Generation (particle physics)

Tiny bits make up everything. There are three groups of these bits. We call these groups families.

Some bits are very light. Other bits are much heavier. The heavy bits want to change.

They change into the light bits. This is why our world is made of light bits. Most things use them.

We only see heavy bits in big machines. We also see them in space. They are hard to find.

Scientists wonder why there are three groups. It is a big mystery to solve.

Everything is made of tiny bits called particles. These particles come in three groups. We call these groups generations. Each group has four kinds of particles. Two are called leptons. The other two are called quarks.

Particles in each group are different. They have different masses. Mass is how much something weighs. Each new group is heavier than the last. For example, the electron is in the first group. It is very light. The muon is in the second group. It is heavier. The tau is in the third group. It is very heavy.

Because they are heavy, these particles change. They decay into particles from the first group. This is why our world is made of first-generation particles. We only see the heavy ones in space. We also see them in big machines. These machines are called particle accelerators.

Scientists do not know why there are three groups. This is a big mystery. Some think there might be a fourth group. But so far, no one has found one. People keep looking for new particles in big machines.

In the world of particle physics, scientists group tiny bits of matter into families. These families are called generations. Each generation contains four specific types of particles. Two of these are leptons, which include things like the electron. The other two are quarks, which are the bits that make up larger particles. These generations are very important for understanding how our universe works.

Each generation works in a similar way, but they have different weights. We call this weight mass. Particles in the same generation have the same electric charge. However, particles in a higher generation have more mass than the ones before them. This means a second-generation particle is heavier than a first-generation one. Because they are so heavy, these particles often decay. Decaying means they break down into lighter particles from the first generation.

We can see how this mass changes by looking at the leptons. The first-generation electron has a very small mass. The second-generation muon is much heavier than the electron. The third-generation tau is even bigger. It is almost twice as massive as a proton. Most of our everyday matter is made of first-generation particles. We only see the heavier ones in special places. These places include cosmic rays in space or particle accelerators on Earth.

Scientists have been studying these families for a long time. A man named Haim Harari first used the word generation in 1976. He shared this idea at the Les Houches Summer School. Since then, experts have used big machines to look for more. For example, the Large Electron–Positron Collider at CERN ruled out some ideas in 1989. Today, scientists use the Large Hadron Collider to look for even more particles. They are searching for a possible fourth generation of matter.

Knowing about these generations helps us understand the world around us. Atoms are made of protons, neutrons, and electrons. These are all particles from the first generation. Even though we do not see heavy particles every day, they are still part of nature. Neutrinos are special because they stream through the whole universe. They rarely hit anything at all. Understanding why these generations exist is still one of the biggest mysteries in science.

In the field of particle physics, scientists organize elementary particles into groups called generations. These groups are also known as families. The Standard Model of particle physics states that there are exactly three generations of these particles. Each generation is composed of four specific types of particles. Two of these particles are categorized as leptons. The other two are categorized as quarks. Generations are a vital concept because they help us understand the structure of matter. They also explain why the universe looks the way it does today.

Particles within the same generation share certain fundamental characteristics. For example, their electric and strong interactions are identical. However, particles differ between generations in two major ways. They differ in their mass and their flavour quantum number. Within a single generation, the leptons are divided into two types. One type has an electric charge of −1, which we call electron-like. The other type is neutral and is called a neutrino. The quarks are also divided into two types. One is a down-type quark with a negative charge. The other is an up-type quark with a positive charge.

A key feature of these families is the mass hierarchy. This means that each member of a higher generation has more mass than the corresponding particle in the previous generation. Neutrinos are a possible exception to this rule. Their masses are small but not zero, though they have not been accurately determined. We can see this mass increase clearly in the leptons. The first-generation electron has a very small mass. The second-generation muon is much heavier. The third-generation tau is even more massive. In fact, the tau is almost twice as massive as a proton. This heavy mass causes higher-generation particles to decay. They break down into particles from the first generation. This process explains why everyday atoms are made only of first-generation particles.

History shows how our understanding of these families has grown. The term "generation" was first introduced by Haim Harari. He used this term at the Les Houches Summer School in 1976. Since then, scientists have used massive machines to test these ideas. For example, researchers at CERN used the Large Electron–Positron Collider, or LEP. In 1989, measurements at the LEP ruled out a fourth generation with a light neutrino. This neutrino would have had a mass less than about 45 GeV. Scientists continue to search for more particles using high-energy colliders. They look for evidence of a fourth generation in places like cosmic rays or particle accelerators.

There is a lot of debate regarding a possible fourth generation of particles. Many theoretical physicists believe a fourth generation is unlikely. This is because extra generations would cause subtle changes to electroweak observables. Current measurements strongly disfavor these changes. However, searches at high-energy colliders continue. Scientists use specific symbols to denote these potential fourth-generation particles. They take the symbols for third-generation particles and add a prime mark. For example, they use b′ and t′ to represent fourth-generation quarks. So far, no evidence for these particles has been observed.

Specific numbers help scientists set limits on what these particles might be. As of 2010, the lower bound for a fourth-generation neutrino mass was about 60 GeV. This is millions of times larger than the upper bounds for the other three neutrinos. In 2019, experiments at the Large Hadron Collider set a lower bound for fourth-generation quarks at 1.4 TeV. In 2012, the lower bound for a fourth-generation charged lepton was 100 GeV. Some theories, like the Koide formula, suggest even higher masses. This formula suggests fourth-generation charged leptons could be 44 GeV, though that was ruled out. It also suggests quarks could be as heavy as 3.6 TeV or 84 TeV.

The reason why multiple generations exist remains one of the greatest unsolved problems in physics. Scientists are looking for a deeper explanation for this count of three. String theory offers some possible reasons for multiple generations. In string theory, the number depends on how D-brane intersections are compactified. Grand unified theories in 10 dimensions also naturally contain three generations of matter. Some models suggest that a single fermion field can produce multiple mass poles. This might explain the large mass ratios we see between generations. Understanding these relationships could eventually explain the very nature of mass itself.