Electron–positron annihilation

Tiny bits of matter can crash.

One bit is an electron. The other is a positron. When they hit, they disappear. They turn into bright light.

This light flies away fast. It can fly in two directions. Sometimes, even more light bits fly out. This happens when they hit with much energy. This light helps doctors see inside you. It is a big, bright surprise!

Tiny bits of matter can crash into each other. This happens when an electron hits a positron. A positron is an antiparticle. This means it is like an electron but different.

When they hit, they disappear. This is called annihilation. They turn into light called photons.

At low speeds, they usually make two photons. These photons fly in opposite directions. They must do this to keep energy balanced. Sometimes, they make three or more photons.

If they hit with high energy, they can make more. They can make heavy particles like B mesons. They can also make W and Z bosons. These are particles that carry the weak force.

Scientists use this to help people. Doctors use it for PET scans. These scans help see inside the body. It also helps scientists study metals. They can look for tiny gaps in the metal. This way of working helps us learn about our world.

Tiny bits of matter can do something amazing. When an electron hits a positron, they collide. A positron is an antiparticle. It is very much like an electron. However, they are not exactly the same. When they meet, they undergo electron-positron annihilation.

This means they disappear completely. They turn into pure energy in the form of light. This light is made of particles called photons. This process is a key part of how our universe works.

How does this work step by step? At low energies, the two particles simply vanish. They transform into energetic photons. Usually, they create exactly two gamma photons. These photons fly away in opposite directions. They do this to keep energy and momentum balanced. Sometimes, they might create three or more photons. This happens to keep other rules of nature in balance. It is also possible to make neutrino pairs. This is very unlikely and happens much less often.

Nature has strict rules for these collisions. These are called conservation laws. One rule is the conservation of electric charge. The total charge must be zero before and after. Another rule is the conservation of linear momentum. This rule is why a single photon cannot be made alone. There is also conservation of angular momentum. Finally, there is a rule for lepton number. This rule tracks the number of leptons and antileptons. These laws ensure that everything stays perfectly balanced during the crash.

Sometimes, these collisions happen at very high energies. This can create much heavier particles. Scientists can see B mesons or D mesons appear. They can also make W and Z bosons. These particles carry the weak force. The heaviest single-charged particle made is the Z boson. It has a mass of 91.188 GeV/c2. Scientists want to use these collisions to find Higgs bosons. These have a mass of 125.09 GeV/c2.

Building the International Linear Collider is a way to do this.

This science is not just for far-away space. It helps doctors help people every day. Doctors use this for PET scans. This stands for positron emission tomography. It helps them see inside the human body. Scientists also use it to study metals. They use a technique called positron annihilation spectroscopy. This helps them find tiny gaps or defects in metals. It can even help them study semiconductors. This tiny collision helps us understand the solid world around us.

Electron-positron annihilation is a fundamental process in particle physics. It occurs when an electron meets its antiparticle, the positron. These two particles collide and disappear. They transform their mass into pure energy. This energy usually takes the form of light particles called photons. This event is a key way that matter and antimatter interact. Understanding this process helps scientists explore the basic building blocks of our universe.

At low energies, the mechanism of annihilation is quite specific. The electron and positron collide and vanish. This results in the creation of energetic gamma photons. In most cases, exactly two gamma photons are produced. Each photon carries energy equal to the rest energy of the electron or positron. To keep the system balanced, these two photons fly away in opposite directions. Sometimes, three photons are created instead. This happens to satisfy the rule of conservation of charge parity. While more than three photons can be made, it becomes less likely each time.

Nature follows strict rules called conservation laws during these collisions. One law is the conservation of electric charge. The total charge before the collision must equal the total charge after. Since an electron and a positron have opposite charges, the net charge is zero. Another law is the conservation of linear momentum. This law forbids the creation of just one single photon. There is also the conservation of angular momentum. Finally, the process must follow the conservation of total lepton number. This tracks the number of leptons minus the number of antileptons. This is sometimes called a conservation of net matter law.

Collisions can also happen at very high energies. In these cases, the particles have high kinetic energy. This extra energy allows for the creation of much heavier particles. Instead of just photons, scientists can see B mesons or D mesons appear. At even higher energies, the particles can become W and Z bosons. These bosons are the carriers of the weak force. When energies reach the mass of these carriers, the weak force becomes very strong. It becomes much easier to produce neutrinos during these high-energy events.

Scientists use particle accelerators to study these high-energy states. They have produced very heavy particle pairs through annihilation. The heaviest pairs produced so far are W-minus and W-plus pairs. The mass of these particles is 80.385 GeV/c2 each. The heaviest single-charged particle ever produced is the Z boson. It has a mass of 91.188 GeV/c2. One major goal in physics is to produce Higgs bosons. These particles have a mass of 125.09 GeV/c2. Scientists hope to create them using the International Linear Collider.

This scientific process has many practical uses in medicine and industry. Doctors use it for positron emission tomography, or PET scans. This technique helps visualize the inside of a human body. Scientists also use it to study the structure of metals. They use a method called Angular Correlation of Electron Positron Annihilation Radiation. This helps them measure the Fermi surface and band structure. It is also useful for studying nuclear transitions.

Furthermore, annihilation helps us understand the materials we use every day. Positron annihilation spectroscopy is a vital tool for researchers. It is used to study crystallographic defects in metals and semiconductors. In fact, it is considered the only direct way to probe vacancy-type defects. By watching how these tiny particles disappear, we learn about the gaps in solid matter. This connection between subatomic collisions and material science shows how deeply these rules affect our world.