Wonderful pictures!

The Super Kamiokande had a terrible engineering event where the delicate sensor bulbs shattered, and the pressure delta from one shattering caused neighbors to shatter, in a chain reaction that destroyed large amounts of sensors.

https://www.youtube.com/watch?v=YoBFjD5tn_E

Unrelated:

>Neutrinos come in three different “flavors” (electron, muon, and tau) and can oscillate, or switch, between them. To do so, neutrinos must have mass

Why? What actually is "Neutrino oscillation" and why does it require the neutrino have mass? My already feeble understanding of particle and quantum physics always breaks down at these sorts of points.

How are we sure that the neutrino is in fact a single particle that should use the same sort of mathematical machinery as all others? Am I even asking a question that means something? I know literally every physicist ever graduated has spent time thinking everything in physics is wrong and tried poking at such ideas, so I guess I'm more interested in what those kids end up finding that brings them back to "No this makes more sense" of neutrinos in the standard model.

Intuition for neutrinos requires at least a little bit of math to really grasp but I'm not a physicist so take this all with a mountain of salt:

Imagine the flavor state of a neutrino is expressed a discrete value: -1 for tau, 0 for electron, 1 for muon. The neutrino is a superposition of the three mass states and the flavor is like speed: it's the magnitude of a vector expressed as i,j,k basis vectors where the "tau", "electron", and "muon" mass states are the components of the flavor. These flavor components are kind of like sine waves that are out of phase with each other. When a neutrino interacts with the weak force, you combine these components (the probability distribution of the mass states technically) and the magnitude gives you which flavor it is. If the waves interfere to create a peak that's one flavor, interfere to create a trough is a different flavor, and if they destructively interfere to zero that's the third state (oversimplifying dramatically here).

Neutrino oscillation happens because these states (the phase of each flavor component) evolve at different rates with respect to time, related to their different masses (f=E/hbar if I remember correctly). Without mass, there would be no neutrino oscillation, as the neutrinos and their state would not experience time.

When a tau neutrino is created, we know the phases of the individual flavor components (where they are in the sine wave), but as the neutrino experiences time, those phases progress at different rates. Essentially, in order to predict the flavor of a neutrino, you need to know both what flavor it was born as, and how much time it has experienced in its reference frame before interacting with the weak force again.

> How are we sure that the neutrino is in fact a single particle that should use the same sort of mathematical machinery as all others?

My understanding is that neutrino oscillation is the physical phenomenon that required new machinery (again I'm not a physicist). If there was no neutrino oscillation, we'd be able to easily predict the flavor distribution of neutrinos we measure because we can predict the flavor that they're born as, but we can't since we don't know how much time each neutrino has experienced between the time it was born and interacted with our detectors.

Disclaimer: I'm not a physicist.

For a particle to "oscillate", it must "experience" time. All massless particles travel at the speed of light. As a consequence of special relativity, they don't "experience" time.

Therefore, neutrinos must be traveling slower than light, and they must have mass.

I remember some of the videos in this playlist doing a very good job explaining neutrino oscillations, and what it means, but I don't remember which ones: https://www.youtube.com/playlist?list=PLCfRa7MXBEsqOlL_g6wTM...

Maybe this one? https://www.youtube.com/watch?v=eBT1-dV1BTM

It's kind of useful to only think of these things as "particles" in a nominal sense. You need to adopt a "quantum imagination". I tend to think of them as a wave or field of probabilities of energy. It sounds weird, but similar to "spin", "flavour" describes a particular relationship between waves or fields of probabilities of energy as it moves through space over time.

A simplified summary: The discovered mass emerges out of this relationship between detection and probability.