertex in the figure. It is simply the only vertex with more than two tracks. Most of the other vertices are just random associations of tracks.
If you have not made out the vertex yet, it is shown in this image (.eps.gz). After the initial vertex search, the remaining tracks are re-analyzed to find secondary vertices and evidence for tau leptons. This image shows a close-up view of the vertex with the gray tau-track (.eps.gz).
This page was last modified on 07/06/00 by Reinhard Schwienhorst.