1934,1947

Meson

• Hideki Yukawa predicted
• that an unstable particle would be found
• heavier than an electron
• and lighter than a proton
• to carry the nuclear force
• that holds atomic nuclei together.
• We name these after the Greek
• μέσος meaning “intermediate.”
• It took thirteen years
• and the end of the war
• for the first meson to be found.
• Investigating the products of cosmic rays
• on mountain peaks, Powell, Lattes, and Occhialini
• discovered the pi meson, or pion.

List of mesons

• About 140 types of mesons,
• all being virtual composites
• of a quark and an antiquark
• bound by strong interactions,
• but differing in spin, parity,
• charge, mass, angular momentum,
• strangeness, charm, beauty,
• and lifetime, including
• pion,
• eta meson,
• eta prime meson,
• bottom eta meson,
• charmed eta meson,
• charmed rho meson,
• neutral rho meson,
• omega meson,
• phi rho meson,
• upsilon meson,
• and kaon.
• Kaon or K meson
• is any of four mesons
• with different strangeness,
• composed of a strange quark
• or strange antiquark
• with an up or down quark
• or antiquark.

The cosmic ray connection

• Pions were discovered as tracks in photographic plates
• exposed for long periods at Pic du Midi de Bigorre
• in the Pyrenees and at Chacaltaya in the Andes.
• Kaons, too, were discovered as products of cosmic rays.
• At first, scientists thought muons were mesons.
• Its discoverers, Carl David Anderson and Seth Neddermeyer,
• had lifelong interests in studying the effects of cosmic rays.