Sunday, May 12, 2019

Here comes the tau?

Simulated tau neutrino event in IceCube.  Each sphere is an optical module that observed light, with the size scaling with the number of photons.  The color indicates the timing of the light, from red (earliest) to blue (latest).
 One of the more interesting/embarrassing holes in IceCube's physics portfolio was the tau neutrino.  Neutrinos come in three flavors: electron, muon and tau, each tied to the charged lepton of the same name.  Over long distances, these neutrinos can oscillate, changing flavors.  So, no matter what flavor ratio a neutrino beam is produced with, over long distances, we expect it to oscillate and reach Earth as a roughly 1:1:1 ratio of electron:muon:tau neutrinos.

Since very few tau neutrinos are produced directly on Earth, the observation of tau neutrinos was considered to be a clear sign of astrophysical neutrinos, and many many papers discussed the signatures and expectations.  A beautiful 1995 paper by John Learned and Sandip Pakvasa   (also available on the arXiv - soon after it was founded) pointed out that sufficiently energetic tau neutrinos could produce a distinctive 'double bang' signature - a large cascade when the neutrino interacted, and a second when the resulting tau lepton decays.  Even though the tau lepton lifetime is very small (3*10-13 s), when it has energies of a PeV (1015 eV) or higher, a Lorentz boost extends its lifetime in the Earth frame of reference so that the two bangs can be separated by an average of (Energy/1 PeV) * 50 meters, making for a distinctive signature seen in the simulation shown above, with two distinctive light clusters.  Unfortunately, IceCube has not seen this signature, and we have also not seen enough PeV-energy neutrinos so that we can expect to see it.
A candidate tau neutrino event seen in IceCube. Each sphere is an optical module that observed light, with the size scaling with the number of photons.  The color indicates the timing of the light, from red (earliest) to blue (latest).  The seven plots show the waveforms (light vs. time) for certain optical modules; several show apparent double-pulse signatures.

However, IceCube is developing techniques that will allow us to see tau neutrinos with lower energies, where the two bangs are closer together.  Even if they are so close together (10-30 m) that we cannot separate the overall light clouds,  there may be some optical modules that see pulses from the two cascades at separate times, producing a double-pulse topology in an individual optical module.   The figure above shows one candidate event, along with waveforms from some of the modules, showing the double-pulse signature.   A word of caution is in order - there are some possible background processes that could mimic these signatures - but this is considered by IceCube to be evidence for tau neutrinos.  "Evidence" typically means that the statistical significance is 3 sigma or more, not the 5 sigma required to claim a detection.  Since we expect to see tau neutrinos, this is reasonably convincing, an it seems safe to say that the holes has largely been filled in.  We look forward to more precise measurements, of course, to check in more detail for consistency with the standard acceleration scenarios.

The tau neutrino work has been presented in several recent conference presentations, including ones with writeups by Daan van Eijk and Logan Wille and Juliana Stachurska.