A Tau Neutrino Runs into a Candy Shop…

We recently discussed some curiosities in the data from the IceCube neutrino detector. This is a follow up Particle Bite on some of the sugary nomenclature IceCube uses to characterize some of its events.

As we explained previously, IceCube is a gigantic ultra-high energy cosmic neutrino detector in Antarctica. These neutrinos have energies between 10-100 times higher than the protons colliding at the Large Hadron Collider, and their origin and nature are largely a mystery. One thing that IceCube can tell us about these neutrinos is their flavor composition; see e.g. this post for a crash course in neutrino flavor.

When neutrinos interact with ambient nuclei through a W boson (charged current interactions), the following types of events might be seen:

I refer you to this series of posts for a gentle introduction to the Feynman diagrams above. The key is that the high energy neutrino interacts with an nucleus, breaking it apart (the remnants are called X above) and ejecting a high energy charged lepton which can be used to identify the flavor of the neutrino.

• Muons travel a long distance and leave behind a trail of Cerenkov radiation called a track.
• Electrons don’t travel as far and deposit all of their energy into a shower. These are also sometimes called cascades because of the chain of particles produced in the ‘bang’.
• Taus typically leave a more dramatic signal, a double bang, when the tau is formed and then subsequently decays into more hadrons (X’ above).

In fact, the tau events can be further classified depending on how this ‘double bang’ is resolved—and it seems like someone was playing a popular candy-themed mobile game when naming these:

In this figure from the TeVPA 2 conference proceedings, we find some silly classifications of what tau events look like according to their energy:

• Lollipop: The tau is produced outside the detector so that the first ‘bang’ isn’t seen. Instead, there’s a visible track that leads to the second (observable) bang. The track is the stick and the bang is the lollipop head.
• Inverted lollipop: Similar to the lollipop, except now the first ‘bang’ is seen in the detector but the second ‘bang’ occurs outside the detector and is not observed.
• Sugardaddy: The tau is produced outside the detector but decays into a muon inside the detector. This looks almost like a muon track except that the tau produces less Cerenkov light so that one can identify the point where the tau decays into a muon.
• Double pulse: While this isn’t candy-themed, it’s still very interesting. This is a double bang where the two bangs can’t be distinguished spatially. However, since one bang occurs slightly after the other, one can distinguish them in the time: it’s a “double bang” in time rather than space.
• Tautsie pop: This is a low energy version of the sugardaddy where the shower-to-track energy is used to discriminate against background.

While the names may be silly, counting these types of events in IceCube is one of the exciting frontiers of flavor physics. And while we might be forgiven for thinking that neutrino physics is all about measuring very `small’ things—let me share the following graphic from Francis Halzen’s recent talk at the AMS Days workshop at CERN, overlaying one of the shower events over Madison, Wisconsin to give a sense of scale:

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