Neutrinos are some of the most mysterious particles in nature, capable of passing through the Earth like it wasn’t there. Neutrinos are tiny subatomic particles, often called ‘ghost particles’ because they barely interact with anything else.
Neutrinos are, however, the most common particle in the universe. Believe it or not, approximately 100 trillion neutrinos pass completely harmlessly through your body every second! Their tendency not to interact very often with other particles makes detecting neutrinos very difficult, but it does not mean that they never interact — the probability that any given neutrino will interact with another particle is just very small.
Neutrinos play crucial roles in the standard model of particle physics, in stellar physics and black holes, and even in cosmology and the nature of the Big Bang. On the family tree of particles, called the Standard Model, neutrinos belong to the family of particles known as leptons. There are three main leptons, namely electrons, muons and tau particles, and each one has an associated neutrino and anti-neutrino.
WHAT EXACTLY ARE THESE ‘GHOST PARTICLES’ ?
Neutrinos have no charge; they are neutral, as their name implies. And while the neutrino mass has yet to be precisely measured, we know it must be very small. At KATRIN(opens in new tab), the Karlsruhe Tritium Neutrino Experiment in Germany, scientists were able to measure the upper limit of the neutrino mass to be 0.8 electronvolts, or eV. (An electronvolt is the amount of kinetic energy acquired by an electron when it is accelerated through a potential difference of one volt).
While it might at first seem strange to be measuring mass using units of energy, Albert Einstein showed us how mass and energy are two sides of the same coin (as described by his famous equation, E = mc2), and extremely small particle masses are often given in eV because the kilogram conversion is so tiny (0.8eV is about 1.4 x 10–36 kg). To put that into context, neutrinos are about ten-thousand times less massive than electrons.
Neutrinos don’t interact at all with the strong nuclear force that binds atomic nuclei together, but they do interact with the weak force that controls radioactive decay. Hence this is how neutrinos are produced; the KATRIN experiment, for instance, measured the mass of neutrinos that resulted from the decay of tritium isotopes.
HOW WERE NEUTRINOS DISCOVERED ?
The conservation of both energy and angular momentum are two fundamental tenets of physics. You can’t produce energy out of nothing, and angular momentum can’t just vanish. Back in 1930, the famous quantum physicist Wolfgang Pauli realized that in order to maintain the conservation of energy and angular momentum in beta decay (in which an electron or its anti-particle, a positron, are emitted from a radioactive atom) it required the presence of a new type of particle with no charge, none or very little mass, and a quantum spin of 1/2. This new, theoretical particle was, of course, the neutrino.
It remained purely theoretical until 1955, when physicists Clyde Cowan and Frederick Reines of the Los Alamos National Laboratory led a team to detect neutrinos for the first time, coming from beta decay inside a nuclear reactor at the Savannah River Site in South Carolina. Their neutrino detector consisted of scintillating fluid and photomultiplier tubes and didn’t detect the neutrino directly. Instead, the detector watched for neutrinos interacting with protons in the fluid, the interactions producing positrons and neutrons.
The positrons annihilated when they encountered electrons, which are their antimatter equivalent, in the fluid. This annihilation converted all their mass into pure energy in the form of two gamma rays, while the neutrons also produced extra gamma rays when they were subsequently captured by another atom. The photomultiplier tubes were able to detect these gamma rays.
These neutrinos were being artificially produced, however, by the nuclear reactor. The first ‘natural’ neutrino to be detected was found in 1965 at an experiment deep underground at the East Rand goldmine in South Africa, but it wasn’t until the famous Homestake Mine detector was built that neutrino physics really came of age.
Homestake Mine, in South Dakota, was once upon a time the largest gold mine in the United States. Physicists John Bahcall and Ray Davis, Jr built an experiment deep in the mine to detect neutrinos coming from the core of the sun, where nuclear fusion reactions turn hydrogen into helium. To do so, Bahcall and Davis filled a tank in the mine with 100,000 gallons (454,600 liters) of a chlorine-rich dry-cleaning fluid — perchloroethylene to be precise. The methodology was simple — on the occasions that a neutrino interacted with an atom of chlorine-37, it turned into a radioactive isotope of argon-37, and by counting how many atoms of argon-37 had appeared every few weeks, Davis and Bahcall could calculate how many neutrinos from the sun had passed through the thank. Because it was 4,850 feet (1,478 meters) underground, the Homestake experiment was shielded from cosmic rays that could interfere with the results.
WHERE DO NEUTRINOS COME FROM ?
As we have seen, neutrinos are produced inside nuclear reactors on Earth and fusion reactions inside the sun. However, they are also produced much further afield. In February 1987 a star exploded as a supernova in the Large Magellanic Cloud, which is a small, nearby galaxy. The supernova, known as SN 1987A, was visible to the unaided eye. However, two to three hours before the visible light of the supernova reached us, a burst of neutrinos was detected coming from the dying star. Only a handful of neutrinos were detected at each detector around the world, but given how weakly neutrinos interact, the two-dozen detections was well above the background level and indicated a huge burst of neutrinos that had been produced as the core of the star collapsed. It was the first time that neutrinos had been detected coming from a supernova, and confirmed various theories about how massive stars end their lives.
Since then, neutrinos have also been detected coming from violent events around active supermassive black holes, such as those found in quasars and blazars. Neutrinos are also relevant to cosmology since primordial neutrinos that formed in the first second after the Big Bang are also prevalent in the universe — one estimate suggests there are about 300 Big Bang neutrinos in every cubic centimeter. These neutrinos from the Big Bang have been detected, as well as how they affect the size of baryonic acoustic oscillations in the cosmic microwave background (CMB) radiation. Therefore, understanding Big Bang neutrinos will help us to understand the CMB and the Big Bang itself better.