Meson << MEHS on or MEE son >> is any of about 150 kinds of unstable subatomic particles. Meson comes from a Greek word meaning middle. Mesons have this name because the first ones found were heavier than an electron but lighter than a proton. However, physicists have discovered that most mesons are heavier than a proton.
A meson is unstable because of its composition. It consists of one quark and one antiquark. An antiquark is a particle of antimatter. An antiparticle has the same mass (amount of matter) as its corresponding particle, but is opposite in electric charge or certain other properties. When a particle of matter and a particle of its antimatter opposite meet, they annihilate each other. A meson’s quark and antiquark can avoid meeting for only a tiny fraction of a second.
Mesons differ in the types of quarks and antiquarks they are made of and in the motions of those particles within the meson. The lightest meson is the pi meson, also known as a pion. It comes in three varieties, with electric charges of 11, 21, and 0. The charged pions are the longest-lived mesons, with an average lifetime of 26 nanoseconds. One nanosecond is one billionth of a second. The mass of a pi meson is about one-seventh the mass of a proton. The heaviest mesons observed, the upsilons, are more than 10 times as heavy as a proton.
Mesons are produced when stable particles, such as protons, collide at high speeds. The collision converts part of the kinetic energy (energy of motion) of the particles into a quark and an antiquark. Such collisions often occur when a cosmic ray, an electrically charged particle from outer space, strikes a particle in Earth’s atmosphere. Mesons are also produced in machines called particle accelerators.
Japanese physicist Hideki Yukawa predicted the existence of mesons in 1935. In 1947, British physicist Cecil Frank Powell and his colleagues observed charged pi mesons, confirming Yukawa’s prediction. Yukawa received the 1949 Nobel Prize in physics for his prediction. Powell received the 1950 prize, in part for his discoveries concerning mesons.
See also Antimatter; Quark.
