With only 150,000th of the mass of an electron, this mysterious "invisible man" can easily penetrate our bodies!

Neutrinos: The mysterious "invisible man"

Neutrinos are a type of elementary particle that is uncharged, small in size, and interacts very weakly with other matter. They are known as the "invisible men" in the universe.

It took more than 20 years for the scientific community to predict the existence of neutrinos and discover them. Ordinary people have never seen the "real body" of this mysterious particle, but in fact it is around us all the time. It can easily penetrate any material, and 100 trillion neutrinos from the sun pass through everyone's body every second. Of course, we are unharmed because they are too small!

Most particle physics and nuclear physics processes are accompanied by the production of neutrinos, such as nuclear reactor power generation (nuclear fission), solar luminescence (nuclear fusion), natural radiation, supernova explosions, cosmic rays, etc.

Since neutrinos interact very weakly with other matter, it is very difficult to detect them. For this reason, neutrinos are the last and least understood of all elementary particles. In fact, most particle physics and nuclear physics processes are accompanied by the production of neutrinos, such as nuclear reactor power generation (nuclear fission), solar luminescence (nuclear fusion), natural radiation, supernova explosions, cosmic rays, etc.

Small particles that shake up the universe

The interaction between neutrinos and other substances is so weak, so is it useless to study neutrinos? The answer is not! For example, the study of neutrinos has made neutrino astronomy a very cutting-edge and popular research field. Take the "martial arts secret book" in the hands of astronomers - the cosmic microwave background radiation, which is actually the residual heat of the Big Bang. It only began to spread in the universe 380,000 years after the Big Bang. In other words, through the cosmic microwave background radiation, humans can know the history of the universe 380,000 years after the Big Bang, but we have no way of knowing the history of the first 380,000 years, which can be achieved by observing neutrinos.

The observation of neutrinos can help us better understand the physical phenomena of microscopic time, and at the same time help us improve the relevant models of stars and black holes in astronomy, and help us grasp more information about the origin and evolution of the universe.

Do neutrinos have mass?

Mass, a physical property that we usually don't pay much attention to, is one of the most difficult mysteries of neutrinos. Do neutrinos have mass?

Sudbury Neutrino Observatory in Canada (Image credit: Sudbury Neutrino Observatory)

In 2012, the international collaboration of the Daya Bay Neutrino Experiment, led by the famous Chinese physicist Wang Yifang, announced the discovery of a new type of neutrino oscillation, which showed that all known neutrinos have mass. The 2015 Nobel Prize in Physics was awarded to Japanese physicist Takaaki Kajita and Canadian physicist Arthur B. McDonald for their discovery of neutrino oscillations, which is a direct manifestation of neutrino mass. In 2020, scientists announced that the mass of neutrinos does not exceed 1.1 electron volts, which is about one 150,000th of the mass of an electron.

Neutrinos come in three flavors

So, how do we measure the mass of neutrinos? This is a very complicated question! Why?

The experiments conducted by Takaaki Kajita's team and the research team led by Arthur McDonald jointly discovered a new phenomenon of neutrino oscillation, proving that neutrinos, which had long been believed to be massless, actually have mass.

Schematic diagram of the three types of neutrinos

Then, these three types of neutrinos (electron, muon, tau neutrino) also correspond to three types of neutrino masses (the first, second, and third mass eigenstates). However, there is not a one-to-one correspondence between them, but there is a certain "mixing". For example, the electron neutrino "mixes" the first, second, and third mass eigenstates together in some way.

There are still many questions about neutrinos that scientists need to solve. I believe that after cracking these "secrets", physics will surely make greater progress!

Editor-in-charge/ Gao Lin

Art Editor/ Wang Chen

Author: Chen Si (Dalian Institute of Chemical Physics, Chinese Academy of Sciences)

Source: Knowledge is Power Magazine