Neutrinos are mysterious particles that are difficult to analyze, yet they may hold the key to unlocking some of the universe’s greatest secrets. Scientists are revealing the secrets of neutrinos by using accelerators to create neutrino beams.
Neutrinos are among the universe’s most numerous particles, although they rarely interact with matter. By researching neutrinos and detecting their interactions with matter, some of today’s outstanding scientific riddles, such as why there is more matter than antimatter in the cosmos, could be addressed.
Every second, billions of neutrinos from natural sources such as the Sun whiz past every square centimeter of the Earth. Scientists, on the other hand, are unable to ascertain their initial type or how far they traveled before reaching a detector.
Scientists use proton accelerators to create high-intensity neutrino beams to better investigate neutrinos. Only a few institutions in the world are capable of producing such neutrino beams: Japan’s J-PARC laboratory, Europe’s CERN research center, and the United States’ Fermi National Accelerator Laboratory. Fermilab launches a trillion neutrinos every two seconds toward particle detectors in northern Minnesota, more than 450 miles away. In the detectors, this powerful beam causes around a thousand neutrino interactions every year.
Scientists start with batches of protons from a bottle of hydrogen gas to create high-intensity neutrino beams. They accelerate each batch to nearly the speed of light before smashing it into a graphite or beryllium target. Protons fracture the atomic nuclei of the target, producing new particles such as short-lived pions, which are the source of neutrinos and anti-neutrinos.
The pions are redirected in roughly the same direction by powerful focusing horns, which produce strong, precisely aligned magnetic fields, resulting in a meter-wide beam of either positively or negatively charged pions. Positively charged pions decay into anti-muons and muon neutrinos after a fraction of a second; negatively charged pions decay into muons and muon anti-neutrinos after a fraction of a second.
The final phases in creating a pure neutrino (or anti-neutrino) beam are aluminum, steel, and concrete blocks. Except for ghost-like neutrinos (or anti-neutrinos), which pass through the blocks unharmed, the blocks stop and absorb all particles. That’s it!
Many particle physics experiments rely on neutrino and anti-neutrino beams. They allow scientists to investigate neutrino interactions with other particles, determine how one type of neutrino oscillates into another, distinguish between neutrino and anti-neutrino behavior, measure the mass differences between the three types of neutrinos that exist, and search for new types of neutrinos that could emerge from neutrino oscillations.
For more than 50 years, scientists have used proton accelerators to create neutrino beams. The Nobel Prize-winning discovery of the muon neutrino was made possible by an experiment at Brookhaven National Laboratory in 1961.
Scientists intend to generate better neutrino beams in the future by employing muons instead of pions. The muon is an electron’s heavier relative. It produces both a muon neutrino and an electron anti-neutrino when it decays. NuSTORM is a planned project that intends to create a neutrino beam from muon decays. Muons are easier to accelerate and focus than pions since they live roughly 100 times longer, but they also travel a longer distance before decaying. The goal is to generate and collect enough muons, then propel and store them in an accelerator ring until they decay.
There are also methods for producing neutrinos without the use of accelerators. Clyde Cowan and Frederick Reines discovered the electron anti-neutrino in a nuclear reactor at the Savannah River Plant in 1956. Nuclear reactors produce just anti-neutrinos, and only one type: electron anti-neutrinos, in contrast to accelerators, which can produce both neutrinos and anti-neutrinos. That’s ideal for experiments like China’s Daya Bay experiment, which investigates electron anti-neutrino oscillations across short distances.
Nuclear reactors are not practical for neutrino oscillation experiments that transmit neutrinos hundreds of kilometers into the Earth. Unlike accelerator-produced focussed neutrino beams, anti-neutrinos from a nuclear reactor’s core go in all directions, similar to how a light bulb shines light in all directions.
Some scientists are already considering how neutrino science could be applied to other fields. Perhaps neutrinos will one day be used to communicate in regions where radio waves can’t reach, such as deep-sea submarines or satellites going across the Moon’s far side. This would necessitate the development of even better neutrino beams as well as supersensitive neutrino detectors.
A group of experts demonstrated what would be required to make this happen earlier this year. They sent a short, encoded message through 240 meters of rock using a neutrino beam at Fermilab. The scientists detected and interpreted the message, which read “neutrino,” using the MINERvA neutrino detector. It took roughly 90 minutes to send this basic message 240 meters using the world’s highest powerful neutrino beam.
Scientists may one day develop more efficient devices to harness neutrinos’ power in novel and exciting ways.