Peter B. Denton: Neutrino Quiz

Solar neutrinos at night:
Are more neutrinos detected from the Sun during the day or during the night?
[answer]
Night.

Solar neutrino flavor ratios:
For electron neutrinos from the Sun, the flavor ratio at the Earth is close to 1:1:1. What is the expected flavor ratio for muon neutrinos produced in the Sun?
[answer]
1:1:1. See this [twitter thread] for more.

Distance to point of neutrino last scattering:
The comoving distance to the photon surface of last scattering is O(10) Gpc. Is the surface of last scattering for neutrinos closer or farther?
[answer]
Closer. The temperature of last scattering is much higher for neutrinos than photons, but they don't travel far in the time it takes to cool down. So for massless neutrinos it's a tiny bit farther, but, for neutrinos heavier than about 0.0002 eV (and certainly some neutrinos are heavier than this), their finite mass moves this surface closer to us: O(1) Gpc away depending on their exact mass. Further details like oscillations make this effect a bit stronger. See [0907.2887]. Inspired by this [tweet]. See this [twitter thread] for more.

Possible solar measurements:
SNO measured a CC to NC ratio of ~1/3 of solar neutrinos. KamLAND's measurement of reactor neutrinos confirmed the measurement of solar neutrinos by SNO and others. Given KamLAND's measurement, what other values if any could SNO have measured?
[answer]
2/3. Long-baseline reactor neutrino experiments like KamLAND and later JUNO measure sin²2θ₁₂ as the depth of the oscillation minima. High energy solar neutrinos E≳5 MeV are measured at a rate sin²θ₁₂ which can take two different values given KamLAND's measurement. See this [twitter thread] for more.

Massless neutrinos (Aug 2023):
Given the current neutrino data, it could be that one of the mass states is massless. The massless state could be ν₁. Which of the other two states, if any, could also be massless? (Define the mass states in the usual way: |U_e1|>|U_e2|>|U_e3|.)
[answer]
Just ν₃. Put another way, we know that ν₂ is massive since solar neutrino data from Homestake, SNO, SuperK, Borexino, SAGE, and GALEX tells us that Δm²₂₁ is positive. Since the atmospheric mass ordering (the sign of Δm²₃₁) is unknown, it could be that either ν₁ or ν₃ is massless. See this [twitter thread] for more (including plots).