Okay, I’ve drafted an update to my list of open questions in physics.
I eliminated a bunch of questions that seem to have been answered. It’s really remarkable how accelerator experiments in the last decade or so have settled questions in particle physics without discovering any new mysterious phenomena. The really big mysteries remain.
I have not gotten around to adding the new questions about black holes raised by LIGO. I have not gotten around to updating the sections on ultra-high energy cosmic rays or gamma ray bursters, both of which sorely need it. But I have updated the section on neutrinos!
Here’s the new version. I still need some more good new general reviews of neutrino experiments and theoretical questions. Do you know some?
What’s going on with neutrinos? Why are all the 3 flavors of neutrino—called the electron neutrino, the muon neutrino and the tau neutrino—so much lighter than their partners, the electron, muon, and tau? Why are the 3 flavors of neutrino so different from the 3 neutrino states that have a definite mass? Could any of the observed neutrinos be their own antiparticles? Do there exist right-handed neutrinos—that is, neutrinos that spin counterclockwise along their axis of motion even when moving very near the speed of light? Do there exist other kinds of neutrinos, such as “sterile” neutrinos—that is, neutrinos that don’t interact directly with other particles via the weak (or electromagnetic or strong) force?
Starting in the 1990s, our understanding of neutrinos has dramatically improved, and the puzzle of why we see about 1/3 as many electron neutrinos coming from the sun as naively expected has pretty much been answered: the three different flavors of neutrino—electron, muon and tau—turn into each other, because these flavors are not the same as the three “mass eigenstates”, which have a definite mass. But the wide variety of neutrino experiments over the last thirty years have opened up other puzzles.
For example, we don’t know the origin of neutrinos’ masses. Do the observed left-handed neutrinos get their mass by coupling to the Higgs and a right-handed partner, the way the other quarks and leptons do? This would require the existence of so-far-unseen right-handed neutrinos. Do they get their mass by coupling to themselves? This could happen if they are “Majorana fermions“: that is, their own antiparticles. They could also get a mass in other, even more exciting ways, like the “seesaw mechanism“. This requires them to couple to a very massive right-handed particle, and could explain their very light masses.
Even what we’ve actually observed raises puzzles. With many experiments going on, there are often “anomalies”, but many of these go away after more careful study. Here’s a challenge that won’t just go away with better data: the 3×3 matrix relating the 3 flavors of neutrino to the 3 neutrino mass eigenstates, called the Pontecorvo–Maki–Nakagawa–Sakata matrix, is much further from the identity matrix than the analogous matrix for quarks, called the Cabibbo–Kobayashi–Maskawa matrix. In simple terms, this means that each of the three flavors of neutrino is a big mix of different masses. Nobody knows why these matrices take the values they do, or why they’re so different from each other.
For details, try:
• Paul Langacker, Implications of Neutrino Mass.
• A. Baha Balantekin and Boris Kayser, On the Properties of Neutrinos.
• Salvador Centelles Chuliá, Rahul Srivastava and José W. F. Valle, Seesaw Roadmap to Neutrino Mass and Dark Matter.
The first of these has lots of links to the web pages of research groups doing experiments on neutrinos. It’s indeed a big industry!