1900: A physics genius wandering around Europe

Chapter 667 Neutrinos Appear! A Shock to the Entire Audience! The Four-Fermion Theory! The First Weak Force Theory!
Under the watchful eyes of the bigwigs, Pauli calmly walked to the front of the round table.

Even someone as unconventional as Pauli became calm and composed in such a high-class conference room.

After nodding to the crowd, he began his report:
"Today, my presentation will focus on some thoughts about beta decay."

"Dr. Chadwick of Cavendish once discovered a very strange phenomenon in his experiments on beta decay."

Pauli then briefly reiterated the issue of the continuity of electron energy.

Those present were all physics experts; even those who didn't specialize in beta decay could easily understand it.

"So, there is a contradiction here."

Why is the kinetic energy of electrons produced by neutron decay not a fixed value, but a continuous value?

"And this value is far lower than the theoretical kinetic energy of an electron?"

"This means that some energy has disappeared."

"I have reviewed all the relevant papers from recent years, and I have not yet found a reasonable explanation."

At this moment, the host, Lang Ziwan, suddenly said:
“I remember Professor Bohr once used the principle of non-conservation of energy to explain this problem.”

Pauli nodded.

"That's right, but this explanation has not been accepted."

"That's a bit too shocking."

Pauli also frequently visited Bohr's Institute of Theoretical Research over the years, and the two had a very good relationship.

So, he can make a little joke.

Bohr gave a helpless smile.

He was ridiculed by many people because of this incident.

"Professor Bohr always wanted to make a big splash."

Pauli continued:

"So, I'm giving a completely new explanation today."

Wow!
Everyone was shocked!
In everyone's opinion, although Pauli has a sharp tongue, his strength is top-notch.

Moreover, he not only likes to question others, but he also holds himself to high standards.

In such a high-level setting as the Bruce Conference, his answer to a lingering question must have been the result of careful consideration.

At least he was convinced.

Moreover, the problem of beta decay is related to the weak force and is extremely important.

Currently, the ideas of quantum field theory have encountered unimaginable difficulties when applied to the strong force, weak force, and gravity.

All the big shots were so tormented that they couldn't sleep.

It's really too difficult.

Therefore, a breakthrough in the weak force domain would be a great joy.

Amidst everyone's anticipation, Pauli said:

"Professor Bruce believes that after a neutron undergoes beta decay, it produces a proton and an electron."

"But I believe that the neutron will also generate an additional new particle!"

"It is this particle that takes away the energy that disappeared and converts it into its own kinetic energy!"

"This would explain the problem of beta decay."

boom!
As soon as he finished speaking, a gasp of surprise erupted in the room!
Pauli was so bold as to predict a new particle!
Heisenberg was shocked upon hearing this!
After several years of silence, is Pauli finally about to make a stunning comeback?
“I know his personality; he would never make wild guesses.”

However, those present were all big shots, and they were able to remain calm.

Everyone knew that Pauli would definitely elaborate further.

Sure enough, he continued:

"I assume that the mass of this new particle is about the same as that of an electron."

"In this way, the kinetic energy it carries away will meet the requirements."

"From the perspective of charge conservation, the new particle is uncharged and is a neutral particle."

"Moreover, from the perspective of spin conservation, it can be analyzed that such a particle must exist."

Pauli then elaborated on the contents he had prepared.

Everyone who heard this was deeply moved.

The introduction of this new particle can perfectly solve the problem of beta decay.

For a time, everyone was talking about it.

Li Qiwei smiled slightly as he looked at Pauli's presentation.

In real history, the new particle that Pauli proposed in 1930 is the famous ghost particle, the neutrino.

Initially, Pauli called this new particle a "neutron" because it was electrically neutral.

However, in 1932, Chadwick finally discovered the neutron in the proton-neutron model, and the two names conflicted.

Later, Fermi changed the name to "neutrino," meaning a smaller, neutral particle.

Research has revealed that the mass of a neutrino is not equal to that of an electron as Pauli conjectured, but rather only one millionth the mass of an electron.

At the time, the electron was the smallest known particle.

Neutrinos have a mass much smaller than that of electrons, which is considered "tiny" even in the field of particle physics.

This has led to the fact that the neutrino particle has remained undiscovered by the physics community since it was first proposed.

Because of its small size and lack of electrical charge, many detection methods and techniques cannot be used.

However, in 1942, Chinese physicist Wang Ganchang proposed a very ingenious detection method.

He used anti-beta decay to detect neutrinos.

The normal nuclear reaction formula for beta decay is: n → p + e + v (n: neutron; p: proton; e: electron; v: neutrino). ①
So clever of you, you must have already figured it out: the nuclear reaction formula for anti-β decay is: p + e + v → n. ②
Congratulations, you answered incorrectly.

This is an important point to explain.

The neutrino in Formula ① should actually be called the antielectron neutrino.

Therefore, the process of positive beta decay is that a neutron produces a proton, an electron, and an antielectron neutrino.

Why would an antineutrino be generated?
As mentioned earlier, there are many conservation principles in the field of particle physics.

The conservation of lepton number is one such law.

Since an electron is a lepton with a lepton number of 1, the right side of the equation must contain an antilepton with a lepton number of -1.

In this way, the number of leptons on the right is 0. And the number of neutrons on the left is also 0.

Both are conserved.

Therefore, the generated neutrinos must be antineutrinos.

Then, out of curiosity, you asked again:

"Then why is it called an anti-electron neutrino?"

Why not just call it antineutrino?

This is because there are 12 types of leptons in the Standard Model.

The three types are: electron (e), muon (μ, equivalent to a large electron), and tauon (τ, equivalent to a super-large electron).

Therefore, you can understand it as there are three types of electronics.

Each of these three types of electrons has a corresponding neutrino: electron neutrino ve, muon neutrino vμ, and tau neutrino vτ.

These six types of leptons each have a corresponding antiparticle:

Anti-electron, anti-muon, anti-tauon, anti-electron neutrino, anti-muon neutrino, anti-tauon neutrino.

These 12 particles are the 12 leptons in the Standard Model.

So now you should understand the true meaning of beta decay.

Back to the topic.

Logically, positive beta decay is n→p+e+v.

Therefore, anti-β decay is p + e + v → n. This process conforms to any conservation law.

However, the truth is: p + v → n + e.

This is equivalent to moving the electron to the right side of the reaction equation, so the electron must become an anti-electron.

That is: one proton plus one antielectron neutrino produces one neutron plus one antielectron.

Wang Ganchang used this reaction pattern to propose a method for detecting neutrinos for the first time.

The specific method is as follows:

First, water is used as a detector because water contains a large number of protons.

If antielectron neutrinos do exist, they must react with protons in water to produce neutrons and antielectrons.

After the positron is formed, it annihilates with electrons in the water, a process that emits two gamma-ray photons.

Therefore, by adding gamma-ray detectors on both sides of the water detector, it is possible to detect the presence of gamma photons.

If gamma photons are present, it means that antielectron neutrinos exist, that is, neutrinos do exist.

Unfortunately, Wang Ganchang's research was not valued or recognized by the physics community at the time because he was Chinese.

In 1956, American physicists Reines and Cowin designed an experiment to verify the principle published by Wang Ganchang.

The entire experimental setup was buried deep underground near the nuclear power plant to minimize environmental interference.

Furthermore, they not only detect anti-electrons and gamma photons produced by electron annihilation.

The two also added a lot of cadmium to the water.

The neutrons produced in the reaction will be absorbed by the cadmium nuclei.

When a cadmium nucleus absorbs a neutron, it enters an excited state, then releases a gamma photon, and finally falls back to the ground state.

Therefore, if two types of gamma photons can be measured simultaneously, then the existence of neutrinos can be proven almost 100%.

The experimental results are certainly positive; neutrinos do exist.

Reines was awarded the Nobel Prize in Physics in 1995 for this, while his colleague Cowen was unable to win the prize due to his early death.

Along with Reines, American physicist Paul Perl, who discovered the tau tau, also won the Nobel Prize in Physics that year.

Professor Wang Ganchang of our nation did not receive the honor he deserved.

Finally, those who are observant might have another question:
"The whole process seems to have nothing to do with Fermi."

Why is he qualified to name the neutrino?

"Why should he?"

That's because Fermi used neutrinos to propose the famous "four fermion theory".

This is the first theory to explain the weak force based on quantum field theory, and it is on par with Hideki Yukawa's meson theory explaining the strong force.

Its importance is self-evident.

As we already know, particles can be divided into fermions and bosons.

Fermions cannot superimpose at the same spatial location, while bosons can (for example, light can superimpose).

It is clear that the four particles associated with the weak force—proton, neutron, electron, and neutrino—are all fermions.

In 1934, Fermi proposed a weak force theory that could explain neutron beta decay.

He believes that the essence of beta decay is a vector flow coupling mechanism in which the four fermions involved in the weak force interact at the same point in spacetime.

Using the Lagrange quantity of the interaction, he innovatively gave the first mathematical formula for the distribution of the electron energy spectrum and the decay probability, which is in good agreement with the experimental results.

The paper caused a huge sensation in the physics community as soon as it was published.

This was the first universal theoretical framework that could explain weak forces, laying the foundation for subsequent developments.

Fermi thus gained great fame and became a leading figure in the theoretical community, which is why he was qualified to name the neutrino.

His theory, which concerns the interaction of four types of fermions, is also figuratively called the "four-fermion theory".

However, with the further development of quantum field theory, the four-fermion theory soon encountered its flaws.

First, it cannot explain parity nonconservation. (See Chapter 563)
Second, it cannot describe the weak force in high-energy states.

It wasn't until later, when the electroweak unification theory replaced the four-fermion theory, that the weak force was officially put to an end.

Fermi's contributions laid a solid foundation for the electroweak unification theory and were a pioneering work in the history of quantum field theory.

This concludes the story of the discovery and proposal of the neutrino, and its relationship with the weak force.

This shows that weak force and strong force are two completely different processes.

At this moment, Pauli boldly proposed this new particle, which aroused great interest among everyone.

Bohr first asked:

"If the mass of the new particle is comparable to that of the electron, why doesn't a corresponding particle stream form?"

Pauli replied:

"The reason why beta decay can form beta rays, i.e., electron streams, is because of the control of an additional electric field."

"These new particles are uncharged, so they are emitted in all directions and cannot be concentrated, which is why their particle stream cannot be observed."

Then, Dirac and the others each raised several questions.

Pauli could answer some questions, but not others.

After all, this is just a guess on his part right now.

He doesn't even know the exact mass of this particle.

However, nearly half of the bigwigs present agreed with this conjecture.

This is because it not only perfectly explains the energy crisis in beta decay, but also serves as a confirmation of spin conservation.

Fermi was the most excited among them.

The moment he heard about this new particle, it was as if some kind of inspiration poured out of him.

It's as if saying, "This particle was created for me."

Fermi shook his head and chuckled to himself, realizing he was getting a little carried away with his thoughts.

At this moment, Heisenberg suddenly smiled and asked:

"Pauli, have you named this new particle yet?"

As a staunch follower of Pauli, Heisenberg certainly supported this theoretical conjecture.

Pauli said:

"not yet."

"Because I don't think it has been proven yet."

However, to everyone's surprise, Li Qiwei suddenly spoke:

Let's call it the neutrino.

Wow!
The whole place was shocked!

This statement carries a rather unusual meaning.

"Professor Bruce actually agreed with Pauli's conjecture!"

"Nine times out of ten, neutrinos definitely exist."

Li Qiwei's words were like a heavy hammer blow to Pauli's heart, making him extremely excited.

With Professor Bruce's endorsement, his conjecture has at least a 9% probability.

That's an honor that comes from predicting or discovering a completely new particle!
To date, only five people in physics have received this honor:
Electronics: Thomson;
Proton: Rutherford;

Neutron: predicted by Ligdwell, discovered by Yu Yin;
Anti-electrons: predicted by Rigdwell, discovered by Zhao Zhongyao.

And now, we need to add one more: the neutrino: the Pauli prediction.

Pauli muttered to himself:

"A neutrino is a neutron that is even smaller than a neutron."

"This name is so fitting."

(End of this chapter)