"Professor Pang, you mean that there may theoretically be a fourth type of neutrino that cannot be observed by the decay of the Z boson?"

In the laboratory of the Institute of High Energy Physics of the Chinese Academy of Sciences, Qiao Anhua, 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 papers in the fields of mathematics and physics.

Some of them are the results of his previous research, and some of them are simply derived from systematic awards.

Therefore, at present, in the scientific community, Pang Xuelin's name is well-known.

This is also the reason why he suggested that there may be a fourth type of heavy neutrino, which Jo Anhua did not directly refute.

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

"But...... Why haven't we found the existence of such neutrinos until now? ”

Jo Anhua asked the crux of the question.

Neutrinos were first detected in 1956 in an experiment conducted by the American physicists Lenis and Cohen's team using a reactor at the Savannah River Plant.

The experimental reactor produces a strong flow of neutrons accompanied by a large number of β decay, emitting electrons and antineutrinos, antineutrinos bombarding protons in the water, producing neutrons and positrons, neutrons and positrons enter the target liquid in the detector, neutrons are absorbed, positrons and negative electrons are annihilated, and high-energy γ rays are generated, so as to determine the generation of the reaction.

Although antineutrino fluxes were as high as 5×10 to the 13th power per square centimeter per second, the number of detections at that time was less than 3 per hour.

In 1983, physicists in Gifu Prefecture, Japan, built the Super Kamiokande Detector using the principle of "Cherenkov radiation".

The main part of the Super Kamiokande detector is a huge water tank built at a depth of 1,000 meters, containing about 50,000 tons of high-purity water, and 11,000 photomultiplier tubes attached to the inner wall of the tank to detect the Cherenkov light emitted by neutrinos as they pass through the water, so as to capture the traces of neutrinos.

The so-called Cherenkov radiation means that when the charged particles travel through the medium, their velocity exceeds the velocity υ of light in the medium, Cherenkov radiation occurs, emitting Cherenkov light.

Specifically, when a neutrino beam passes through water, it reacts with the nucleus of a water atom to form a high-energy negative μ. Since the negative μ travels at 0.99 times the speed of light in the water, exceeding the speed of light in the water (0.75 times the speed of light), the "Cherenkov effect" occurs when it traverses a path six or seven meters long in the water, radiating the so-called "Cherenkov light".

This light not only encompasses all continuously distributed 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 with a highly sensitive photomultiplication array, the neutrino beam can also be detected.

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

Now, in the year 2075, different kinds of neutrino detection technology have long been matured, but in addition to the three neutrinos mentioned earlier, humans have not discovered the existence of a fourth neutrino.

Theoretical parts and experiments, 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 now?" ”

Jo Anhua thought for a moment and said, "Experimentally, neutrinos are classified according to the leptons that always accompany them (the probability effect of quantum mechanics) to participate in weak reactions. ”

"For example, in the Cowan-Reines experiment, which discovered neutrinos, scientists hypothesized that neutrinos would be produced by the β decay reaction taking place in a nuclear reactor. After these neutrinos fly out of the reactor, appropriate detection devices are placed outside the reactor for detection. The liquid (cadmium chloride) contained in the device contained a large number of protons, and it is theorized that neutrinos and protons would have an inverse β decay reaction. The positron can annihilate with the electrons in the detection liquid to produce light, and then read out the light signal (as well as the time and energy of the light signal to arrive, etc.) through the photoelectric effect sensor. Neutrons, on the other hand, can be absorbed by heavy metals (cadmium) in the liquid and emit light, a slightly slower process. The Cowan-Reines experiment saw two light signals before and after, and the light signals were as expected, then it was said that there was an inverse β decay reaction, which proved the existence of neutrinos. ”

Further analysis of this experiment shows that the light signal produced by the annihilation of positrons and negatives shows that the neutrinos produced by the nuclear reactor are accompanied by the appearance of positrons, so this is actually an anti-electron neutrino. Ray Davis, an early discoverer of solar neutrinos, tried to detect neutrinos from nuclear reactors in the same way. But from the nuclear reactor he did not get the expected results. This same reaction was later used to detect solar neutrinos, and the results can be seen. This illustrates that the neutrinos that accompany the e- and e+ reactions are different. Nuclear reactors produce anti-electron neutrinos, whereas solar nuclear reactions produce electron neutrinos. The fundamental reason for this comes from the fact that the left and right sides of the nuclear reaction require the conservation of leptons in addition to the conservation of charge. The number of leptons of positrons and anti-electrons is denoted as -e, and the number of leptons of electrons and electrons is +e. ”

Later, Lederman et al. studied neutrinos produced in accelerators. The neutrinos produced in the accelerator are mainly from π meson decay. They expect two inverse β decay reactions. However, they did not observe reaction 1, only reaction 2. This shows that the neutrinos produced by accelerators are always accompanied by positrons rather than positrons during the anti-β decay reaction. Muons and electrons have similar properties but are more massive. They are classified as leptons. This shows that the conservation of lepton number should be subdivided into conservation of electron lepton number and conservation of muon lepton number. So what they observed had to be antimund neutrinos. ”

"A third neutrino was discovered on a higher energy accelerator, Tevatron (DONUT experiment). Similar to before, they react with pottery. Pottery is also a type of lepton, but it is more massive, even larger than proton, so it requires more energy to manufacture (by Einstein's mass-energy equation), which is why pottery and pottery neutrinos were discovered later. Similarly, a pottery lepton number is also introduced for pottery. Among them, the neutral current channel can detect all kinds of neutrinos, and the current channel can only detect electron neutrinos, and the reaction probability of electron neutrinos is higher in the elastic scattering reaction with electrons. In this way, the total amount of neutrinos of all species can be obtained by analyzing the detection results of the neutral current channel, and the number of electron neutrinos can be obtained by analyzing the detection results of the band current detection, so as to calculate the conversion probability of electron neutrinos. ”

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

Pang Xuelin smiled slightly and said: "Professor Qiao, you should know that neutrinos with different flavors can be converted into each other through neutrino oscillations, so have you considered whether new neutrinos will be generated in the process of transformation?" ”

Qiao Anhua was slightly stunned, looked at Pang Xuelin in puzzlement, and said, "Professor Pang, what do you mean?" ”

Pang Xuelin said: "My idea is whether there is an inert neutrino, such as the transformation of electric neutrinos into ceramic neutrinos, first through neutrino oscillation, into this inert neutrino, and then from this inert neutrino to Tao neutrino, Tao neutrino into Miao neutrino, the same through this inert neutrino transformation, but the process is too short, so that we do not have enough ways to detect it now!" ”

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