Everything about this particle is a mystery, beginning with its very existence, which was theoretically predicted in 1930 by Wolfgang Pauli to solve a problem that was puzzling physicists: beta decay. It is a type of radioactive decay whereby an unstable atom emits an electron (or its antiparticle, a positron). Pauli had a compelling reason to affirm that it should exist, because if it were not so, one of the most sacrosanct laws of physics, the conservation of energy, did not apply to this type of radioactive decay. The new and hypothetical particle had to be in the nucleus of the atom and be emitted together with the electron when the disintegration took place. That led to another problem, because inside the nucleus there are no electrons. Where they came from?
In 1934 another of the greats of physics, Enrico Fermi, raised his theory of beta decay: a neutron in the nucleus disintegrated producing a proton, an electron and the Pauli neutrino. He sent an article explaining this model to the prestigious journal Nature , but it was rejected because the evaluators considered that it was an idea “very far from reality”. But he was right.
Of course, it is one thing to predict its existence and quite another to find it: the neutrino must have been quite elusive because no one had come across it before. More than two decades later, in 1956 , Clyde Cowan, Jr. and Fred Reines found the Pauli particle, a discovery that earned them the 1995 Nobel Prize . But only Reines could receive it; Cowan had died in 1974.
The neutrino already had its birth certificate, but what were its properties? It had no electrical charge , which is obvious, and it hardly interacted with matter . In fact, Cowan and Reines had to use a nuclear reactor that was emitting 50 trillion neutrinos per second per square centimeter and two 500-liter water tanks with 50 kilograms of cadmium chloride dissolved in them in order to detect it. Was it possible for something this singular to have mass? That was the million dollar question.
In 1967 a physical chemist named Raymond Davis decided to study the neutrinos that originate inside the Sun. By then it was already fairly well known what the Sun looked like; is the Standard Solar Model. Due to nuclear fusion reactions, each time four hydrogen nuclei become one helium nucleus, two neutrinos are produced, which immediately escape into space . Davis had an enormous number of neutrinos: the Sun produces more than 200 trillion trillion trillion of them every second. The problem was how to catch them , since they are capable of passing through a wall of lead several hundred billion kilometers thick as if it were air. Or put another way: of the neutrinos coming from the Sun we only detect one in 5,000 million… once they have crossed the Earth!
Such difficulty forced Davis to make two decisions . The first, use a very large detector. Davis filled a tank with 600 tons of perchlorethylene, a compound used in dry cleaning. The second is to bury the detector under tons of rock and, in this way, shield it so that nothing disturbs it because cosmic rays reach us from outer space , which produce an annoying background noise in the detector and mask the detection of neutrinos. But what Davis could not expect is that his experiment would only detect a third of the neutrinos predicted by the Solar Model . Had I done something wrong? Or was the theory wrong? Thus was born what has since been known as ‘the solar neutrino problem’.
Davis didn’t know he had the solution at hand long before this puzzle appeared . It had been provided in 1957 by an Italian physicist named Bruno Pontecorvo , who had defected to the Soviet Union seven years earlier. From an incomparable physical intuition, he predicted that there were different types of neutrinos , depending on the particle with which they were associated ( there are three types: the electron, the muonic and the tauonic neutrino ) and that they can change their suit and become another, a phenomenon known as the neutrino oscillation .
Inside the Sun, nuclear reactions produce electron neutrinos, and Davis’s team detected only that type. But what if on their way to the mine they changed, becoming one of the other two? In this way the Solar Model was saved and the problem of the scarcity of neutrinos was solved. The only ‘but’ was that it introduced a problem: for neutrinos to oscillate they must have mass (in fact, they do so with a frequency that is proportional to their mass) and the Standard Model of particle physics says they don’t. In other words, we have not solved the problem, but we have placed it in another place.
Was this the solution? At the beginning of the 21st century, the answer came from Canada . There is the Sudbury Neutrino Observatory (SNO) , which has a tank with 1,000 tons of heavy water (water made with deuterium, an isotope of hydrogen), buried in a nickel mine at a depth of 2,100 m. The main virtue of the SNO is that it is capable of separately observing the total number of all types of neutrinos. The SNO began to measure in April 1999 and in August 2001 it demonstrated by direct observation that electron neutrinos coming from the Sun were converted into muon and tauon neutrinos.
This opened the door to a mystery that, at the moment, theoretical physicists are unable to explain: if the oscillation of the neutrino implies that it has mass, what about the Standard Model, which says that it should not? And second, why is its mass so small?