To first approximation, neutrinos weigh nothing, interact with nothing and are impossible to detect. Why, then, would anyone have predicted their existence? Well, it all started with beta decay, a form of radioactive decay in which a nucleus of atomic number Z transforms to one of atomic number Z+1 and an electron is emitted. An example of beta decay is the decay of carbon-14 to nitrogen-14 used in archaeological dating: 6C14 → 7N14 + e–.
Beta decay takes place because the daughter nucleus has less mass than the parent, and therefore the decay is energetically favoured. By Einstein’s E = mc2, early nuclear physicists expected that the electron would carry off the difference in masses in the form of kinetic energy. However, it turned out that the electron always carried off less energy than expected, and instead of all electrons having the same energy, there was a continuous distribution, as shown in figure 1.
Figure 1: electron energy spectrum for beta decay of carbon-14. The red line marks the expected electron energy if only an electron were emitted. The blue line shows the observed electron energies.
This was a very unexpected result, as energy conservation is much beloved of all physicists. At first, nobody could think of an explanation (there were even suggestions that energy conservation did not hold at the atomic level) but in December 1930 Wolfgang Pauli wrote a famous letter (translated here) to a conference in Tübingen, in which he proposed the existence of a light neutral particle of spin 1/2 emitted alongside the electron in beta decay. This explains the continuous spectrum – the available energy is split between the electron and the undetected neutral particle – and also solves a couple of more technical non-conservation problems. Pauli originally called his particle the “neutron”, but when this name was given to the particle we now call the neutron (the proton-like neutral hadron discovered by James Chadwick in 1932), Fermi renamed Pauli’s particle the “neutrino” (Italian for “little neutral one”) – the name it still bears today.
The discovery of the neutrino
Fermi incorporated the neutrino into his ground-breaking theory of beta decay, published in 1934[1]. The success of this theory established the existence of the neutrino in the eyes of nuclear and particle physicists, but the particle itself remained elusive: indeed, Pauli worried that he might have postulated a particle which could never be detected (contrary to the principle that scientific theories should always be testable). Fortunately, the advent of nuclear fission in the 1930s and 1940s offered an unprecedentedly intense source of (anti-)neutrinos, which for the first time made the experimental detection of these elusive particles a realistic proposition.
Nuclear fission produces neutrinos because the fission of heavy elements makes isotopes that have too many neutrons to be stable. They therefore find it energetically favourable to convert the excess neutrons into protons, resulting in a cascade of beta decays. The enormous number of neutrinos produced greatly increases the chance of detecting some: as with winning the lottery, even though the chance of any given neutrino being detected (any given ticket winning the jackpot) is very small, if enough neutrinos are produced it is likely that some will be detected (if enough tickets are bought, at least one will probably win the jackpot).
During and after the Second World War, physicist Fred Reines was working at Los Alamos, involved in the testing of nuclear weapons. In 1951, as he describes in his Nobel lecture, he decided that he would like to do some fundamental physics, and after racking his brains for some months “all I could dredge up out of the subconscious was the possible utility of a bomb for the direct detection of neutrinos”. Teaming up with the experimentalist Clyde Cowan, he set to work on designing the necessary detector, which would have to be very large by 1950s standards – a whole cubic metre! (In contrast, the Super-Kamiokande neutrino detector has a volume of about 50000 cubic metres.) Not only would the detector have to be very large, it would also have to be capable of surviving in close proximity to a nuclear explosion, and would have only a few seconds in which to take data before the nuclear fireball dissipated. This presented a serious technical challenge!
The reaction Reines and Cowan planned to use was inverse beta decay, νe + p → e+ + n, detected using the newly developed technology of organic liquid scintillators. This reaction produces an initial prompt burst of light when the positron annihilates with an electron to create two gamma rays (high-energy photons). The neutron bounces around for a few microseconds and is then captured by an atomic nucleus, producing another gamma ray as the nucleus releases excess energy. It was with some relief that Reines and Cowan realised that this “delayed coincidence” of two signals separated by a characteristic time lag would greatly reduce the experimental background, making it possible to use the less intense neutrino flux from a nuclear reactor instead of having to rely on a bomb.
An initial experiment at Hanford in 1953 gave tantalising hints of a signal, but was plagued by a background level much higher than expected. Tests back at Los Alamos showed that this was due to cosmic rays, and could be reduced by locating the detector underground (so that the overburden of earth absorbs many of the cosmic rays). The detector was redesigned, incorporating cadmium to improve the efficiency of neutron capture – cadmium has a very high affinity for neutrons, which is why it is used in nuclear reactor control rods. The team also relocated to the new Savannah River reactor in South Carolina, where there was an available space conveniently located 11 m from the reactor core and 12 m underground. With a more powerful reactor, a more sensitive detector, and much improved shielding from cosmic rays, the signal improved to an unambiguous 3.0±0.2 events per hour. The neutrino had finally been observed.
Different types of neutrino
In 1956 there were two known charged leptons, the electron and the muon (the tau was not discovered until 1975), easily distinguishable by their different masses. Each of these also had an antiparticle with opposite electric charge. Therefore, the immediate questions following the experimental observation of the neutrino were:
- Is the neutrino distinguishable from its antiparticle?
- Is the neutrino produced in association with the muon, e.g. by π+ → μ+ ν, different from the neutrino produced in association with the electron, e.g. in beta decay?
The first question was answered very quickly. The heavier isotope of chlorine, chlorine-37, can convert to argon-37 by inverse beta decay, ν + Cl37 → Ar37 + e–. According to the law of lepton number conservation, which states that the total number of leptons minus the total number of antileptons is always constant, this reaction must involve a neutrino (not an antineutrino); in contrast, beta decay must involve an antineutrino (because an antilepton must be produced to balance the electron). Therefore, if the neutrino and antineutrino are different, reactor neutrinos will not convert chlorine-37 to argon-37.
Ray Davis, later to become famous for his work on solar neutrinos, investigated this process using carbon tetrachloride (CCl4) as the target and the Brookhaven research reactor as the source. Over the period 1955–1960 he was able to show that the probability of this reaction was less than 10% of that expected on the assumption that the neutrino and the antineutrino were identical. This work indicated that the neutrino and antineutrino were different particles, and that the law of lepton number conservation was obeyed in weak interactions.
The second question can be answered by investigating whether the neutrinos produced in pion decays, which are always associated with a muon (the pion decays by π → μ ν, never by π → e ν) can subsequently be converted into electrons. If they can, then the two types of neutrino are not distinct. This experiment requires an accelerator, because the energies involved in radioactive decays are not nearly high enough to produce pions or muons directly.
The experiment was carried out at Brookhaven in 1962[2]. The accelerator used was the Brookhaven AGS, which produced a beam of 15 GeV protons. These protons strike a target (beryllium in this case), producing pions which are allowed to decay in flight into muons and neutrinos. A steel shield, 13.5 m thick, then absorbs all the particles except the neutrinos. The result is a beam of muon-associated neutrinos with energies up to about 1 GeV (at higher energies there is also a contribution from neutrinos produced in kaon decay, which are not guaranteed to be associated with muons).
The experiment detected 34 muon tracks, with an estimated background of 5 from cosmic-ray muons. If the neutrinos produced by pion decay were identical to those produced in beta decay, they would therefore have expected to produce about 29 electron events (they would actually see about 20, because the experiment’s efficiency in detecting such events was about 2/3). If the neutrinos were different, they would expect to see perhaps one or two electrons produced by electron-associated neutrinos from kaon decays such as K+ → e+ + νe + π0. In fact, the events that were not muon-like were consistent with background from neutrons, and did not look like electrons (they had exposed their detector to a 400-MeV electron beam, so they knew what electron events should look like).
Therefore, by 1962 it appeared to be clear that neutrinos were different from antineutrinos, that neutrinos associated with electrons and muons (now called electron-neutrinos, νe, and muon-neutrinos, νμ, respectively) were different, and that the law of conservation of lepton number was obeyed separately for electrons and muons. These properties were well described by a model invented almost simultaneously by Lee and Yang, Landau, and Salam, in which the neutrinos had exactly zero mass.
Ironically, the advances in neutrino physics over the last 15 years or so have been entirely devoted to overturning all of these long-held certainties! We now know that neutrinos do have mass, and that neutrinos produced in association with electrons can subsequently interact as muon-neutrinos. We strongly suspect, but do not yet know for certain, that neutrinos and antineutrinos may not, after all, be distinct particles. This does not mean that the experiments described above were wrong, or that the Nobel Prize awarded to Leon Lederman, Mel Schwartz and Jack Steinberger for the Brookhaven two-neutrino experiment was undeserved. The masses of neutrinos are extremely tiny, and therefore the oscillation between different types (known as flavours) is very small under most experimental conditions. Similarly, the distinction between neutrinos and antineutrinos would be absolute if neutrinos were massless, and is very nearly so given that they are very nearly massless. Only very sensitive experiments, analysed with great care over many years, have allowed us to demonstrate these subtle effects.
Neutrinos and the Nobel Prize
Neutrino experiments are difficult and often ground-breaking. In (sometimes long-delayed) recognition of this, a number of pioneers of neutrino physics have been awarded the Nobel Prize for Physics.
| 1988 | Leon Lederman, Melvin Schwartz, Jack Steinberger | for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino |
| 1995 | Frederick Reines | for the detection of the neutrino |
| 2002 | Raymond Davis and Masatoshi Koshiba | for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos |
| 2015 | Takaaki Kajita and Art McDonald | for the discovery of neutrino oscillations, which shows that neutrinos have mass |
In addition, Wolfgang Pauli (1945), Enrico Fermi (1938), and Lee and Yang (1957), who made major contributions to neutrino theory, won the Nobel Prize for work not directly connected with neutrinos. Clyde Cowan did not share in the belated prize for the discovery of the neutrino because Nobel prizes are not awarded posthumously.
