Did you play with bricks when you were a kid? If you haven't played with bricks, you should have built a house with bricks! Okay, so let's imagine a scenario where you buy back a set of bricks and finally put them together and build a beautiful house.
But in the end, it was found that there was still one block left that was not used, and you studied it for a while, and you found that this block was an enlarged version of one of the blocks, that is, the remaining block looked exactly like one of the blocks, it felt like a repetition, the difference was that the remaining block was a little heavier, a little larger, and the other blocks looked incongruous.
At this time, you will definitely think that this block must be superfluous, it should not exist, it must have been mixed in during production, and then you bought a few sets of blocks, but found that each set is like this, which you can't explain clearly, you say that something that should not exist, why it really exists. This is the real situation in our universe, the Creator is the one who produces the building blocks, and the extra blocks are the half of our protagonists today μ.
To say μ, we have to start with π mesons, to say π mesons, we must first mention the nuclear force, which we have already said in the previous article, after Chadwick discovered neutrons in 1932, this people thought, what kind of force you say can resist the electrostatic repulsion between protons, glue them together, then it didn't take long for Heisenberg to say, I think: the force that sticks the nucleus together is produced by the exchange of electrons between protons and neutrons.
This claim did not stand at all, and was quickly rejected by experiments, because it was found that there were no electrons between protons and protons, but the force between them was as strong as the force between protons and neutrons. Therefore, people think that the nuclear force has nothing to do with the charge, but a new type of force that is much stronger than the electromagnetic force, so this force is called strong force.
By 1935, Hideki Yukawa of Japan said that the particle that transmits nuclear power is indeed not an electron, because I found that this electron is too light, and the particle that transmits nuclear force needs to be 200 to 300 times heavier than the electron, which is a particle that we have never found before, because its mass is between leptons and baryons, so it is called a meson, and when you take out a Greek letter, it is called π meson.
So now the nuclear force transfer process becomes, a neutron emits a π-meson, becomes a proton, and then a proton absorbs this π-meson, becomes a neutron, then in the process of emitting and absorbing π-meson, the force is exchanged, and the nucleon is glued together.
Regardless of whether this process is right or wrong, the question is how did Yukawa know that particles that transmit nuclear force need to be more than two hundred times heavier than electrons? This is the most critical issue.
Do you remember that in quantum mechanics there is an uncertainty relation, which is the relationship that two conjugate variables must satisfy, for example, energy and time are two conjugate variables, so ΔEΔt ≥ћ/2
ΔE is the indeterminate range of energy, Δt is the uncertain range of time, if the π meson as a particle transmitting nuclear force, then the maximum range of its propagation is the scale of the nucleus, which is about 10^-15 meters.
If the speed of the π meson after its birth is close to the speed of light, then we can know that its time in the nucleus is about 10^-15 meters/C, C is the speed of light, and this calculates the indeterminate range of time π mesons exist.
Now that we know Δt, we can calculate the uncertain range of ΔE based on the uncertainty relationship, so Yukawa estimates that the mass of the π meson is about 200 to 300 times that of an electron.
Yukawa also said that not only π-mesons, but also π0 and π+ mesons, where π0 is responsible for transmitting the force between neutrons and neutrons, because the two neutrons are not charged, and all need a neutral particle π0.
π+ transmits the force between protons and protons, because both protons are positively charged, so a positively charged particle π + meson is needed, which is Yukawa Hideki's prediction of π mesons.
Similarly, if others now predict new particles, experimental physicists will need to find this new particle, but there was no decent accelerator in the laboratory at that time, and according to the prediction, the energy of the π mesons would be at least a few hundred megaelectron volts, and the energy of the particles released by the commonly used radioactive sources at that time was only a few megaelectron volts.
So it's basically impossible to find π meson in the lab, but before the particle collider was made, nature had already provided humans with a high-energy particle device, that is, a cosmic ray.
These cosmic rays are a stream of high-energy particles from outer space, which are basically composed of protons, and these extremely energetic protons will produce some new particles after colliding with oxygen and nitrogen molecules in the atmosphere.
So the experimental sites of particle physicists at that time were all on the mountain, because the higher you go, the closer you get to the collision site, the greater the chance of discovering new particles.
In 1936, Anderson and Nader Mayy loaded the experimental equipment on a flatbed truck and pulled it to the summit of Pyx Peak in the Rocky Mountains, where Anderson had already tasted the sweetness in the cosmic ray because it had discovered the positron of Dirac's prophecy in the cosmic ray in 1932 and won the Nobel Prize.
So his goal this time is also very clear, that is, to run to the π meson, and in 1937, they found a new particle in the cosmic ray, and his behavior is very similar to the behavior of the electron, according to the electromagnetic deflection, it can be known that it is the same as the electron, it is also negatively charged, and it can also be positively charged like the positron. And measured that its mass is 207 times that of electrons.
In addition to the difference in quality, this thing is completely an electron after gaining weight, so people first called it heavy electron, and then felt that the name was not good, and at that time, it was felt that from the quality point of view, this may be the π meson predicted by Yukawa, so let's refer to it as a meson.
However, it was soon discovered that the force between this new particle and the nucleon was very weak, only the strength of the electromagnetic force, and it was obvious that this newly discovered particle was not the π meson predicted by Yukawa, so the new particle was renamed μ.
Now we know that μ is indeed not the so-called meson, it belongs to the lepton, and the spin of the electron is 1/2, just like the previous mentioned, the charge and the situation of the electron, it is completely the electron after eating fat, in addition to the difference in mass, it is almost the same as the electron.
Also, μ it is unstable and decays in 2×10^-6 seconds, such as a negative μ, it decays into an electron, an anti-electron neutrino, and a muctor neutrino.
A positive muse decays into a positron, an electron neutrino, and an anti-muse neutrino. It can be seen that the charge and the number of leptons in the above two reactions are conserved.
It can also be seen that in the process of decay, the muse will always produce a neutrino associated with it, so we call it a mucus neutrino, the mucchi neutrino is the same as the electron neutrino, but the taste is different, and the electron neutrino is always related to the electron, and the mucchi neutrino is always related to the mucchi.
At this point we already know four leptons, of which electrons and electron neutrinos are called the first generation of leptons, and muse and muse neutrinos are called second generation leptons, and the only difference between the first generation of leptons and the second generation of leptons is that the second generation is a little heavier, and all other properties are the same. It's like, there are two you in this world, and the other you weigh more than two hundred times more than you, and apart from this difference, you two are exactly the same.
It can be seen that this elementary particle has a repetitive phenomenon, just like the building blocks we just said, having the first generation of leptons is enough, the second generation is completely redundant, of course, now we know that there are third generations of leptons, they are pottery and pottery neutrinos, scientists do not know why elementary particles repeat the appearance, why there are three generations, instead of two generations, four generations, five generations? These are unsolved mysteries.
So scientists are not happy about discovering new elementary particles, they hate the emergence of new particles, just like in 1955, physicist Lamb said in his Nobel Prize speech, in the beginning if you find a new particle, you will definitely win the Nobel Prize, and now if someone finds a new particle, you should first fine $10,000. It's enough to see that these particles are about to drive physicists crazy.
Well, now that we have found μ, but still have not found π meson, does the π meson exist or not? This Yukawa was a little anxious, he said, everyone believes me, this π meson must exist, and I estimate that this μ is produced after the decay of the π meson, so everyone is walking to a high place, looking for it.
In 1947, british physicist Powell and his team tied the probe to a hot air balloon and sent it to the upper atmosphere, where they found the figure of π meson.
Its mass is 273 times that of an electron, and it can be seen from experiments that π the meson is indeed produced directly from strong interactions, so it is the particle predicted by Yukawa.
However π mesons and μ are unstable and decay in 2.6×10^-8 seconds, such as a π+ can decay into a positive mucchi and a mucular neutrino, a π - can decay into a negative mucil and an anti-mucil neutrino.
In the above process, the generated muse is unstable and will continue to undergo decay, and at about the same time, people also discovered the π0 meson predicted by Yukawa, which is electrically neutral, its mass is 264 times that of an electron, and its average lifespan is about 0.84× 10^-16 seconds, and then it decays into two gamma photons.
From the product of decay, it can be seen that the decay of the π0 meson is a process dominated by electromagnetic force, and since the electromagnetic force is much stronger than the weak force, the decay process led by it takes less time than the decay dominated by the weak force. Later we will also encounter decay dominated by force, which will be shorter.
It can be seen that particle physics has developed to the present stage, new particles are constantly emerging, and in addition to the decay of the β that we thought before, there are several more ways of decay, you see the decay of the π meson, the decay of the muse. Therefore, people feel that it is very important to understand the decay dominated by weak forces, which also accelerates the study of weak forces.
Of course, strong force is also a problem that people think about, but this time is not the focus, because we have not completely uncovered the internal structure of hadrons, and still treat protons and neutrons, including the newly discovered π mesons, as elementary particles, so we will put forward the prediction of π mesons, and I did not expect that this thing really existed. Nature is amazing.
To add to this, what I just said about hadrons means that particles involved in strong interactions are called hadrons, including baryons and mesons.
In the previous article we said that there is a particularly important quantum number for particles involved in strong interactions called isospines, the isospin of protons and neutrons is 1/2, and the projection of the isospine spin space, the proton is upwards, that is, +1/2, the neutron is downwards, that is, -1/2, and the two projections represent two states of charge.
That π meson also has isospines of 1, which have three projections, +1, 0, and -1, representing the charge states of the three π, respectively, π+, π0, and π-, which can be remembered and will be used later.
Okay, so that's it for today, and we'll see the hadron explosion in the next lesson.