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The original title: "have we only found half of physics?"

Physics has an ultimate goal: to find some relevant basic theories and then combine them to form a theory of everything. So far, what we are closest to the theory of everything is the standard model of particle physics, and the equation behind this theory is the most accurate equation we have in physics. However, there are many things that cannot be explained by theory, such as the quantum theory of dark matter, dark energy and gravity.

There is a theoretical idea that many people think can bring us closer to the theory of everything and fill several gaps in the standard model. This theory, called supersymmetry, is the idea that particles of matter and particles carrying forces are connected by a new symmetry. Although there are no experiments to confirm this symmetry, it is still important because in physics, symmetry can be broken and supersymmetry can still explain a lot of things.

If supersymmetry is true, it can solve several problems in our current understanding of the universe. For example, it can explain why the mass of the Higgs boson is so small, it can combine three fundamental forces: electromagnetic force, strong force and weak force, and it can provide the perfect candidate for elusive dark matter particles. So, what exactly is supersymmetry? Why is symmetry so important in our search for the theory of everything?

Supersymmetry supersymmetry (SUSY) is the symmetry between matter particles (fermions) and force particles (bosons) in the standard model. The main difference between these two particles is spin. In the standard model, the spin of the fermion is 1max 2, while the spin of all bosons in the standard model is 0 or 1. The rule followed by fermions is that they cannot be in the same position with the same spin at the same time, which is the Pauli incompatibility principle. But this rule does not apply to bosons because many of the same bosons can appear in the same place.

If you look at some standard models, you will find that there are much more particles of matter than those carrying forces. does the universe prefer matter to forces that allow them to interact? Because the standard model is full of symmetry, it seems to many physicists that the basic symmetry between fermions and bosons should also exist. So they use supersymmetry to describe the symmetry between matter and force.

Although there are many different versions of supersymmetry theory, the basic concept of supersymmetry theory is the same: each particle has a mirror particle or supersymmetric corresponding particle. Each mirror particle is of the opposite type, for example, the mirror particle of a fermion is a boson, and the mirror particle of a boson is a fermion. Through the simple concept of supersymmetry, we can bring balance to the standard model so that we have the same number of bosons and fermions.

What's the point of all this, you might ask? Is it really important to have the same number of bosons and fermions? Do we really need this symmetry? The simple answer is that if it is correct, it will help us understand the universe better.

The benefits of supersymmetry as early as the 1980s, physicists began to notice some problems with the standard model. Generally speaking, supersymmetry will solve three problems: first, it can solve the low quality of the Higgs boson, second, it can unify the three basic forces-electromagnetic force, strong force and weak force, and third, it provides a solution to dark matter.

The first problem is simply that the mass of the Higgs boson looks unnatural. The mass of the Higgs particle is 125GeV, which seems unlikely in theory. Why? This is related to how quantum correction affects the quality of the Higgs boson. We can write down the mass of the Higgs boson, as shown in the following picture. M_measure is a measure of mass, which we can measure in experiments. M_corrections is a mass correction calculated according to the equation of the standard model. Masks 0 is a tuning parameter called bare mass, which the standard model does not tell us should be.

We get the m_measure from the experiment of the large Hadron Collider. The problem is that when we use the equation of the standard model to calculate the m_corrections, the result is several orders of magnitude larger than the measured mass. Based on this, MQ0 needs to be numerically equal to m_corrections, but the symbol is opposite. In other words, the muni0 parameter needs to be so high that it almost completely offsets the huge quantum correction calculated in theory, and the high value of muni0 seems unnatural, so we need other theories to fine-tune it.

This is where supersymmetry works, and it can better explain the quality of Higgs' measurements. Extra particles from supersymmetry can offset these very large quantum corrections because bosons contribute the opposite Higgs mass compared to fermions, as shown in the following equation.

Then there is the question of the unity of forces. Physicists believe that all forces are unified on some basic levels, and there are some theories that unify forces and explain physics in a unified way. Each force in the standard model has some coupling constants that reflect the strength of the force, which are actually not constant because they change at higher temperatures. Physicists predict that the coupling constants should be combined at a very high temperature, so at some point, different coupling constants of different forces should become equal in the standard model.

The original standard model does not seem to meet this requirement, but adding additional particles from supersymmetry can change the way the coupling changes at higher temperatures, making them possible to unify at some point. This is because the temperature dependence of the coupling depends on the number of particles in the theory, and supersymmetry will at least double the number of particles in the theory.

Dark matter may be a stable electrically neutral particle, and undiscovered supersymmetric particles may be candidates for dark matter.

Have you found supersymmetry? the theory sounds so good, so why haven't we found any supersymmetric particles yet? The first reason may be that particles are too heavy for us to produce them in the best accelerators. The second reason is that if they are indeed dark matter particles, they must be weakly interacting massive particles. So we can produce them, but we don't detect them because their interaction is very weak.

Although the concept of supersymmetry is very good, it can explain some important aspects. But with the passage of time, physics thinks that it is less and less likely to be correct, because we have not seen its shadow so far.

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

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