The Science Fiction World of Xueba

For the next month, Pang Xuelin locked himself in the expert building of the Institute of High Energy Physics, Chinese Academy of Sciences, and studied the basic physics of this world and the neutrino theory brought out from the world of "Whale Song".

From this point of view, the harvest is not small.

In the world of "Earth Cannon", the most widely used field of neutrino technology is neutrino communication.

In the world of "Whale Song", the most widely used neutrino technology is in the field of detection. Scientists use neutrino detectors to detect drugs in various interlayers.

In principle, both use high-energy proton accelerators to accelerate protons to obtain high-energy electron beams of hundreds of billions of electron volts. It is then used to bombard the target, producing unstable particles.

These particles undergo continuous changes, eventually forming neutrinos and other particles, and then passing them through thick steel plates, sieving out the charged particles, and obtaining a beam of uncharged neutrinos.

Neutrino communication is to let these neutrinos pass through the water. At that time, the water will emit blue light, which can be received by a photomultiplier to obtain information.

Neutrino detection is to determine the composition of different media through different photoelectric signals radiated by neutrinos passing through different media.

There is no big difference between the two in fundamental principles.

However, Pang Xuelin discovered that in the research of particle physics in the world of "Whale Song", compared with the world of "Earth Cannon", there is an additional type of neutrino, that is, the heavy neutrino.

As we all know, neutrinos belong to leptons like electrons, muons and tauons, and there are several ways to produce cosmic neutrinos. One is the original one, produced in the Big Bang, and now it is the cosmic background neutrinos with very low temperature.

The second type is produced during the gravitational collapse process of giant celestial bodies in supernova explosions. The SN1987A neutrinos are of this type.

The third type is neutrinos below a dozen MeV produced by light nuclear reactions on stars like the sun.

The fourth type is that high-energy cosmic ray particles shoot into the atmosphere and undergo a nuclear reaction with the atomic nuclei therein to produce π and kaons, and these mesons decay to produce neutrinos. These neutrinos are called "atmospheric neutrinos".

Fifth, high-energy protons in cosmic rays collide with photons from cosmic microwave background radiation to produce π mesons. This process is called "photoinduced π mesons". The decay of π mesons produces high-energy neutrinos, which have extremely high energy.

The sixth is that cosmic ray high-energy protons hit the nuclei of stellar clouds or the interstellar medium to produce nuclear reactions, and the mesons generated by nuclear reactions decay into neutrinos.

Such neutrinos can be produced especially on some neutron stars, pulsars and other celestial bodies.

The seventh kind is the neutrinos produced by the spontaneous or induced fission product β decay of matter on the earth. This kind of neutrinos is very rare.

Although produced in different ways, through the observation of Z bosons, scientists have found that neutrinos have three "flavors": electron neutrinos (νe), muon neutrinos (νμ) and tau neutrinos (ντ) .

For each flavor of neutrino there exists a corresponding antineutrino that is also electrically neutral and has a spin quantum number of ?.

In the Standard Model, the production process of neutrinos obeys the law of conservation of lepton number.

Since neutrinos are electrically neutral and also a kind of lepton, they do not participate in strong and electromagnetic interactions, but only participate in gravitational and weak interactions.

The weak interaction distance is very short, and the gravitational interaction is very weak at the subatomic scale, so neutrinos will not be hindered too much when passing through ordinary matter, and it is difficult to detect.

At present, neutrinos can be produced in many ways such as radioactive decay and nuclear reactions.

Nuclear reactions are happening inside the sun all the time, and processes such as supernovae are also accompanied by violent nuclear reactions, so the existence of neutrinos can be detected in cosmic rays.

Most neutrinos detected near Earth originate from the Sun.

In fact, about 65 billion neutrinos from the sun pass through each square centimeter every second on the part of the earth facing the sun.

It is now recognized that neutrinos oscillate between flavors during their flight, such that electron neutrinos produced in beta decay, for example, may become muon or tau neutrinos when detected.

This phenomenon indicates that neutrinos have mass, and that neutrinos of different flavors have different masses.

According to the current cosmological detection data, the sum of the neutrino masses of the three flavors is less than one millionth of the mass of an electron.

Further studies have found that neutrinos with definite masses (i.e., mass eigenstates) m1, m2, and m3 are not identical to taste eigenstates—electron neutrinos, mu neutrinos, and tau neutrinos. correspond.

For example, m1 with a certain mass can be regarded as a combination of neutrinos of three flavors in a certain proportion, and an electron neutrino with a certain flavor is also a combination of neutrinos of three different masses.

It is this mixing that causes neutrinos to oscillate.

The oscillations of the three generations of neutrinos can be described by six parameters, including two mass square differences, three mixing angles and one CP violation phase angle.

The solar neutrino experiment measured m2^2-m1^2=7.5×10-5eV^2 and the mixing angle sin^2β12=0.86, and the atmospheric neutrino experiment measured |m3^2-m2^2|=2.4× 10^-3eV^2 and sin^2β23≈1.

In the real world, the Daya Bay reactor neutrino experiment led by Wang Yifang, an academician of the Chinese Academy of Sciences, measured the last mixing angle sin^2β13=0.09.

In the world of "Whale Song", human beings have measured the parameters of neutrino CP destruction phase angle. At the same time, the problem of which quality order (or quality level) is heavier among m1, m2, and m3 is determined.

And on this basis, the properties of electron neutrinos, muon neutrinos and tau neutrinos were thoroughly clarified.

But there is a problem here. The scientists in the world of "Whale Song" found that according to the measured results, there should be a fourth type of neutrino in theory. They named this neutrino as heavy neutrino , also known as sterile neutrinos.

Under current conditions, the best determination of the number of neutrino species comes from observations of Z boson decays.

This particle decay produces various types of light neutrinos and their antineutrinos. And the more types of light neutrinos produced, the correspondingly shorter lifetime of Z bosons.

But the existence of sterile neutrinos cannot be determined by observing the decay of Z bosons.

The observed data of the cosmic microwave background radiation obtained by the microwave anisotropy detector are compatible with the situation of three or four neutrinos at the same time.

...

Heavy neutrinos!

Pang Xuelin wrote down these four big characters on the manuscript paper, and then circled them.

Neither in the world of "Whale Song" nor in the world of "Earth Cannon", human beings have failed to observe the existence of heavy neutrinos in experiments.

Pang Xuelin faintly felt that this heavy neutrino might be the key to complete the research and development of a new generation of formation neutrino CT detection instrument.

But the question is, how to find the heavy neutrinos described in the paper?

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