The experiment I work on, EXO, is one of many experiments studying the neutrino. While the neutrino rarely gets a title as lofty as the “God particle”, understanding its nature is an essential quest in modern particle physics. The neutrino isn’t a “rare” particle only produced in supernovae or accelerators – trillions of neutrinos from the sun are passing through your body every second. It is one of the particles we have known about for the longest, yet we seem to know the least about it.
The neutrino was first proposed by Pauli in 1930 to resolve issues in the understanding of nuclear beta decay. At the time it was known that in radioactive decays the nucleus was changing and that an electron was coming out. The neutron was not known at the time, so it was thought the nucleus was made up of protons and electrons, and the decay process was one of these electrons being emitted. However, that process didn’t make sense due to conservation of energy, momentum, and angular momentum. Pauli proposed a new particle which was neutral, had spin 1/2, and was about the same mass as the electron. This was the first particle proposed by theory and later found experimentally – essentially, the beginning of particle physics.
Eventually the nucleus was better understood and other particles (the muon, positron, mesons…) were discovered. Neutrinos were successfully “observed” in 1956, but were still not understood. It was thought that neutrinos were massless – like the photon. In the 1990’s it was accepted that neutrinos have mass and (therefore) one type (electron, for instance) changes to another (muon). It wasn’t until 2000 that the last predicted type of neutrino was observed – the tau neutrino.
So where do we stand today? We have measurements of the mass differences between types of neutrinos, but that doesn’t tell us the whole story. It is like knowing siblings are 3 years apart – you have no idea if they are both schoolchildren or grandparents. We don’t know if neutrinos are their own antiparticle, or if there are types of neutrinos we haven’t seen, perhaps because they don’t interact with ordinary matter. We know that massive neutrinos do not cleanly fit into the Standard Model, which means we may find new physics and new particles by studying them. I find neutrino physics one of the most interesting puzzles to study. I remember the excitement over the oscillation discoveries in the 1990’s, which was my first hint that there were still mysteries left in the Universe. I am optimistic that we will soon answer many of the main questions, such as making a direct measurement of the neutrino mass.
I am also hopeful that the neutrino will continue to surprise us. Neutrino physics will lead us to new technologies and discoveries. Since neutrinos barely interact with matter, new approaches have been developed to detect them. New detector technology will use neutrinos to monitor what is going on in a reactor. Changes in the neutrino spectrum would indicate a change in the fuel, which would indicate any illicit uses of a reactor. The neutrino has been used for decades to study solar physics, and large neutrino detection arrays allow astrophysicists to map the neutrino sources in the sky. It has been proposed that neutrino beams could be used to destroy nuclear weapons. Have no fear, we are very far away from having a neutrino death beam! But what haven’t we thought of yet? Could we develop a technology to create power from solar neutrinos? Could neutrino beams provide a treatment for cancer superior to what we currently achieve with other particle beams? I believe I will know much more about the neutrino within my lifetime, perhaps in time for my thesis!
