Pion

                                    In particle physics, a pion (short for pi meson, denoted with π) is any of three subatomic particles: π0, π+, and π−. Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force.

Pions are unstable, with the charged pions decaying with a mean life time of 26 billionths of a second and the uncharged pion decaying with an even shorter lifetime. Charged pions tend to decay into muons and muon neutrinos, and uncharged pions into gamma rays.

Pions are not produced in radioactive decay, but are produced commonly 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, detection of characteristic gamma rays originating from decay of neutral pions in two supernova remant stars has shown that pions are produced copiously in supernovas, most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays.