Pion

In <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, a pion or pi meson, denoted with the <a href="/facts/Greek_alphabet/1cHwB8Dg">Greek</a> letter <a href="/facts/Pi_(letter)/4UpopgBE">pi</a> (π), is any of three <a href="/facts/Subatomic_particle/tMEzcTWQ">subatomic particles</a>: π0, π+, and π−. Each pion consists of a <a href="/facts/Quark/hwuuVkV5">quark</a> and an <a href="/facts/Antiquark/hwuuVkV5">antiquark</a> and is therefore a <a href="/facts/Meson/9R0kIc1q">meson</a>. Pions are the lightest mesons and, more generally, the lightest <a href="/facts/Hadron/C0sd1zty">hadrons</a>. They are unstable, with the charged pions π+ and π− decaying after a <a href="/facts/Mean_lifetime/wLB1zYDP">mean lifetime</a> of 26.033 <a href="/facts/Nanosecond/gARLTp4c">nanoseconds</a> (2.6033×10−8 seconds), and the neutral pion π0 decaying after a much shorter lifetime of 85 <a href="/facts/Attosecond/G1ynD8f9">attoseconds</a> (8.5×10−17 seconds). Charged pions most often <a href="/facts/Particle_decay/3koAz34P">decay</a> into <a href="/facts/Muon/BP2fLVIR">muons</a> and <a href="/facts/Muon_neutrino/A8kPHDKd">muon neutrinos</a>, while neutral pions generally decay into <a href="/facts/Gamma_ray/5Drf913J">gamma rays</a>.
The exchange of <a href="/facts/Virtual_particle/CfHFkVL3">virtual</a> pions, along with <a href="/facts/Vector_meson/T87cuauz">vector</a>, <a href="/facts/Rho_meson/55vNvssd">rho</a> and <a href="/facts/Omega_meson/6Ua73RTW">omega mesons</a>, provides an explanation for the <a href="/facts/Nuclear_force/A2MyJoWE">residual strong force</a> between <a href="/facts/Nucleon/6fguU9Xk">nucleons</a>. Pions are not produced in <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a>, but commonly are in high-energy collisions between <a href="/facts/Hadron/C0sd1zty">hadrons</a>. Pions also result from some matter–antimatter <a href="/facts/Annihilation/tm55RWnf">annihilation</a> events. All types of pions are also produced in natural processes when high-energy <a href="/facts/Cosmic-ray/uxCVdTKd">cosmic-ray</a> protons and other hadronic cosmic-ray components interact with <a href="/facts/Matter/osyc04UN">matter</a> in Earth's <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a>. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in two <a href="/facts/Supernova_remnants/tYboe2n4">supernova remnants</a> 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 pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the <a href="/facts/Cosmic_microwave_background/ThB2HkLS">cosmic microwave background</a>, through the <a href="/facts/Greisen%25E2%2580%2593Zatsepin%25E2%2580%2593Kuzmin_limit/2haM9XP9">Greisen–Zatsepin–Kuzmin limit</a>.

Pion open-in-new

Pion