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We know that unbound free neutrons decay in about 15 minutes, producing protons, electrons and antineutrinos. By contrast, protons are a more noteworthy particle for two reasons. First, the standard model of particle physics shows that protons are 100% stable, which means they will not decay. On the other hand, other theories predict that protons will decay, or at least eventually. Some scientists hope that these new theories will replace the standard model, so it is very important to look for proton decay.

First of all, let's take a look at why the standard model claims that protons are stable, which all boils down to conservation laws. The law of conservation states that some attributes will not change anyway. When talking about proton decay, three specific conserved quantities are important-- charge, energy, and baryon number.

We are all familiar with charge, and the conservation of charge means that the amount of charge after decay must be the same as that before decay. Conservation of energy has many meanings, but the key one has to do with Einstein's equation E=mc ², which means that energy and mass are the same. For particle decay, the conservation of energy means that particles can only decay to lighter particles.

The number of baryons is less known. Baryons are particles made up of three quarks. Protons and neutrons each contain three quarks, so both protons and neutrons are baryons, but there are many kinds of baryons. The baryon number is very simple, the baryon number of each baryon is + 1, and the baryon number of antimatter baryon is-1. To find the total number of baryons, just add them up. For example, at the large Hadron Collider, when you hit two protons with the baryon number + 1, the number of baryons before the collision is + 2. After the collision, no matter how chaotic and complex particles are produced, the number of baryons is still + 2.

According to the standard model, the baryon number, charge and energy are all conserved. There are more conserved quantities, but we only focus on these three. For example, in the case of neutron decay, neutrons become protons, electrons, and electron antineutrinos. Since the baryon number of both protons and neutrons is + 1, the baryon number is conserved before and after decay. Because neutrons have zero charge, protons have positive charge, electrons have negative charge, and antineutrinos have no charge, so the charge is zero before decay and zero after decay, which conforms to the charge conservation. In terms of energy, when we add up the mass of protons, electrons and antineutrinos, it cannot be greater than the mass of neutrons.

Now let's think about proton decay, so we need to introduce two kinds of particles first. The first particle is the positron, which is the antimatter counterpart of the electron. The positron has a positive charge and the mass is about 0.05% of that of the proton. Because the positron is not a baryon, its baryon number is zero. The second particle is the pion meson, which has no charge and has a mass of about 15% of the mass of a proton. Moreover, it is not a baryon, so its baryon number is zero. Finally, there is an important fact that protons are the lightest baryons.

The standard model shows that any decay must obey the conservation of charge, energy and baryon number. Therefore, for protons with baryon number + 1, positive charge and fixed mass, if they decay into a certain number of particles, then the total mass of these particles cannot be greater than that of protons, the total charge is equal to positive one and the total baryon number is + 1. But that's the problem. Protons are the lightest baryons, so any baryons that decay into them will be heavier than protons, which would violate the conservation of energy. Therefore, according to the standard model, protons cannot decay and protons are therefore stable.

However, in some proposed standard model substitution theories, some of the conservation laws have been relaxed. For example, these models often say that protons can decay into positrons and pion mesons. We see that in this decay, the charge is conserved, and the mass of the particles after decay is not larger than that before decay. So proton decay can occur on the grounds of charge and energy. On the other hand, we see that the number of baryons is + 1 before decay and 0 after decay. This decay is prohibited in the standard model, but is allowed in some possible alternative theories of the standard model. Therefore, the observation of proton decay will be an important verification of the new physical theory.

Third, we know that protons do not decay too quickly. If that were the case, the atom wouldn't exist and we wouldn't be here. Fortunately, these new substitution theories show that protons have a very long life. Even in the 1980s, when these theories were new, proton lifetimes were predicted to be about 10 ^ 31 years. The universe has existed for about 13.7 billion, and protons live much longer than the universe.

But particle decay is a statistical process, and if there are enough protons, we will observe that some will decay ahead of time. Calculate that 30000 tons of water contains about 10 ^ 31 protons, and the SuperK detector is such a cylindrical tank filled with ultra-pure water. In 1996, SuperK scientists turned on their detectors and began to look for decaying protons. Although they have made a large number of measurements of neutrinos, they have not yet seen the decay of individual protons. Based on this, the researchers concluded that if a proton decays, its life span should be more than 2 × 10 ^ 34 years.

So does this mean that protons don't decay? All we can say is that they don't decay into positrons quickly. We need to build larger detectors to see if the decay life of protons will be longer than that. Now, the study of proton decay continues.

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

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