Weak interaction

In nuclear physics and particle physics, the weak interaction, also called the weak force or weak nuclear force, is one of the four known fundamental interactions. The other three are electromagnetism, the strong interaction, and gravitation.

The weak interaction is very important because it helps subatomic particles change. This change causes atoms to undergo radioactive decay.

This force plays a key role in processes like nuclear fission and nuclear fusion. These processes help create power and make stars shine. Even though the weak force is powerful, it only works over very tiny distances—smaller than the width of a single proton.

Scientists study the weak force using something called quantum flavordynamics (QFD). They also study it as part of electroweak theory. This theory helps us understand how the weak force and electromagnetism are connected.

Background

The Standard Model of particle physics helps us understand how tiny particles interact with each other. These interactions happen when particles exchange special carriers called bosons. The particles can be simple, like electrons or quarks, or more complex, like protons or neutrons.

In the weak interaction, particles exchange three types of bosons: W⁺, W⁻, and Z bosons. These bosons are very heavy, so the weak force only works over short distances. The weak force is called "weak" because it is much weaker than other forces, like the electromagnetic force.

The weak interaction can change the types of quarks. For example, during beta-minus decay, a down quark in a neutron changes into an up quark, turning the neutron into a proton. This process also releases an electron and an antineutrino.

The weak interaction helps stars produce energy. It helps turn hydrogen into helium, which makes stars shine. It is also responsible for the decay of carbon-14 into nitrogen-14, which is used in radiocarbon dating.

History

In 1933, Enrico Fermi created the first idea about how the weak interaction works. He thought that beta decay happened because of a special force between tiny particles.

Later, scientists noticed something unusual about how these tiny particles spun. In the 1950s, Chen-Ning Yang and Tsung-Dao Lee guessed that this spinning might break a certain rule. An experiment in 1957 showed they were right.

In the 1960s, three scientists combined what we know about two forces — the electromagnetic force and the weak interaction — into one idea called the electroweak force. We didn’t see the particles that carry this force until 1983.

Properties

The weak interaction is one of the four basic forces in nature. It can change the type of tiny particles called quarks and leptons. For example, it can turn one kind of quark into another.

This force breaks some rules that other forces follow. It works through particles called W and Z bosons, which are very heavy and don't last long. Because of this weight, the weak interaction is weaker than other forces and only works over very short distances.

One big effect of the weak interaction is in radioactive decay. For example, a neutron can change into a proton by turning one of its quarks into a different type. This process can't happen through the strong force or electromagnetism, only through the weak interaction.

All tiny particles have a property called weak isospin, which affects how they interact through the weak force. Left-handed particles have weak isospin, while right-handed ones do not. This property helps scientists understand how particles change during weak interactions.

Left-handed fermions in the Standard Model
Generation 1			Generation 2			Generation 3
Fermion	Symbol	Weak isospin	Fermion	Symbol	Weak isospin	Fermion	Symbol	Weak isospin
electron neutrino	ν _e	⁠++1/2⁠	muon neutrino	ν _μ	⁠++1/2⁠	tau neutrino	ν _τ	⁠++1/2⁠
electron	e⁻	⁠−+1/2⁠	muon	μ⁻	⁠−+1/2⁠	tau	τ⁻	⁠−+1/2⁠
up quark	u	⁠++1/2⁠	charm quark	c	⁠++1/2⁠	top quark	t	⁠++1/2⁠
down quark	d	⁠−+1/2⁠	strange quark	s	⁠−+1/2⁠	bottom quark	b	⁠−+1/2⁠
All of the above left-handed (regular) particles have corresponding right-handed anti-particles with equal and opposite weak isospin.
All right-handed (regular) particles and left-handed antiparticles have weak isospin of 0.

Interaction types

There are two types of weak interaction. The first type is called the "charged-current interaction" because it involves particles that have an electric charge. The second type is called the "neutral-current interaction" because it involves particles with no electric charge. These interactions help explain how tiny particles change and move.

Charged-current interaction

Main article: Charged current

In one kind of charged current interaction, a charged particle like an electron can change into a neutrino by sending out a special particle called a W boson. A particle called a quark can also change into a different type of quark by sending out or taking in a W boson. These changes happen very quickly.

Neutral-current interaction

Main article: Neutral current

In neutral current interactions, a quark or a particle like an electron can send out or take in a Z boson, which has no charge. The Z boson changes quickly after it is made. This type of interaction can happen with many different particles.

Electroweak theory

Main article: Electroweak interaction

The Standard Model of particle physics explains the electromagnetic interaction and the weak interaction as two parts of one electroweak interaction. This theory was created around 1968 by Sheldon Glashow, Abdus Salam, and Steven Weinberg, who received the 1979 Nobel Prize in Physics for their work. The Higgs mechanism helps explain why some particles have mass. It describes how three particles, called W⁺, W⁻, and Z⁰, get their mass, while another particle, the photon, remains massless.

According to this theory, at very high energies, the universe had four parts of a special field, and four particles that carried interactions. But at lower energies, these interactions change, and three of these particles combine to give mass to the W and Z particles. The fourth particle remains the photon. This theory has helped scientists predict the masses of the Z and W particles before they were discovered in 1983. In 2012, scientists at the Large Hadron Collider found a new particle that looks like the Higgs boson, and by 2013, they believed it existed.

Violation of symmetry

Scientists used to think that the rules of nature worked the same way even if you looked in a mirror. They called this idea parity conservation. It worked for forces like gravity, electricity, and the strong force.

But in the 1950s, two scientists suggested the weak force might break this rule. An experiment later proved this, and those scientists won a Nobel Prize.

At first, people used an older theory to explain the weak force. But new discoveries led to a better idea in 1957. This new idea showed that the weak force only affects certain particles. This helps explain why the mirror rule doesn’t work for it.

Later, scientists found that another rule, called CP, could also be broken in rare cases. This discovery helped explain why there is more matter than antimatter in the universe.