The Science Fiction World of Xueba

But soon, the excited expression on Qiao Anhua's face subsided.

"Professor Pang, it is undeniable that your theory is wonderful, but the problem is that we must find the inertial neutrino you mentioned to prove your theory is correct. According to the calculation results in your paper, this This kind of neutrino exists for a very short time, and it is difficult to react with other substances, just how to design experiments to find it is a big problem!"

Pang Xuelin smiled lightly: "Professor Qiao, do you still remember the mystery of the disappearance of solar neutrinos?"

"The Mystery of the Missing Solar Neutrinos?"

Qiao Anhua was slightly taken aback, and his brows frowned slightly.

Of course he knew about this famous problem in the history of science.

During the first half of the 20th century, physicists generally believed that the sun's light is due to the continuous fusion reaction from hydrogen to helium in its interior.

According to this theory, every 4 hydrogen nuclei (i.e. protons) inside the sun are transformed into 1 helium nucleus, 2 positrons and 2 mysterious neutrinos.

It is the energy released by this nuclear fusion reaction that the sun glows and heats up, feeding everything on the earth.

As the thermonuclear reaction proceeds, neutrinos are continuously released.

Since the mass of 4 protons is greater than that of 1 helium nucleus plus 2 positrons and 2 neutrinos, the reaction releases a large amount of energy.

A small fraction of this energy ends up on Earth in the form of sunlight.

This nuclear reaction is the most frequently occurring reaction inside the sun.

Neutrinos can easily escape from the interior of the sun, and their energy does not appear in the form of light and heat.

Sometimes the energy of neutrinos produced by thermonuclear reactions is relatively low, and the energy taken away is relatively small, so the sun has obtained more energy.

If the energy of neutrinos is relatively high, the energy received by the sun will be relatively less.

Neutrinos have no charge and no internal structure.

In the Standard Model of elementary particle physics, neutrinos are massless.

About 100 billion solar neutrinos reach every square centimeter of the earth's surface every second, but we don't feel them because the probability of neutrinos interacting with matter is very small.

Only 1 out of every 100 billion solar neutrinos passing through Earth will interact with the material that makes up Earth. Because neutrinos have a very small chance of interacting with other particles, they can easily escape from the interior of the sun and directly bring us important information about nuclear reactions inside the sun.

There are three different types of neutrinos in nature. The neutrinos produced by nuclear reactions in the sun are electron-type neutrinos, which are associated with electrons. The other two types of neutrinos are the muon and tau neutrinos, which can be produced in accelerators or exploding stars and are associated with charged muons and taions, respectively.

In 1964, Raymond Davis and John Bacow proposed an experimental scheme to test whether the nuclear reaction that provides the sun's energy is fusion.

John Bacow and his colleagues used an elaborate computer model to count solar neutrinos at different energies.

Because solar neutrinos react with chlorine to release radioactive argon atoms, they also counted the number observed in a giant vat filled with perchlorethylene.

Although the idea seemed unrealistic at the time, Davis believed that a detector the size of a swimming pool filled with pure perchlorethylene could measure the amount of argon produced each month as predicted by theory.

Davis's earliest experimental results were published in 1968.

The number of cases he detected was only one-third of the value predicted by theory. The problem that the number of cases predicted by the theory is inconsistent with the experiment was later called the "solar neutrino problem", and the more popular term is "the mystery of the missing neutrinos".

In order to explain the solar neutrino problem, three possible solutions have been proposed.

The first solution is that there may be a problem with the theoretical calculation, and there may be errors in two places: or there is a problem with the solar model, which leads to an incorrect number of solar neutrinos predicted by the theory, or a problem with the calculated production rate.

A second explanation suggests that perhaps Davis' experiment went wrong.

The third option, the most daring and the most discussed, holds that the solar neutrinos themselves change as they travel through space from the sun to the earth.

Over the next 20 years, many people carefully recounted the number of solar neutrinos produced. The precision of the data used for calculations is constantly improving, resulting in more accurate results.

It was found that there was no apparent error in the number of neutrinos derived from the solar model and in the calculation of the number of neutrino events detectable by Davis' experimental setup.

At the same time, Davis increased the precision of the experiment and performed a series of different tests to confirm that he was not ignoring certain neutrinos.

No errors were found in his experimental setup. The discrepancy between experiment and theory remains unresolved.

The third explanation mentioned above was put forward by former Soviet scientists Bruno Pontekov and Vladimir Glibov in 1969.

This idea holds that the nature of neutrinos is not as simple as physicists originally imagined. Neutrinos may have a rest mass and different types of neutrinos can transform into each other. The latter is the so-called neutrino oscillation.

When the idea was first proposed, it was not embraced by most physicists. But over time, more and more evidence began to lean toward the existence of neutrino oscillations. This is a new kind of physics that goes beyond the framework of the standard model.

In 1989, 20 years after the results of the first solar neutrino experiment were published, a Japanese-American experimental group (the Kamioka Cooperation Group) led by Masatoshi Koshiba and Yoji Totsuka reported their experimental results. They filled giant detectors with pure water to measure the rate of scattering between electrons in the water and energetic neutrinos from the sun.

This experimental device is very precise, but it can only detect high-energy solar neutrinos. The high-energy neutrinos come from the decay of elements, a relatively rare process in thermonuclear reactions inside the Sun. Davis's original experimental setup used chlorine, but could also detect neutrinos in this energy region.

The Kamiokande experiment confirmed that the number of neutrinos observed is indeed less than the theoretical prediction value of the solar model, but the degree of inconsistency between theory and experiment revealed by it is smaller than that of Davis' experiment.

Over the next 10 years, three new solar neutrino experiments further complicated the problem of missing neutrinos.

The GALLEX laboratory led by German Til Kirstein and the SAGE laboratory led by Vladimir Glibov respectively used detectors filled with gallium to detect low-energy solar neutrinos and found low-energy neutrinos There is also the problem of loss.

In addition, the Super-Kamiokande experiment, led by Yoji Totsuka and Yoichiro Suzuki, made more precise measurements of high-energy solar neutrinos using a massive detector containing a total of 50,000 tons of water, convincingly confirming Davis' The neutrino loss phenomenon observed by the experiment and the Kamiokande experiment.

In this way, both high-energy solar neutrinos and low-energy solar neutrinos are missing, but the missing ratio is different.

At 12:15 noon on June 18, 2001, the neutrino experiment team composed of scientists from the United States, Britain and Canada led by Canadian Arthur Macdonald announced an exciting news: they solved the neutrino in the sun. sub-puzzle.

The international team used 1,000 tons of heavy water to detect neutrinos.

The detector was placed in a mine 2,000 meters below the southern Canadian city of Sudbury. They detected solar neutrinos in the high-energy region with a new method different from the Kamiokande and Super-Kamiokande experiments. This experiment is called the SNO experiment.

In SNO's original experiments, the heavy water detector they used was in a state that was only sensitive to electron neutrinos.

The number of electron neutrinos observed by scientists at SNO is about one-third of the value predicted by the standard solar model, and the previous Super-Kamiokande experiment is not only sensitive to electron neutrinos, but also to other types of neutrinos to a certain extent. Sensitivity, so the number of observed neutrinos is about half of the theoretically expected value.

If the Standard Model is correct, the SNO experiments should agree with Super-Kamiokande that all neutrinos from the Sun should be electron neutrinos. The results of the two experiments were inconsistent, suggesting that the Standard Model describing the properties of neutrinos is problematic, at least incomplete.

Combining the experiments of SNO and Super-Kamiokande, the SNO cooperation team not only determined the number of electron neutrinos, but also determined the total amount of three types of neutrinos from the sun, and the results were consistent with the predictions of the solar model.

Electron neutrinos make up one-third of all neutrinos.

In this way, the problem is clear: Although the number of electron neutrinos observed on the ground accounts for only one-third of the total number of solar neutrinos, the latter has not decreased; the lost electron neutrinos have not "disappeared" ", only transformed into the hard-to-detect muon neutrinos and tau neutrinos.

This landmark result was published in June 2001 and was soon supported by a series of other experiments.

The SNO collaboration measured the number of all three high-energy neutrinos on their heavy water detector, which was unique at the time. Their experimental results show that most neutrinos are produced inside the sun, and they are all electron neutrinos.

Upon reaching Earth, some of the electron neutrinos transform into muon and tau neutrinos.

The key to the SNO experiment is the measurement of the total number of three types of neutrinos. It is precisely because of the determination of the total amount of three types of neutrinos that physicists can convincingly explain the mystery of the disappearance of solar neutrinos without relying on specific theoretical models.

...

"Professor Pang, what do you mean, the existence of such inert neutrinos can be found through solar neutrino experiments?"

Qiao Anhua looked at Pang Xuelin and frowned.

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