Neutrinos are classified into three flavors by physicists: electron neutrino, muon neutrino, and tau neutrino. This makes sense because when neutrinos interact with charged particles in detectors, they often generate their signature charged particle, which makes the neutrino flavor clear right away. Electron neutrinos interact with muon neutrinos to produce muons, and tau neutrinos produce taus.
However, neutrinos can also be thought of in terms of their mass, or “mass state.” These neutrinos have been given the names mass 1, mass 2, and mass 3, though they can also be called v1, 2, and 3. Although it may appear that each of the three masses has a distinct flavor, the truth is that the mass state of a neutrino does not exactly match the flavor state of a neutrino.
A neutrino’s flavor (such as the electron neutrino) is made up of a mix of masses (1, 2, and 3), and a neutrino of a certain mass (such as the lightest neutrino) has a certain chance of interacting in a detector to produce a particular flavored charged particle (electron, muon, or tau).
Although many issues about neutrino masses remain unanswered, scientists do know a few things. The masses of the three neutrinos are known to be tiny. They also understand how the flavor combination for each mass neutrino degrades. The flavor of mass 1 is largely influenced by electrons; mass 2 is a more balanced mix of electrons, muons, and tau; and mass 3 is dominated by muons and tau. And the masses of 1 and 2 are nearly identical, whereas 3 weighs significantly more or less than the other two.
In an intriguing way, mass neutrinos and flavor neutrinos coexist. When a neutrino—say let’s an electron neutrino—is formed, it combines (or superpositions) all three mass states quantum mechanically. Each particle is likewise a wave, with a slightly different mass and thus a slightly different speed. The multiple mass levels of the electron neutrino go out of phase as it travels; the lightest state has a little faster velocity than the heavier states. The original composition of mass 1, mass 2, and mass 3 that made up the electron neutrino changes along the route of the neutrino as a result of this. When the neutrino interacts again after a certain distance, it has a chance of interacting as an electron neutrino, but it also has a chance of interacting as a muon or tau neutrino. The likelihood of interacting as a muon or tau neutrino could be significantly greater than the probability of interacting as an electron neutrino, depending on how far out of phase the mass states have gone.
Another way to think about it is that an enormous food war has erupted inside an ice cream parlor. Your nearby friend hurls a scoop at you, which lands squarely in your open mouth. In the same way that a detector can taste the incoming neutrino flavor, you can clearly recognize it as chocolate (rather than vanilla or strawberry) (as an electron and not muon or tau neutrino). However, something strange happened with the ice cream. It began as chocolate ice cream in your friend’s hand and ended up in your mouth as chocolate ice cream, but along the way, the ice cream transformed into gold, silver, and copper coins.
These coins, like mass neutrinos, have different masses: mass 1, mass 2, and mass 3. They also travel at slightly different speeds due to their differing weights. Some of the gold coins begin to lag behind, whereas the lighter copper coins fly a little further. Because you were near your friend when she tossed the ice cream, the coins didn’t separate too much. When they got to you, their ratio was about the same, and when they got to your tongue, they turned back into delicious chocolate ice cream.
You watch your friend hurl chocolate ice cream across the store out of the corner of your eye. POP: the ice cream drops off her palm and transforms into a blob of coins. The weight starts to matter after such a long distance. The silver and copper coins arrive in quick succession, followed by the gold coins a split second later. PLOP. When the ice cream hits the target’s lips, he declares that it tastes like strawberry. The coin ratio in the ice cream is different than it was when it started, and it turns out that this has an impact on the flavor.
If you had a neutrino with only gold coins, silver coins, or copper coins, you’d have a pure mass state neutrino. Because things get pretty strange at small scales, these pure mass states would still have a chance of interacting in a specific flavor. That is, even if you had all gold coins, you wouldn’t be able to tell if the ice cream you smelled was chocolate, strawberry, or vanilla. However, given a particular coin kind, there would always be the same chance of experiencing a specific flavor. Similarly, each of the pure mass state neutrinos is likely to yield a different ratio of flavors.
Neutrino physics is both chaotic and tasty.