In this, the third in a series of short essays about the structure of our universe, I will introduce you to a mysterious and elusive particle, the neutrino. This essay will do the work of three, since there are three different kinds of neutrinos. Their histories are closely linked, so it is easier to present a broad picture of this fascinating and indispensable fundamental particle.
The neutrino is a *lepton*, much like the electron and the muon. Unlike these other leptons, the neutrino is electrically neutral. It feels the presence of other matter through only the weak interaction and gravity; lacking an electric charge, it cannot communicate via electromagnetism, and lacking a color charge it is blind to the strong interaction. The fact that it can only interact through the weakest long-range force (gravity) and the weakest short-range force (weak interaction) makes it uniquely difficult to detect. The fact that it went unnoticed for so long is not a surprise. However, despite its slippery nature our universe would not function without it.
The history of the neutrino takes us back to the early days of the 20th century. The discovery of radioactivity led to a great number of experiments to understand this unique process, one with no analog in the macroscopic world. Atomic elements, produced in some quantity, would eventually decay away over time through the process of radioactivity. There were three kinds of radiations identified: alpha, beta, and gamma. Beta radiation became the vessel through which we glimpsed the presence of the unseen neutrino.
Beta radiation is just a fast electron emitted from an atomic nucleus. In 1911, Lisa Meitner and Otto Hahn studied the energy of this electron and found that it had a spectrum, rather than a definite value. This was very strange. Why? Electric charge was always conserved in every reaction, and it was believe that so was energy. Since radioactive decay involved transforming from a nucleus of one definite energy to a nucleus of another definite energy, you would expect the energy of the electron to be exactly the same every time beta radiation occurs. If the electron actually has a *spectrum* of energies – that is, a range of available energies – that means that energy is not conserved by the reaction!
This was a challenging discovery. The long-cherished and long-observed conservation of energy was rocked to its foundation. Could radioactivity violate this principle that once occupied the status of a law of nature? Not only that, the transformations between nuclei in beta decay appeared to violate the conservation of spin angular momentum. Momentum was another quantity long observed to be conserved in every interaction in nature. Nuclei and electrons have internal angular momentum, spin, which wasn’t conserved in nuclear beta decay.
In order to explain these violations of energy and momentum conservation, Wolfgang Pauli proposed in 1930 – 19 years after the Meitner and Hahn experiments – that there was an unseen particle as yet undetected in the nucleus. If another particle was produced with the electron it could carry energy, too, and explain the electron’s spectrum. From charge conservation, he hypothesized that it was neutral. It would be light, since its mass was constrained by the energy spectrum of the electron to be tiny, and be produced at the same time as the electron during beta decay. When James Chadwick announced the discovery of the neutron in 1932, a heavy neutral particle residing in the nucleus (and distinct from Pauli’s hypothesized particle), Pauli announced his idea. The name of Pauli’s particle was coined by Enrico Fermi. *Neutrino* is a play on the Italian word “neutrone”, or “neutron”. In italian, it means *little neutral one*.
Since no neutral particle was ever detected in the process of beta decay, it was clearly not easy to prove or disprove Pauli’s hypothesis. In fact, confirmation or refutation of this idea had to wait two decades. In 1956, Fred Reines and Clyde Cowan devised an experiment to identify what should have been the unique signature of the neutrino. Since the neutrino is produced by beta decay, there’s no reason you couldn’t expect that process to be *reversed*. In the intervening time since the 1930s, it had become clear that the reaction underlying beta decay was the transformation of a neutron into a proton and an electron. If the neutrino was real, it should also be produced by this decay. However, if the neutrino could then be made to pass through a large number of protons, one might expect it to react and produce a neutron and a positron, the electron anti-particle.
The seemingly simple realization that, if the neutrino was real then beta decay was invertible, meant that one could detect the neutrino. But there was no doubt it would be hard. The signature of the reaction – a prompt pair of gamma rays from the positron, a delayed burst of energy from the neutron capture – was unique, but expected to happen very rarely. However, Reines and Cowan succeeded and detected evidence for the presence of the neutrino. Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published the article “Detection of the Free Neutrino: a Confirmation” in Science, and received the 1995 Nobel Prize in physics for their work.
The neutrino detected by this first experiment was just one of three neutrinos that have, to date, been detected. In 1962, a second kind of neutrino was detected – the muon neutrino – by Leon M. Lederman, Melvin Schwartz and Jack Steinberger. How can you distinguish two kinds of neutrinos? Consider the beta decay reaction. The neutrino produced there, if it is a unique particle, will only ever participate in reactions involving electrons. Likewise, if the muon is associated with a different neutrino, reactions involving it will never produce anything but muons. When this was observed, the hypothesis that there were two unique neutrinos was confirmed.
Since the neutrino appeared to have at most a vanishingly small mass, it was assumed that it was completely massless. This was the simplest assumption, and it held until the 1990s. The story of the neutrinos mass goes back to 1964. It was at that time that Ray Davis first measured the flux of electron neutrinos emitted from the sun’s nuclear reaction. Comparing this to a prediction by John Bahcall and his solar model, the results seemed to indicate that the sun’s neutrino output was much less than expected. At first, this was taken to mean that Bahcall’s solar model was incomplete or inaccurate. But Bahcall pressured scientists to study this problem more closely. From this first set of experiments, neutrino astronomy – the study of the universe by looking for neutrino production by stars, galaxies, and even the big bang – was born.
In the late 1990s and early this century, experiments in Japan (Super Kamiokande and KamLAND) and Canada (SNO) revealed a new property of the neutrino: it could oscillate from one flavor to another. “Oscillation” is the process of one flavor, or definite state, of particle changing into another spontaneously. Quantum mechanical flavor oscillation was not a new phenomenon; it had first been observed in neutral K-mesons. What quantum mechanical oscillations mean is that there is a slight mass difference between the two definite states. But if neutrinos oscillate, and if that can only happen if there is a mass difference between different neutrinos states, neutrinos must have mass.
This seemingly simple revelation has huge consequences for the universe. Neutrinos contribute to the mass of the universe; their abilty to change flavor may mean they harbor an asymmetry between their matter and anti-matter states, an idea that will be tested in the next decade; their weak interactions with normal matter makes them sensitive to exotic processes at high energies, and may be our first glimpse into new principles of nature. We’ve really learned very little about the neutrino by comparison with other particles. But it wasn’t for lack of very hard work. To crack the secrets of the neutrinos, many hundred of physicists have made great technological leaps, pushing the edge of the scientific frontier further and further. The neutrino challenges us as humans, a riddle of nature, and invites us to try harder to learn more about it.
No doubt that as we continue to learn more about this elusive particle, we will gain more insight, and raise more questions, about our universe.
Riordan, Michael. “The Hunting of the Quark, a True Story of Modern Physics”. Simon and Schuster/Touchstone. 1987.
Segre’, Emilio. “Nuclei and Particles, an Introduction to Nuclear and Subnuclear Physics”. W.A. Benjamin, Inc. 1964.