Pion

vipera/spinner-330,000t



Composition π+:ud, π0:uu or dd, π−:du
Statistics Bosonic
Interactions Strong, Weak, Electromagnetic and Gravity
Symbol π+, π0, and π−
Theorized Hideki Yukawa (1935)
Discovered César Lattes, Giuseppe Occhialini (1947) and Cecil Powell
Types 3
Mass π±: 139.57018(35) MeV/c2
π0: 134.9766(6) MeV/c2
Electric charge π+: +1 e
π0: 0 e
π−: −1 e
Spin 0
Parity −1

In particle physics, a pion (or a pi meson, denoted with the Greek letter pi:
(π) is any of three subatomic particles:
π0, π+, and π−. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions π+ and π− decaying with a mean lifetime of 26.033 nanoseconds (2.6033×10−8 seconds), and the neutral pion π0 decaying with a much shorter lifetime of 8.4×10−17 seconds. Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.

The exchange of virtual pions, along with the vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but are commonly produced in high energy accelerators in collisions between hadrons. All types of pions are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth's atmosphere. Recently, the detection of characteristic gamma rays originating from the decay of neutral pions in two supernova remnants has shown that pions are produced copiously after supernovas, most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays.

The concept of mesons as the carrier particles of the nuclear force was first proposed in 1935 by Hideki Yukawa. While the muon was first proposed to be this particle after its discovery in 1936, later work found that it did not participate in the strong nuclear interaction. The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950.