Tru Physics

Pions

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Tru Physics
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Introduction

Pions, also known as pi mesons, are a type of subatomic particle in the meson family. They were first proposed in 1935 by Japanese physicist Hideki Yukawa as a particle mediator for the strong nuclear force that holds protons and neutrons together in the atomic nucleus.

Pions formed by two quarks.
Artisitic representation of two quarks forming a pion. Not an actual image.

Basic Properties of Pions

Pions come in three varieties, each associated with a different charge: \pi^+, \pi^-, and \pi^0, which are positively charged, negatively charged, and neutral, respectively.

Each pion consists of a quark and an antiquark. Specifically, the \pi^+ is composed of an up quark and a down antiquark (u \bar{d}), the \pi^- is composed of a down quark and an up antiquark (d \bar{u}), and the \pi^0 is composed of either an up quark and up antiquark or a down quark and down antiquark (u \bar{u} \text{ or } d \bar{d}).

Mass and Spin

The pions are relatively light particles. The charged pions (\pi^+ and \pi^-) each have masses of approximately 139.57061 MeV/c^2, while the neutral pion (\pi^0) is slightly lighter with a mass of approximately 134.9770 MeV/c^2. They all have a spin of zero, meaning they are bosons.

The masses of each pion are shown again below:

m_{\pi^+} = m_{\pi^-} = 139.57061 \dfrac{\text{MeV}}{c^2}

m_{\pi^0} = 134.9770 \dfrac{\text{MeV}}{c^2}

Role in the Strong Force

The pions play a fundamental role in the strong nuclear force, which binds protons and neutrons together within atomic nuclei. The strong force is approximately 100 times stronger than the electromagnetic force that binds electrons to the nucleus, but it operates over very short distances, on the order of a femtometer (1 \text{ fm} = 10^{-15} \text{ m}).

According to the Yukawa interaction, which models the strong force, the pions act as the exchange particles for this force. When a neutron and proton, for example, interact via the strong force, they do so by exchanging pions.

Pions formed by two quarks.
Artisitic representation of two quarks forming a pion. Not an actual image.

Pions in Particle Physics

The discovery and study of pions were significant milestones in the development of particle physics. The confirmation of Yukawa’s prediction of the pion’s existence gave strong support to the model of particle interactions via exchange forces.

Moreover, pions are frequently produced in high-energy particle collisions, and they have been instrumental in exploring the properties of other particles and interactions. For example, the decay of pions to muons and neutrinos has provided important tests of the weak nuclear force and the universality of leptonic interactions.

The decays can be represented by the following equations:

\pi^+ \to \mu^+ + \nu_\mu

\pi^- \to \mu^- + \bar{\nu}_\mu

\pi^0 \to \gamma + \gamma

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