Neutrino Interaction Topologies

The key to finding the tau neutrino lies in being able to recognize event topologies, which is precisely what DONUT physicists did.

Topologies are essentially snapshots of event occurrences. They are the physical manifestations of particle transformations which are recorded in detectors. Certain particles interact to form specific tracks at specific angles, which can be used to determine their identity. This is how we uncover the nu tau.

Neutrino interactions, although very rare, can be reduced to two categories: Charged Current (CC), when they weakly interact through the exchange of a +/-W boson to form charged particles, and Neutral Current (NC), when they produce uncharged particles through the weak exchange of Z particles. Each of these interactions has a distinct track type which scientists analyze. Knowing these topologies allows DONUT physicists to identify which interactions they observe in the emulsion data.

The first figure is a charged current muon neutrino interaction to a muon particle. These penetrating events are characterized by the long tracks of the muon.

The second figure is also a charged current interaction, this time of an electron neutrino. Unlike the long product tracks of the muon neutrino, the electron produced in the neutrino interaction produces showering, having a short lifetime. This is a called a short event.

Any neutrino can undergo a neutral current interaction. In this case there are no charged lepton products, only multiple hadron products, which are generally short events.

The tau neutrino interactions are a bit more difficult to observe, namely for the reason that the tau lifetime is so short. The tau neutrino will interact and form a tau that has an 18% probability of decaying either to a muon and two neutrinos (long event) or an electron and two neutrinos (short event). 86% of all tau decays involve only 1 charged particle--called a kink---this is what DONUT scientists were looking for.

Here are examples of two charged current tau neutrino events

Last updated: 6/29/01 comments