The most elusive of particles, the ghostly neutrinos, are again making the headlines. Yesterday, physicists from the OPERA experiment at Gran Sasso mountain in Italy announced that they witnessed a remarkable metamorphosis; a muon neutrino morphing into a tau neutrino. This apparently mysterious statement may hold the key to a whole new window into the universe.
Neutrinos have always been shrouded in mystery. They were predicted to exist by Austrian physicist Wolfgang Pauli in 1930 in a somewhat desperate attempt to save the most sacred of all laws in physics, the conservation of energy. Experiments at the time had noticed that certain kinds of radioactive decays involving electrons seem to have some energy missing. As some of the big names of the time, including Niels Bohr, were ready to give up on energy conservation, Pauli came up with this strange new particle that had no mass or electric charge: the piccolo neutron or “neutrino,” as Enrico Fermi called it. Their existence was confirmed only in 1956.
No one thought neutrinos could have a mass. But once again, from the 60s all the way to 2002, something weird was going on. The story starts right at the heart of the Sun. It turns out that our life-giving star is also a major neutrino factory. Every time hydrogen nuclei (i.e. protons) are fused together to become a nucleus of helium, neutrinos are produced. In fact, each one of us is bombarded by trillions of solar neutrinos per second! There are many ways to think of our connection to the Sun…
The problem was that theory predicted more neutrinos than what was being observed. Ray Davis had set up an experiment to “count” the neutrinos coming from the Sun as a way to check on the theoretical models. To everyone’s surprise, there were fewer neutrinos than expected, about 1/3 of the total. The knee-jerk reaction was to discount Davis’s experiment as being wrong; it was a very tough measurement. But with time, it became clear that Davis was counting things right. The fault was once again with the bizarre neutrinos that had a property few had anticipated.
Imagine if ice cream could change flavor from strawberry to vanilla to chocolate. Neutrinos are kind of like that. They come in 3 flavors and are able to morph between them. Electron neutrinos can change to muon neutrinos that can change to tau neutrinos. Muons and tau are particles related to electrons, similar in many ways but heavier. So, the reason Davis was counting less neutrinos was that his experiment was only sensitive to electron neutrinos: on their trip from the Sun to his detector, these neutrinos morphed into muon and tau neutrinos and passed uncounted. Davis shared the 2002 Nobel Prize for his discovery.
This bring us to the recent results at the OPERA experiment. Physicists claim to have witnessed (results need to be confirmed with more detailed statistics and validation) a direct metamorphosis of a muon neutrino into a tau neutrino. If confirmed, there will be no more doubt that neutrinos can change flavor. The exciting part is that this morphing can only occur if neutrinos have a mass. Since their prediction by Pauli, they were thought to be massless, like photons, the particles of electromagnetic radiation. Well, it looks like neutrinos do have oscillations between their three flavors and hence have masses, albeit tiny ones.
If neutrinos have mass, the model that accounts for all that we know of the properties of matter, the Standard Model of particle physics, must be amended. This is the first real window we have into new physics, pointing beyond what we know at present. Once again, a new tool has revealed new vistas into Nature.
The results also show that neutrinos may contribute to the dark matter in the universe, linking the very small to the very large. We have known for a while that there is more matter out there than what produces light in the form of stars and nebulae. Some of this matter is common stuff, like faint starts, planets and chairs, things that don’t shine on their own. But most of the matter out there, in a proportion of 6:1, is dark: not only it doesn’t produce its own light as it’s not made of electrons and proton, like ordinary matter. Massive neutrinos are a very strong candidate for dark mater, although they are probably not the only suspects. The hunt for what else is out there continues unabated.
So, the results are a cause for celebration. Once again our instruments allowed us to peer into the darkness that surrounds us. As the small flashes of energy detected as neutrinos morphed into one another, each discovery allows us to see a bit farther away. Science, as a candle in the dark.