Pions

Pions or pi mesons are subatomic particles consisting of a quark and an antiquark that are bound together. They are part of the meson family, which in turn is a member of the broader hadron class of particles. Pions are denoted by the symbols π^+, π^0, and π^−, representing the positively charged, neutral, and negatively charged variants, respectively. They play a crucial role in the strong force that holds the atomic nucleus together, acting as the force carriers between nucleons (protons and neutrons).

Discovery

The existence of pions was first proposed by Hideki Yukawa in 1935 as part of his theory on the strong force. Yukawa's prediction included a particle that mediated the strong force, with a mass about 200 times that of the electron. The discovery of the charged pions (π^+ and π^−) in cosmic rays by Cecil Powell, César Lattes, and Giuseppe Occhialini in 1947 confirmed Yukawa's theory. The neutral pion (π^0) was discovered a few years later in 1950 by Clyde Cowan and Frederick Reines through its decay into gamma rays.

Properties

Pions are mesons with a spin of 0, making them bosons. The masses of the charged pions are approximately 139.6 MeV/c^2, while the neutral pion has a slightly lower mass of about 135.0 MeV/c^2. Pions have a very short lifetime, with the charged pions decaying with a mean lifetime of 2.6×10^−8 seconds and the neutral pion even shorter at about 8.4×10^−17 seconds. The primary decay mode of the charged pions is into a muon and a muon neutrino, while the neutral pion most commonly decays into two gamma rays.

Role in the Strong Force

Pions are the lightest mesons and are thus the most easily produced particles in high-energy nuclear reactions. They are essential in the study of the strong force, as they mediate the force between nucleons in the nucleus. The concept of pion exchange allows physicists to understand how protons and neutrons are held together in the nucleus despite the repulsive electromagnetic force between the positively charged protons.

Applications

Beyond their fundamental role in particle physics and nuclear physics, pions have applications in medical physics, particularly in cancer treatment. Pion therapy, although not as common as other forms of radiation therapy, has been explored for its potential to deliver highly localized doses of radiation, minimizing damage to surrounding healthy tissues.

See Also

• Meson
• Hadron
• Quark
• Strong force
• Nucleon