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The first stage of the analysis process occurred in Japan at Nagoya University, the leading institution for emulsion exposure and digitization. Emulsion analysis has come a long way since the 1950s when all examination was done by hand. Today, even though sophisticated digital scanning techniques, pioneered at Nagoya, have made emulsion data much more accessible, it nonetheless took 18 months to process the DONUT emulsion sheets.
The previously used method of scanning, the "scan back" technique, involved identifying a high momentum track using the spectrometer and then tracing it back through the emulsion to its vertex. Because of the intense muon background created in the DONUT experiment, this method was not appropriate for the experiment, so a new technique was developed at Nagoya called "net scan". Physicists use information from fiber trackers and the spectrometer to predict a vertex within the emulsion sheets. Using this information, a 5x5x20 mm3 volume is scanned around the predicted vertex. This volume is divided into 16 layers to determine track angles and positions. Within a typical scan, physicists find 105 tracks which they need to thin to create a usable data set. In order to do so, they throw out any tracks that traverse the entire scanning distance (because they are looking for tracks that originated inside the scanning volume) and all vertices with one or two tracks (because they have a high probability of being coincidental track formations). Throwing out a portion of the data is termed 'making cuts', which becomes a balance between the number of events and the intensity of the background you accept. These cuts leave all vertices that have 3 or more tracks emanating from them for examination.
Once the vertices are located, Nagoya scientists do another scan, this time a 2.6x2.6 mm2 area around the vertex perpendicular to the beam with a depth of 2 emulsion sheets upstream of the vertex and 20 sheets downstream. Then the scanned volume is digitized and the data is sent to other DONUT physicists. This process has a track resolution of better than .6 micrometers.
First DONUT physicists had to locate neutrino interactions of any flavor, let alone tau. These interactions are characterized by event vertices with no tracks leading into them. Muon and electron neutrinos have easily observed topologies. Muons are characterized by long, penetrating tracks, and electrons by short, showering events. By recognizing muon and electron neutrino events, DONUT physicists could discount them as tau events, and check to make sure the
In order to observe the tau neutrino, DONUT scientists looked for tau decays, which are products of tau neutrino reactions. 86% of all tau decays involve a 'kink' topology, resulting in only one charged particle product. The easiest to observe, this topology is divided into two categories: long and short decays. Long decays are characterized by kinks occurring outside of the plate where the interaction was recorded. The daughter (particle the tau decayed to) track usually spans several emulsion sheets. The parent track can be found inside the emulsion and, to constitute tau decay, there must be no electrons or muons originating from the primary vertex. Physicists expect 75% of all tau decays to be long.
Short decays, on the other hand, are characterized by the decay occurring in the very close to the primary vertex. Oftentimes the tau decays in the steel so that the kink is not recorded in the emulsion. These cases are identified as a tau based on two characteristics: its impact parameter (a specific area around the projected event location where two tracks meet), and the minimum transverse momentum of the daughter track (the momentum of the daughter traveling away from the parent track).
While examining tracks DONUT physicists have an important consideration to make: background. Because of the intense muon flux through the target area creating many unnecessary tracks, background poses an unpleasant problem through mimicking tau neutrino events. There are four categories that kinked tracks can originate from, random background association, re-scattering, charm decays in charged current interactions, and tau decays.
The first type, random background association, occurs when two separate tracks happen to meet, forming what looks like a kinked track. Physicists calculated the probability of this phenomenon occurring based on the impact parameters around each track starting within two plates of the primary vertex. Based on those probabilities they were able to accept or reject candidates.
Tau interactions can also be faked by re-scattering, an alteration due to coulomb forces, of the primary track to either a neutral current event or a charged current event with an unidentified lepton product. The probability of this occurring, however, decreases rapidly with increasing transverse momentum, so suitable candidates can be selected within certain cuts.
Lastly, charm decays can often look like tau decays. They are differentiated from tau decays by the identification of lepton products. If the lepton products cannot be identified these decays are classified as background because there is no way to determine if they are charm or tau decays. Charm particles are produced in 8.1% of all electron neutrino and muon neutrino charged current interactions. After examining branching ratios, the probability that a given event without lepton identification would be a charm is calculated and is factored into the final analysis.
For more event analysis and information you can visit our collaborators in japan at Nagoya University for their interaction images.
Last updated: 6/29/01 comments