Question

A free neutron decays into a proton and an electron (and an anti-neutrino). A free proton has never been observed to decay into anything. Why then do we consider the neutron to be as "fundamental" (at the nuclear level) a particle as the proton? Why do we not consider a neutron to be a proton-electron composite?

Short answer

Additionally, why do we not consider a neutron to be a proton-electron composite? Answer: Neutrons are considered "fundamental" at the nuclear level because they are basic building blocks of the atomic nucleus, just like protons. Both protons and neutrons are composed of quarks, and their properties and interactions depend on their quark compositions. A neutron is not considered a proton-electron composite because it has a different quark composition than a proton, and electrons are leptons, which are fundamental particles not built from quarks. Neutron decay highlights these particle interactions and weak forces involved but does not imply that a neutron is a composite of a proton and an electron.

Step-by-step solution

Brief Introduction to Fundamental Particles

In particle physics, an elementary particle or fundamental particle is a particle with no substructure, meaning that it is not composed of any smaller particles. As of now, the fundamental particles we know are divided into 3 categories: quarks, leptons, and gauge bosons. The standard model of particle physics describes the interactions between these particles and their properties.

Protons and Neutrons: Composition

Protons and neutrons, collectively known as nucleons, are not fundamental particles. They are considered composite particles because they are made up of elementary particles called quarks. A proton consists of two up quarks and one down quark, while a neutron consists of two down quarks and one up quark. These quarks are held together by the strong nuclear force mediated by the exchange of gluons (another type of elementary particle).

Neutron Decay

When a free neutron decays, it breaks down into a proton, an electron, and an anti-neutrino. This process occurs due to the weak interaction, which relates to the conversion of one type of quark to another. During the decay, one of the neutron's down quarks changes into an up quark by emitting a W- boson (responsible for the weak interaction). The W- boson then decays further into an electron and an antineutrino.

Why is Neutron considered fundamental at the nuclear level?

Although neutron can decay into a proton and an electron, it is still considered as fundamental at the nuclear level due to its quark composition. In a nucleus, protons and neutrons are closely packed, and it is their quark composition that dictates their properties and interactions through different forces. The term "fundamental" in this context does not mean that neutrons are elementary particles but rather that both protons and neutrons are the basic building blocks of the atomic nucleus.

Neutron: not a Proton-Electron composite

A neutron cannot be considered a composite of a proton and an electron because it has a quark composition different from the proton. Electrons are leptons and are considered fundamental particles, not built up from quarks. The process of neutron decay demonstrates the weak interaction and the particle interactions involved in that process, including the emission of a W- boson, but it doesn't imply that a neutron is a composite of a proton and an electron. Instead, it highlights that both protons and neutrons are built from quarks, and their properties and interactions depend on their quark compositions.

Key concepts

Fundamental Particles

In the world of particle physics, fundamental particles are the simplest building blocks of our universe. Unlike larger particles, these fundamental particles have no substructure. This means they aren't made of any smaller components.

There are three main categories:

• Quarks
• Leptons
• Gauge bosons

These particles and their interactions are described by the Standard Model of particle physics. This model explains how fundamental particles interact with each other through forces, defining properties such as mass and charge. Despite being called 'fundamental' at the nuclear level, protons and neutrons aren't truly fundamental particles since they are composed of even smaller components - the quarks.

Quarks

Quarks are the basic constituents of protons and neutrons. They are fundamental particles with intrinsic properties like charge, mass, and spin.

There are six types of quarks, known as "flavors":

• Up (u)
• Down (d)
• Charm (c)
• Strange (s)
• Top (t)
• Bottom (b)

In protons and neutrons, or nucleons, up and down quarks are the primary players. They combine to form these composite particles:

• A proton is made up of two up quarks and one down quark (uud).
• A neutron is composed of two down quarks and one up quark (ddu).

Quarks are held together inside nucleons by the strong nuclear force, mediated by particles called gluons, which ensure that nucleons remain intact within the nucleus.

Weak Interaction

The weak interaction is one of the four fundamental forces in nature and plays a critical role in processes like neutron decay. This force is unique in its ability to change the type (or flavor) of quarks.

In neutron decay, the weak interaction causes one of the down quarks in the neutron to transform into an up quark, effectively turning the neutron into a proton. This transformation involves the emission of a W- boson, a particle that mediates the weak force. The W- boson then decays further into an electron and an anti-neutrino.

The weak interaction is much weaker than the strong force, which binds quarks together, but it is fundamental in processes like nuclear fusion and radioactive decay, essential in stars like our sun.

Nucleons

Nucleons are the particles that make up the atomic nucleus, mainly consisting of protons and neutrons. Even though they are not fundamental, they are vital components of atoms.

Protons and neutrons are responsible for the mass of the atom and hold the nucleus together by exerting the strong nuclear force. The term 'fundamental' at the nuclear level often refers to how essential they are as building blocks within atoms, rather than being indivisible particles themselves.

While neutrons can decay into protons, this does not imply they are merely a proton-electron composite. Instead, this decay process highlights the intricate interactions between quarks and the weak force. Thus, protons and neutrons are deeply intertwined in their roles in atomic structure and nuclear reactions, showcasing the elegance and complexity of particle physics.