Almost everything that surrounds neutrinos is a mystery, including their very existence. In 1930, the Austrian physicist Wolfgang Pauli predicted it to solve a problem that led his colleagues headlong, beta decay, a radioactive phenomenon in which an unstable atom emits either an electron or its antiparticle, a positron. Pauli gave a compelling reason for saying that neutrinos had to exist : otherwise, one of the most sacrosanct laws of physics, the conservation of energy, would not work for this type of decay. This peculiarity, on the other hand, was precisely what Niels Bohr, one of the fathers of quantum theory, defended. That new – and still hypothetical – particle had to be found in the nucleus of the atom, and it would be emitted along with the electron when decaying took place. There was, however, another problem: inside the nucleus there are no electrons, so where did it come from?
Two decades just to prove they really existed
In 1934, another of the greats of physics, Enrico Fermi, raised his own theory about beta decay: a neutron in the nucleus disintegrated, producing a proton, an electron, and a neutrino. The Italian sent an article explaining this model to the journal Nature , but it was rejected. It was claimed that it was something “very far from reality.” The little interest that his proposal aroused caused Fermi to abandon theoretical physics. Meanwhile, that same year, Bohr’s idea that energy could not be conserved in the subatomic world was losing the battle. And it is that if this were the case, there should not be a limit to the energy of the electrons that left the nucleus; just what could be measured experimentally. One way or another, the Pauli neutrino had to exist.
But it is one thing to say it and quite another to try it. The neutrino was very elusive until 1956, when two subatomic particle experts, Clyde Cowan and Fred Reines, found it at the Savannah River atomic power plant in South Carolina. The finding earned Reines the Nobel Prize in 1995 ; Cowan had died twenty-one years earlier.
The neutrino already had its birth certificate, but what were its properties? It had no electrical charge, which was obvious, and it hardly interacted with matter . In fact, it was so elusive that, to detect it, Cowan and Reines used a nuclear reactor that emitted fifty billion of them per second per square centimeter and two 500-liter water tanks with fifty kilos of cadmium chloride dissolved in them. The million dollar question was whether something this peculiar could have mass. The issue could not be resolved until Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Canada, this year’s Nobel winners, came into play. In order to understand his extraordinary work, we must first look at the sun.
In 1967, a physicist-chemist named Raymond Davis, who had joined the Brookhaven National Laboratory in the US to find peaceful applications for nuclear energy, decided to study the neutrinos that originate inside the sun king. By then, the standard solar model, which describes the interior of our star, was well established. One of its architects was the American astrophysicist John Bahcall.
Neutrinos pass through matter like a mirage
Due to the nuclear fusion reactions that take place in the heart of the star, every time four hydrogen nuclei turn into one helium nucleus, two neutrinos are born, which immediately escape into space. More than two hundred trillion trillion trillion of them originate from the Sun every second , so measuring the amount of neutrinos reaching Earth from it seemed like a good way to test Bahcall’s theoretical calculations. Similarly, determining how many the sun actually emits would allow astrophysicists to improve their model. The problem with such an illustrious proposal is that neutrinos 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 nuclear reactions that take place inside our star, we only detect one in every 5,000 million … and that, once it has crossed the Earth!
Such difficulty forced Davis to make two decisions. The first is to use a very large detector. So he filled a tank with six hundred tons of perchlorethylene, a compound used in dry cleaning. The second decision was to bury the detector under tons of rock . With this, he tried to avoid the disturbances caused by the cosmic rays that reach us from outer space. This high-energy radiation produces annoying background noise in instruments used to detect neutrinos. Something similar happens when we want to chat with a hoarse friend in a crowded bar: the sound of other conversations prevents us from hearing their voice. Davis placed his detector 1,500 meters underground in the abandoned Homestake gold mine in South Dakota. It was a shocking situation: to observe solar neutrinos you had to bury yourself under tons of rock.
Under these conditions, Davis and Bahcall began their experiment. The surprise came in 1968, with the first results: they detected only a third of the neutrinos predicted by the latter’s model. Their first reaction was to think that they had done something wrong : either the theoretical calculations were not as good as believed or the essay was flawed. Davis thoroughly checked the experimental design, and Bahcall ran through the accounts over and over again; everything was fine. Thus was born what is known since then as the problem of solar neutrinos. What the hell was going on in the Sun?
Interestingly, the solution was known long before the enigma . It had been provided in 1957 by the Italian physicist Bruno Pontecorvo, who had settled with his family in the Soviet Union. Pontecorvo proved to have an incomparable scientific intuition. Among other great ideas, he showed Reines and Cowan the path to the Nobel, by suggesting how to detect the neutrinos that are produced in nuclear reactors. Furthermore, he predicted that neutrinos associated with electrons differ from those that accompany other particles, such as muons, and proposed that they can change suits, so to speak, and become other types of neutrinos, a phenomenon known as oscillation.
Well, what happens inside the Sun is that nuclear reactions produce electronic neutrinos , so Davis’s team only detected those of that type. But what if on their way to the Homestake mine they changed and became one of the other two? That hypothesis saved Bahcall’s solar model, which worked very well, and solved the neutrino shortage issue. However, it introduced a formidable 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 ensures that they do not. Was the neutrino oscillation the solution?
To find out, it was necessary to confirm that less than expected was actually received. In the early 1980s, Japanese physicist Masatoshi Koshiba proposed building a detector inside an old mine near the city of Kamioka – today Hida. He called it Kamiokande II. In essence, it was a huge tank of water surrounded by photoreceptors, sensors capable of capturing the faint flashes of light that appear when a neutrino collides with the nucleus of one of the atoms of water molecules. The underground laboratory went unnoticed until 1987, when it suddenly became famous .
The standard model of particle physics, challenged
On the night of February 23-24 of that year, one of the scientists at the Las Campanas astronomical observatory in Chile spotted a bright spot in the Large Magellanic Cloud, a nearby galaxy. He had just discovered the first supernova visible to the naked eye since 1604. That same day, the Kamiokande’s detectors went off unexpectedly twelve times . At the same time, another detector buried in a salt mine near Faiport, Ohio, counted eight neutrinos; and a third, located under Mount Andyrchi, in the Caucasus, recorded five. After hundreds of thousands of years of travel, the enormous flux of neutrinos from that stellar explosion had swept across the Earth. What happened gives us an idea of the difficulty involved in studying neutrinos: of the ten trillion trillion that were produced in the supernova, twenty-five were detected .
In 1989, Koshiba confirmed Davis’s results. It remained to be verified that Pontecorvo’s ideas about the neutrino oscillation were real. To do this, Koshiba coordinated the construction of an even more sensitive detector, equipped with 50,000 tons of water and more than 11,000 sensors. The Super-Kamiokande , as it was called, began to function in 1996. It was then that one of Koshiba’s collaborators, the also physicist Takaaki Kajita, took over. In 1998, he discovered that when cosmic rays hit the atmosphere, the muon neutrinos that are generated oscillate – change suits – before reaching the detector. It was the experimental confirmation that everyone expected .
Four years later, physicist Arthur McDonald made a new contribution from Sudbury, Canada. There is the SNO detector (Sudbury Neutrino Observatory), buried in a mine at 2,100 meters deep . The main virtue of the SNO, which contains a thousand tons of heavy water – water made with deuterium, an isotope of hydrogen that has a neutron in its nucleus – is that it is capable of separately capturing the total number of electron neutrinos and that of all the types of neutrinos. In 2001, the SNO showed that electron neutrinos from the Sun turn into muonic and tauonic neutrinos . In this way, the problem of solar neutrinos was explained, although, as often happens, this generated others that, for the moment, theoretical physicists are unable to explain: if the oscillation of the neutrino implies that it has mass, does it why is it so small? And, above all, what will happen to the famous standard model, which claimed that it should not have it?
Images: Super-Kamiokande (Kamioka Observatory / ICRR); supernova SN1987A (ESO / L. Calçada); SNO detector (Snolab).
