1930, physicist Bao Li imagined an unobservable new particle to solve the problem of energy conservation in β decay. This idea aroused the interest of theoretical physicist Fermi. ...

The first part of this series: unpredictable but ubiquitous particle-neutrino (1)

Fermi theory: Fermi, a neutrino that can be produced or destroyed out of thin air, adopts the speculation of some people at that time and assumes that the nucleus is composed of protons and neutrons (just discovered not long ago); As for electrons in beta decay and hypothetical neutrinos, they did not exist in the nucleus originally, but were produced in the process of neutron transformation into protons. In other words, beta decay involves the following reactions:

Neutron → proton+electron+(anti) neutrino [1]

Fermi refers to the phenomenon that charged particles can emit photons, so that theoretically described particles can be produced or destroyed out of thin air! This was a breakthrough concept at that time. No wonder the editor of Nature thought this paper was full of speculation and refused to submit it.

Beta decay was originally thought to release only electrons (upper left), but later in Fermi's theory, it was described as neutron decay into protons, plus electrons and antineutrons (lower right). (Image source) In theory, you can wear everything thoroughly! Can neutrinos really be observed? Although Bao Li's proposal and Fermi's theory seem to perfectly explain the problems caused by beta decay, as long as neutrinos are not observed, everything is an attic in the air and nothing is solved. Scientists began to think about whether it is possible to prove the existence of neutrinos through experiments.

1934, Bert [2] and Pailes [3] estimated the possibility of detecting neutrinos. Compared with the fact that alpha particles (helium nuclei) can be blocked by a piece of paper, beta particles (electrons) need to be shielded by aluminum sheets with a thickness of several millimeters, and gamma rays even need lead with a thickness of one centimeter or concrete with a thickness of six centimeters to reduce the intensity by about 50%. However, they found that neutrinos can travel 10- 16 km (1kloc-0/6 km) in general solids [4], which is about 2.2 million times the distance from Neptune to the sun, that is, 1000 light years! Obviously, no experimental instrument can capture such a strong penetration.

Bette and Peierls came to the conclusion that there is no practical way to observe neutrinos!

Three different main decays have different penetrating power. (Image source) Neutrino observation project: In the spirit of buying lotto! The so-called method was invented by people. Even if neutrinos can pass through almost any object without hindrance, as long as there are enough neutrinos, there is always a chance to see neutrinos react with other substances-just like it is difficult to win the lottery, but as long as you buy more, you will still win more or less.

195 1 year, Raines [6], who worked with Feynman [5] in the Manhattan Project, invited Cowen [7], a colleague of Los Alamos National Laboratory, to carry out a neutrino observation project together. At first, they wanted to dig a deep well, place a detector only 40 meters away from the explosion point of the nuclear test, and use a large number of neutrinos produced by the experimental explosion to improve the detection probability. But considering that the explosion is only a short one or two seconds, once it fails, we have to wait for the opportunity again; Background noise from neutrons and gamma rays is quite high, which makes it more difficult to collect data.

Finally, the two decided to study near the nuclear reactor instead: although the number of neutrinos is much less than that of nuclear explosions, the source is stable-it is estimated that only a few neutrino reaction cases can be detected every hour, but it will be enough to wait a few months and accumulate more data.

Raines and Cowen originally wanted to dig a hole only 40 meters away from the nuclear explosion point and conduct neutrino detection experiments. (Image source) Searching for Neutrinos by Nuclear Power Station1At the end of 955, reines and Cowen set up experimental instruments near the nuclear reactor at the savannah river site in South Carolina. They put cadmium chloride (CdCl? ) is dissolved in 1400 liters of water, in which theoretically about ten trillion neutrinos pass through the water meter area per square centimeter. And if an antineutrino passes by, the protons in the water will react with the antineutrino, producing positrons and neutrons (anti-β decay):

Antineutrino+proton → positron+neutron

Positron will immediately annihilate with electrons in water, releasing two gamma-ray photons; Neutrons will be captured by cadmium nuclei in the next few millionths of a second, and gamma rays will also be produced. Therefore, if the experimental detector sees two different scintillation signals in a short time ── it means that (anti) neutrinos have been observed.

Antineutrinos interact with protons in water to produce anti-β decay, and the generated positrons and neutrons will also emit photons due to annihilation and capture respectively. (Image source) Bao Li, you lost the bet! 1956, Raines and Cowen sent a telegram to Paulie, who was meeting at the European Institute of Particle Physics, informing him that neutrinos had been discovered. Bao Li read the telegram, immediately interrupted the meeting, excitedly read the contents of the telegram to others and delivered a speech. Not only that, because Bao Li once bet with astronomer Bud that neutrinos would never be detected by human beings-now he has to give up the bet and buy Bud a case of champagne.

39 years later, Raines was awarded the Nobel Prize in Physics for discovering neutrinos. Cowen, on the other hand, died young and missed this meaningful moment. With neutrinos confirmed, the suspense caused by beta decay can finally be put down-it would be boring if things developed like this.

The second neutrino is 1962. Lederman [9], Schwartz [10], Stajnberger [1 1] and others used π meson decay to generate neutrino beams from the particle accelerator in Brookhaven National Laboratory, USA, and confirmed that they were the same as 65438+. However, neutrinos discovered by Lederman and others appear in another particle-muon correlation reaction; So it was later called electron neutrino and muon neutrino respectively.

Because Lederman, Schwartz and Steinberg's method of generating neutrino beams can help scientists better study the weak interactions involved in neutrinos; Also, because they discovered new neutrinos, they let future generations know more about the pairing relationship between two different neutrinos and electron muons. These three people won the 1988 Nobel Prize in Physics.

The main decay of π mesons produces anti-muons and neutrinos corresponding to muon-muon neutrinos. (Source: New particles heavier than muons.

Since the discovery of muons in 1930' s, it has taken decades for people to gradually understand that muons are very close to electrons (only with large mass) in nature-they are later classified as "leptons" and have two kinds of neutrinos. So some people speculate that there will be leptons heavier than muons?

197 1 year, Yung-su Tsai, an American scientist and professor of Stanford University, who was born in Taiwan Province Province, China, published a paper to discuss the possible effects of heavier leptons in the experiment, which led to the changes from 1974 to 1977, and Tao Zi (. Pearl [12], the discoverer of Tao Zi, therefore shared the 1995 Nobel Prize in Physics with Raines, who discovered neutrinos.

Since electrons and muons have neutrinos, Tao Zi should be no exception-most people are so sure. But after a long time, it was not until 2000 that Tao Zi neutrino was officially discovered.

According to the current standard model of particle physics, leptons are composed of three families, namely charged electrons, muons and Taozi, and corresponding (electrically neutral) neutrinos. Because there is strong evidence that the mass of neutrinos must be very small, even if it is not zero, the standard model directly sets the mass of neutrinos to zero, and there is no mass source mechanism of neutrinos.