This is a follow-up of the question asked here.

After going through the Wikipedia page on SNO experiment, I need further clarifications about the accepted answer here.

First, the SNO experiment was able to detect $\nu_e$-type neutrinos via neutron absorption by heavy water, $n+\nu_e\to p+e^-$.

Second, it can also detect all three flavours of neutrinos $\nu_{e,\mu,\tau}$ via the deuteron dissociation $\nu_{e,\mu,\tau}+d\to n+p+\nu_{e,\mu,\tau}$. Then, the neutrons are captured by another deuteron producing a gamma ray. The latter scatter with electrons via Compton scattering $\gamma+e^-\to\gamma+e^-$, and the resulting Cherenkov radiations could tell how many neutrino events did take place.

Question It looks like SNO could measure only total flux of $\nu_e+\nu_\mu+\nu_\tau$, and established the phenomenon of neutrino oscillation but could not measure the separate fluxes.

But as of now, do experiments have enough technology or ingenuity to measure the separate fluxes for each of the three flavours of neutrinos? If yes, how do they distinguish between $\nu_e$, $\nu_\mu$ and $\nu_\tau$ events?


The limitations on SNO were not fundamentally technological, they were driven by simple energy concerns.

The neutrinos generated by solar fusions events are low energy (just a few MeV), and that simply isn't enough energy to create heavy charged leptons (you need more than 100 MeV to create a muon and more for a tauon).

That is why SNO could (and did) measure

  1. The electron neutrino flux (in the charged current channel)
  2. The total neutrino flux (in the neutral current channel)

but could not differentiate the muon and tau contributions to the non-electron component.

For higher energy neutrino sources (beam sources, the high energy component of atmospheric neutrinos, and the high energy components of primary cosmic neutrinos) the techniques of extrating good estimates of all three neutrino flavor fluxes are well developed and have been deployed in many distinct experiments (IceCube, Super-K, T2K, etc).

The different flavor are experimentally distinguished by observing what flavor of charged lepton is created in charged-current events (note that for energies not much above threshold a correction has to be applied for the different phase spaces available to different flavors).

  • $\begingroup$ So is it fair to say that unlike $n+\nu_e\to p+e^-$ used to detect $\nu_e$, there is no such straightforward process to detect and differentiate between $\nu_\mu$ and $\nu_\tau$? @dmckee $\endgroup$
    – SRS
    Oct 10 '19 at 13:30
  • 1
    $\begingroup$ No. Heavy-flavor charged-current reactions like $n + \nu_\mu \to p + \mu^-$ are a thing. They just require more energy than solar neutrinos have. That’s why beam experiments can disentangle all three contributions. $\endgroup$ Oct 10 '19 at 14:19

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