OPERA has a new paper on the arXiv. They report finding a fourth candidate ντ event.
The tau neutrino (ντ) is probably the rarest particle in the Standard Model. Well, that technically isn’t true. The tau neutrino should be extremely common but is the hardest to identify, so very few candidate events have ever been found. In years of running, OPERA only has four candidate events.
There are several things that make the ντ hard to find:
- Neutrinos don’t interact very much, so huge numbers of them are required to have measurable numbers of events. The mean free path for neutrino interactions is often quoted as being more than a light year in lead.
- Neutrinos can interact via neutral current (the neutrino stays a neutrino) and charged current (the neutrino turns into a charge lepton). Neutral current events cannot be used to distinguish the neutrino type on an event-by-event basis.
- The τ (tau) is much heavier than the other charged leptons. It has over 10 times the mass of the muon and over 1000 times the mass of an electron. Thus, much more energy is needed for charged current interactions to occur. A high energy proton synchrotron is needed to create neutrinos with energies high enough to produce taus.
- Accelerator-based neutrino experiments primarily produce muon neutrinos from decays of mesons produced in hadronic showers. A tau neutrino experiment must then look for ντ appearance due to the muon neutrinos (and any electron neutrinos) to oscillate into tau neutrinos. This requires the detector to be at an appropriate distance from the target to maximize the number of tau neutrinos passing through the detector.
- Even if taus are produced in charged current events, they quickly decay to other particles. The neutrino detector must be able to identify tau candidates, which will typically require a high enough energy and fine enough spatial resolution to distinguish the primary interaction vertex from the secondary vertex created when the tau decays.
This result further strengthens OPERA’s case for definitively confirming the discovery of νμ to ντ oscillations.