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The standard model of particle physics collects all the elementary particles that make up matter and their interactions. However, when the model was first completed, not all particles had been measured, but one experiment after another filled these gaps. The W and Z bosons were discovered at CERN in 1983, the top quark was discovered at Fermi Lab in 1995, and the last was the Higgs boson found at CERN in 2012, the last nail in the standard model coffin.

But particle physicists believe there is more to be discovered. As early as the 1970s, they said that the standard model was not good enough because it contained three different fundamental forces, namely, electromagnetic force, strong force and weak force, which particle physicists wanted to unify into one force. A better theory that combines these three forces is called the grand unified theory, which can be obtained by assuming greater symmetry than the standard model. The grand unified theory reproduces the standard model in the range that has been tested, but deviates in the untested range.

What I want to say is that the grand unified theory was a reasonable attempt at that time, because the principle of symmetry worked well in physics in the past. The standard model itself is born from the principle of symmetry, even if Einstein himself does not use symmetry argument, we still understand his theory as the realization of some symmetry. But this time, more symmetry doesn't work.

The grand unified theory predicts that protons, one of the components of the nucleus, are unstable. Since the 1980s, there have been experiments looking for proton decay, but we have never seen this happen. This excludes several models of the grand unified theory, but some models choose to add more complex components in order to be compatible with the experimental results.

The next problem is the axion. The standard model contains some parameters that must be determined by experiments, one of which is called the θ parameter, which is found to be zero or too small to be distinguished from zero. If it is not zero, the force violates the CP symmetry. But the standard model does not force it to be zero, so particle physicists have added a mechanism to allow a smaller value for this parameter, but the result of this modification is the existence of a new particle called an axion.

Unfortunately, it turns out that axions don't exist. If the axon exists, the neutron star cools quickly, and we can observe this phenomenon. With this argument, the axion was eventually excluded from the experiment in 1980. But physicists did not give up the axion, they changed the theory to avoid the limitations of the experiment. In 1981, a new type of axion was launched, and many experiments continue to look for these invisible axions.

Another flawed idea put forward by particle physicists in the 1970s was supersymmetry. Supersymmetry assumes that all particles in the standard model have a partner particle. However, the idea failed when it was proposed, because these partner particles have the same mass as the particles of the standard model to which they belong. If they exist, they will appear in the first Particle Collider. But they did not appear, so the supersymmetry was immediately modified so that the partner particles had a higher mass. It takes a lot of energy to produce heavy particles, so it takes a larger particle collider to see them.

In the 1990s, the predictions made by the first supersymmetric model were tested on the large Electron Positron Collider at CERN, and they were immediately falsified. The supersymmetry was modified again and then denied by the larger Collider. In this way, physicists continue to modify these supersymmetric models to avoid conflicts with the new data.

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

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