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Chapter 39: Problem 12

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

Expert verified
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

01

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.
02

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).
03

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.
04

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.
05

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.

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