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 results in three basic natural forces: electromagnetic force, weak force, and strong force. Although symmetry is important, breaking it is also important. Without these broken symmetries, we would not have the familiar universe, and we would not have the life we knew.
In physics, symmetry means that the properties of particles will not change after transformation. The simplest example is that the laws of physics are the same whether you are on the surface of the earth or on China Space Station , and this is the spatial translation symmetry. Or maybe today's laws of physics 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: elementary particle mass
is shown in the figure below, which depicts the energy potential in physics. There are two minimum values on the left and right, and a local maximum value in the middle. If we place a mirror vertically along the center, the sides are symmetrical. Now, if I asked you to put a ball and it doesn't destroy its symmetry, where would you put it? There is only one option, which is to put it in the middle as much as possible. If the ball is slightly disturbed and slides down to one side, the symmetry will be broken. In fact, this is an intuitive representation of the Higgs mechanism that imparts mass to elementary particles. Some elementary particles are not in the center, destroying symmetry in the non-zero potential of the Higgs field.
Why can quality be explained only by symmetric breaking? The quality problem is the basis of the standard model structure, and its core has three symmetry: U(1), SU(2) and SU(3), each symmetry is associated with the basic forces of nature, namely electromagnetic force, weak force and strong force respectively. In other words, these symmetries lead to three of the four basic forces. These constitute the basis of our universe. Without them, there would be no atomic nucleus , no atom , no chemistry, no structure of any kind, and of course there is no life we know.
These forces are mediated by force-carrying particles, namely photon in electromagnetic force, W and Z bosons in weak force, and gluon in strong force. Now the problem is that according to the equations of the Standard Model, these particles must all be massless. This is the requirement of equation symmetry, and what it gives us is a massless boson, not a mass boson. This also includes Higgs boson , which according to the equation must also be massless.
This is obviously not true, because the theory of describing weak forces is only effective if the W and Z bosons have mass. This is related to the fact that the range of weak forces is very limited, and if these two particles have no mass, their range of action becomes infinite. So how do the mass of these elementary particles come into being? 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 that interact with the Higgs field under this non-zero potential will obtain a static mass. All the masses associated with the particles of the Standard Model 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.
Symmetrical 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 the internal quark and the binding energy between the nucleus . In other words, it comes from the gluon interaction associated with force.
If the mass of all the elementary particles that make up the atom is added up, it only accounts for about 1% of the atomic mass.Due to the powerful effect, 99% of the mass comes from binding energy . The question now is what kind of symmetry breaking explains this symmetry breaking. It turns out that besides giving us the normative symmetry of the three basic forces, there are other symmetry in the standard model, such as the existence of chiral symmetry. Chiral symmetry theory treats left-handed and right-handed particles likewise, for example, if we exchange left-handed particles with right-handed particles in any reaction, the result is that nothing will change.
Consider a left-handed quark, its mass is 2.3MeV, and the antiparticles of this quark, which is right-handed. When these two quarks are combined, this combination is called meson . If left-handedness is treated equally, this meson will be annihilated with a net energy of approximately zero. In other words, a combination of quark antiquark pairs will result in a mass of zero. But if chiral symmetry is destroyed, they will not be treated equally and the net energy is not zero.
In fact, when these two particles are bound together by gluons, the mass changes a little significantly. If the individual masses of the quark are added together, it is not 4.6MeV, but it is as high as 135 to 140MeV. Where does this increase in mass come from? This is due to the destruction of chiral symmetry.
When quarks are strongly restricted through the exchange of gluons, chiral symmetry will be broken. Gluons form a cloud around the quark that limits the quark and produces a binding energy that is measured as the mass of the meson. The same thing happens inside protons and neutrons.
