The Science Fiction World of Xueba

"Professor Pang, you mean that there may be a fourth type of neutrino in theory, and this kind of neutrino cannot be observed through Z boson decay?"

In the laboratory of the Institute of High Energy Physics, Chinese Academy of Sciences, Qiao Anhua, the director of the Institute of High Energy Physics, looked at Pang Xuelin and frowned.

In the past six months, Pang Xuelin has not been idle.

While proposing the Earth Cannon Project, he also contributed many top-level papers in the fields of mathematics and physics.

Some are his previous scientific research results, and some are simply derived from system rewards.

Therefore, in the scientific world, Pang Xuelin's name is considered very famous.

This is why Qiao Anhua did not directly refute his proposal that there might be a fourth type of heavy neutrino.

Pang Xuelin nodded and smiled: "According to the calculation results of the model I gave, such a heavy neutrino should indeed exist."

"But... why haven't we discovered the existence of such neutrinos until now?"

Qiao Anhua got to the crux of the problem.

The first time humans detected neutrinos was in 1956, when American physicists Lenis and Cohen's team used the reactor at the Savannah River Plant to conduct an experiment.

The experimental reactor produces a strong flow of neutrons accompanied by a large number of beta decays, emitting electrons and antineutrinos, antineutrinos bombarding protons in the water, producing neutrons and positrons, neutrons and positrons entering the detector When the target liquid is used, neutrons are absorbed, positrons and negative electrons are annihilated, and high-energy γ-rays are generated to determine the occurrence of the reaction.

Although the antineutrino flux was as high as 5×1013 per second per square centimeter, the detection count at that time was less than 3 per hour.

In 1983, physicists built the Super-Kamiokande detector using the principle of "Cherenkov radiation" in Gifu County, Japan.

The main part of the Super-Kamiokande detector is a huge water tank built at a depth of 1,000 meters underground, which contains about 50,000 tons of high-purity water, and 11,000 photomultiplier tubes are attached to the inner wall of the tank to detect neutrinos. Cerenkov light emitted as it travels through water, thus capturing traces of neutrinos.

The so-called Cherenkov radiation means that when the charged particle travels through the medium, its speed exceeds the speed ν of light in the medium, Cherenkov radiation will occur, and Cherenkov light will be emitted.

Specifically, when a beam of neutrinos passes through water,

A nuclear reaction occurs with water nuclei to generate high-energy negative muons. Since the negative muon travels at 0.99 times the speed of light in water, exceeding the speed of light in water (0.75 times the speed of light), the "Cherenkov effect" will occur when it travels through a path of six or seven meters in water, and the so-called "Cherenkov effect" will be radiated. Cherenkov light".

This kind of light not only covers all continuous visible light in the range of 0.38-0.76 microns, but also has a definite directionality.

Therefore, as long as all the "Cherenkov light" is collected by a high-sensitivity photomultiplier array, the neutrino beam can be detected.

In a sense, this is also the basic principle of neutrino communication technology.

Now, it is already 2075, different types of neutrinos detection technology has long been mature, but in addition to the three types of neutrinos mentioned before, humans have not discovered the existence of the fourth type of neutrinos.

The theoretical part and the experiment, either there is a problem with the theory, or there is a problem with the experiment!

From Qiao Anhua's point of view, there is something wrong with Pang Xuelin's theory.

Pang Xuelin smiled slightly and said, "Professor Qiao, how do we determine the different classifications of neutrinos?"

Qiao Anhua thought for a while and said: "From an experimental point of view, neutrinos are classified according to the leptons that always (the probability effect of quantum mechanics) accompany them to participate in weak reactions."

"For example, in the Cowan-Reines experiment that discovered neutrinos, scientists first assumed that the beta decay reaction in the nuclear reactor would produce neutrinos. After these neutrinos flew out of the reactor, appropriate detection devices were placed outside the reactor for detection. Detection. The liquid (cadmium chloride) contained in the device contains a large number of protons, and the theory predicts that neutrinos and protons have a reverse beta decay reaction. Among them, the positrons can annihilate with the electrons in the detection liquid to generate light, and then pass through the photoelectric effect sensor Read out this light signal (and the arrival time of the light signal, energy, etc.). The neutrons can be absorbed by the heavy metal (cadmium) in the liquid and then emit light, which is a little slower. The Cowan-Reines experiment saw the two before and after A light signal, and the light signal is in line with expectations, then it is said that there is a reverse beta decay reaction, which proves the existence of neutrinos."

"Further analysis of this experiment, the optical signal produced by the annihilation of the positron and electron indicates that the neutrino produced by the nuclear reactor is accompanied by the positron, so this is actually an anti-electron neutrino. Ray, the early discoverer of solar neutrinos Davis has tried to use the same reaction to detect neutrinos from nuclear reactors. But he could not get the expected results from nuclear reactors. Later, this same reaction was used to detect neutrinos from the sun, and the results can be seen This shows that the neutrinos accompanying e- and e+ reactions are different. Nuclear reactors produce anti-electron neutrinos, while solar nuclear reactions produce electron neutrinos. The root cause of this comes from the left and right sides of the nuclear reaction except In addition to requiring the conservation of charge, the conservation of lepton number is also required. The lepton number of positrons and antielectron neutrinos is recorded as -e, and the lepton number of electrons and electron neutrinos is +e."

"Subsequently, Lederman et al. studied neutrinos produced in accelerators. Neutrinos produced in accelerators mainly come from the decay of pions. They expected two reverse beta decay reactions. However, they did not observe reaction 1, only reaction 2 .This shows that neutrinos produced by accelerators are always accompanied by positive muons instead of positrons in the process of reverse beta decay reactions. Muons and electrons have similar properties, but are more massive. They are classified as leptons. This It shows that the lepton number conservation should be subdivided into electron lepton number conservation and muon lepton number conservation. Therefore, what they observed must be antimuon neutrinos.”

"A third type of neutrino was discovered on the higher energy accelerator Tevatron (DONUT experiment). Similar to the previous one, they accompanied the tau in the reaction. The tau is also a kind of lepton, but with a greater mass, even larger than the proton , so more energy is needed to manufacture (according to Einstein's mass-energy equation), which is also the reason why tau and tau neutrinos were discovered later. Similarly, a tau lepton number is also introduced for tau. Among them , the neutral current channel can detect all types of neutrinos, and the current-carrying channel can only detect electron neutrinos, and in the elastic scattering reaction with electrons, the probability of electron neutrinos’ reaction is higher. In this way, by analyzing neutral The total amount of all types of neutrinos can be obtained from the detection results of the flow channel, and the amount of electron neutrinos can be obtained by analyzing the detection results of the band current, so as to calculate the conversion probability of electron neutrinos."

Qiao Anhua was not in a hurry, and told Pang Xuelin how to distinguish the three different types of neutrinos.

Pang Xuelin smiled slightly and said: "Professor Qiao, you should know that neutrinos of different flavors can be transformed into each other through neutrino oscillations. Have you ever considered whether new neutrinos will be produced during the transformation process?" Where is the son?"

Qiao Anhua was taken aback for a moment, looked at Pang Xuelin in confusion and said, "Professor Pang, what do you mean?"

Pang Xuelin said: "My idea is whether there is a kind of inert neutrino, such as electron neutrinos transformed into tau neutrinos, first through neutrino oscillation, converted into this kind of inert neutrinos, and then by This kind of inert neutrino transforms into tau neutrino, and when tao neutrino transforms into muu neutrino, it is also transformed through this kind of inert neutrino, but the time of this process is so short that we do not have Enough for testing!"

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