We haven’t pinned down the masses of any individual neutrino, and we don’t even know which ones are heavier than the others. When it comes to our ability to collect raw data, neutrinos present a triple threat: they’re incredibly lightweight (even the electron weighs over 5 million times more than all the neutrinos combined), they shift their identity as they travel (and their rate of flavor oscillation changes as they travel through different substances, so there’s no one-size-fits-all solution), and they barely interact with anything in the first place…
Scientist: Made up a particle.
Also scientist: Can’t find that made up particle.
(This is not related to the post, it’s just something that pops up in my mind that I think is funny).
Technically all particles are “made up”, since the effects of the energy they carry can also be expressed as a waves too and altogether, indeed their nature in our current model is dual, see the utterly interesting double slit experiment.