The Present State Of Neutrino Masses Essay

, Research Paper

One of the current questions in physics is whether or not neutrinos have mass and what this mass is. Neutrinos are subatomic particles that have no electrical charge and interact only via the weak nuclear force. They are products of radioactive decay processes, and thus are produced abundantly in our Sun, our atmosphere, and in other astrophysical sources such as supernovae and active galactic nuclei. Millions and millions of them are crossing through the Earth every second, but only very few of them will interact with the Earth. In practice you can say they are invisible. But fortunately we can detect them by building a very large detector and waiting long enough.

There are several reasons to search for a possible non-zero neutrino mass. Fermion masses in general are one of the major mysteries/problems of the standard model. Observation or nonobservation of the neutrino masses could introduce a useful new perspective on the subject. Nonzero neutrino masses are predicted in most extensions of the standard model. They therefore constitute a powerful probe of new physics. Also, there may be a hot dark matter component to the universe. If so, neutrinos would be (one of) the most important things in the universe. The observed spectral distortion and deficit of solar neutrinos is most easily accounted for by the oscillations/conversions of a massive neutrino.

The largest neutrino detector is the Super-Kamiokande and is located in the Kamioka Mine, about 200 km north of Tokyo. It is water cerenkov detector, which means it is a large (40 meters diameter by 40 meters tall) tank of ultra-pure water viewed by thousands of sensitive phototubes. Super-Kamiokande will address some of the most important open questions in physics today, such as: why does the Sun appear to produce only half as many neutrinos as theory would predict? Do neutrinos have mass? Do protons decay, as predicted by Grand Unification Theory?

One source of neutrinos are nuclear reactions. Inside our Sun nuclear reactions are occurring on a gigantic scale. Lots of neutrinos are produced. There are enough of them, that when they reach the Earth they can still be detected. Since physicists can calculate how many of them should be seen, there is a big problem because we see too few, roughly two times too few. This is so called the solar neutrino problem.

There can be several solutions to the puzzle. One is that we do not understand the Sun well enough. We may be predicting the wrong number of neutrinos produced inside the Sun. This is ruled out easily – we know exactly how much energy is produced by the Sun, and we know how many neutrinos we should expect per unit energy. But a modified version of this explanation is more difficult to reject: we may be predicting the wrong shape of the neutrino energy spectrum. It turns out that the efficiency of our detectors drops down very quickly as we go down in neutrino energy. So measuring the shape of the neutrino energy spectrum is important.

Another explanation is that there is something about neutrinos that we do not know about. This something, which many people suspect may solve the problem, is neutrino oscillations. There are three types of neutrinos in nature: electron-, muon-, and tau-neutrinos. Inside the Sun, electron-neutrinos are produced. We know that all the neutrinos are light particles, possibly massless. But if they have some (small) mass, there is a possibility of ‘mixing’ between them if the so-called ‘mixing angle’ is non-zero. Mixing (and oscillations) of particles is nothing new in physics, it has been observed a long time ago in the neutral kaon system. The idea is that an electron-neutrino on its way from the Sun to the Earth can transform itself into e.g. a muon-neutrino and which will escape detection.

Another problem physicists are having is with atmospheric neutrinos. These are produced by energetic cosmic ray particles. The particle after entering the atmosphere interacts with air atoms and produces several other particles, which in subsequent interactions with air produce even more particles, and so on. It is called a cosmic ray shower. Some of the produced particles are unstable. They are mostly pions which decay into muons, which then decay to electrons, which are stable and don’t decay any further. At each of these decays neutrinos are produced. They are electron-and muon-neutrinos (and anti-neutrinos). The problem is that we can calculate how many neutrinos of each type are created. We can detect them in our detectors and compare the measurement with the prediction, and they disagree.

This could be also explained by neutrino oscillations. Another explanation is that maybe we do not understand interactions of neutrinos with our detectors. The trouble is that neutrino interactions are made complicated by the fact that most of the neutrons and protons are bound together in oxygen nuclei in water.

The ever-so important question of whether or not neutrinos have mass and how much mass they do have will not be answered tomorrow. More data has to be taken, the theories have to tweaked, and many physicists must continue to work together.