Shulou(Shulou.com)11/24 Report--

Looking in the mirror, we will find that the left side of the body is almost the same as the right side, which is called symmetry. Not only can we see it in nature, but more fundamentally, symmetry seems to be written in the blueprint of the universe. For example, symmetry in quantum mechanics leads to three basic natural forces: electromagnetic force, weak force and strong force. Although symmetry is important, it is also important to break it. Without these broken symmetries, we would not have a familiar universe and there would be no life as we know it.

In physics, symmetry means that the properties of particles do not change after transformation. The simplest example is that whether you are on the surface of the earth or on the Chinese space station, the laws of physics are the same, which is the symmetry of space translation. Or maybe the laws of physics today are the same as they were a hundred years ago, which is the symmetry of time translation. Such symmetry shows us that there are some simple rules for building the universe, but it sometimes breaks some of them.

Symmetry breaking one: the mass of elementary particles is shown in the following figure, which depicts the energy potential in physics. There are two minimum values on the left and right, and a local maximum in the middle. If we place a mirror vertically along the center, the two sides are symmetrical. Now, if I asked you to place a ball without breaking its symmetry, where would you put it? The only option is to put it in the middle as much as possible. If the ball is slightly disturbed and slides to one side, then the symmetry will be broken. In fact, this is an intuitive representation of the Higgs mechanism that assigns mass to elementary particles. Some elementary particles are not in the center and break the symmetry in the non-zero potential of the Higgs field.

Why can quality be explained only by symmetry breaking? The mass problem is the basis of the standard model structure, and its core has three symmetries: U (1), SU (2) and SU (3). Each symmetry is related to the basic force of nature, that is, electromagnetic force, weak force and strong force, respectively. In other words, these symmetries lead to three of the four fundamental forces. These form the foundation of our universe, without which there would be no nuclei, no atoms, no chemistry, no structure of any kind, and certainly no life as we know it.

These forces are mediated by force-carrying particles, that is, photons in electromagnetic forces, W and Z bosons in weak forces and gluons in strong forces. Now the problem is, according to the standard model equation, these particles must be massless. This is the requirement of the symmetry of the equation, and it gives us massless bosons, not massive bosons. This also includes the Higgs boson, which must also be massless according to the equation.

This is obviously not in line with reality, because the theory describing weak forces is valid only if W and Z bosons have mass. This has something to do with the fact that the range of action of a weak force is very limited. If these two particles have no mass, then their range of action will become infinite. So how does the mass of these elementary particles come from? The emergence of mass is due to the Higgs mechanism, which is the result of symmetry breaking.

The expected value of the Higgs field is not zero, and the elementary particles interacting with the Higgs field under this non-zero potential will obtain a rest mass. All masses associated with standard model particles are due to the fact that they interact with the Higgs field. Particles that do not interact with the Higgs field are still massless, just like massless photons.

Symmetry breaking 2: atomic mass, in fact, all mass is caused by symmetry breaking, not just the mass of elementary particles. The Higgs field cannot explain most of the mass in the universe. Neutrons and protons account for almost the mass of the entire atom. Their mass comes from the binding energy of internal quarks and the binding energy between nucleons. In other words, it comes from gluon interactions related to strength.

If you add up the mass of all the elementary particles that make up the atom, it accounts for only about 1% of the atomic mass. Due to the strong effect, 99% of the mass comes from the binding energy. The question now is what kind of symmetry breaking explains it. It has been proved that in addition to the canonical symmetry of the three fundamental forces, there are other symmetries in the standard model, such as chiral symmetry. The theory of chiral symmetry treats both left-handed and right-handed particles. For example, if we exchange left-handed particles with right-handed particles in any reaction, nothing will change.

Consider a left-handed quark whose mass is 2.3MeV, and the antiparticle of this quark, which is right-handed. When these two quarks are combined, this combination is called meson. If left and right chirality are treated equally, the meson will be annihilated with a net energy of about zero. In other words, the combination of quark antiquark pairs will result in zero mass. But if the chiral symmetry is broken, they will not be treated equally and the net energy will not be zero.

In fact when the two particles are bound together by gluons there are some significant changes in mass. If you add up the masses of the quark, it is not 4.6MeV, but as high as 135to 140MeV. Where does this increase in quality come from? This is due to the destruction of chiral symmetry.

When quarks are strongly restricted by the exchange of gluons, the chiral symmetry is broken. Gluons form a cloud around quarks that limits quarks and produces a binding energy, which is measured as the mass of mesons. The same thing happens inside protons and neutrons.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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