Universe Puzzle No. 12

How did you do in last week’s Universe Puzzle? Did you easily find out where the green valley is, but have no clue as to why it’s called a ‘valley’?

Do you enjoy these puzzles? What do you particularly like? Dislike? Would like to see changed? Would like to see more of? Let me know please!

Once again, this week’s puzzle requires you to cudgel your brains a bit and do some lateral thinking (five minutes spent googling likely won’t be enough). But, as with all Universe Puzzles, this is a puzzle on a “Universal” topic – astronomy and astronomers; space, satellites, missions, and astronauts; planets, moons, telescopes, and so on.

Which is the “odd one out”? And why?
α, β, γ, μ, ν, and τ.

In case the Greek symbols don’t display properly, these are (lower case, or small) alpha, beta, gamma, mu, nu, and tau.

UPDATE: Answer has been posted below.

There are, of course, many answers. For example, τ (tau) is alone, because neither of its two neighbors in the Greek alphabet are in the list. But that’s not a particularly good answer, and this week’s puzzle asks for the best answer (which may be τ (tau)!), so the explanation of your choice is what counts.

I think Hon. Salacious B. Crumb’s answer is very good (“tau is the odd one out. The rest are al rings of the planet Uranus by increasing radius from the planet’s disk. alpha, beta, gamma are inner rings, nu and mu are the outmost rings.“)

I also like Navneeth’s (“alpha, because it’s a composite “particle” while the others are truly elementary (as far as we know)“) – gopher65 gave much the same answer, iantresman’s (“alpha particles are the only non-fermions. Neutrinos (nu) are the only ones to come in different flavours“), and the several of you who picked gamma because the photon is massless (though I think iantresman’s “it’s the only gauge boson” is a better reason for choosing it; you could also say it’s the only force carrier particle).

Star designations? If someone had come up with a good answer along those lines, they’d’ve got my vote; but, as far as I know there is nothing “odd” about any of these Greek letters.

My own answer was the tau … it’s the only particle as yet undetected, directly, by any spacecraft or Earth-based facility, originating out in space (Fermi detects gammas; plenty of instruments on spacecraft detect alpha particles, electrons, and muons; and neutrinos from the Sun and SN1987A have been detected; but no taus!)

Check back next week for another Universe Puzzle!