antiprotones

An antiproton (symbol p̄ or "p-bar") is the antiparticle of the proton: it has the same mass and spin as a proton but opposite electric charge and baryon number. Like all antiparticles, when an antiproton encounters its corresponding particle (a proton) they annihilate, producing other particles such as pions and photons.

Antiprotons were first observed in 1955 by Emilio Segrè and Owen Chamberlain during experiments at the Lawrence Berkeley National Laboratory; their discovery provided direct evidence for antimatter in the baryonic sector and earned them the 1959 Nobel Prize in Physics. Antiprotons occur naturally in cosmic rays in very small numbers but are most commonly produced artificially in high-energy particle accelerators.

Experimental facilities such as CERN’s Antiproton Decelerator create, slow and store antiprotons for research. Techniques like magnetic confinement in Penning traps, stochastic cooling and electron cooling are used to accumulate low-energy antiprotons suitable for precision measurements and antimatter synthesis, for example antihydrogen atoms.

Research with antiprotons tests fundamental symmetries (notably CPT symmetry), measures intrinsic properties (mass, charge-to-mass ratio, magnetic moment) and probes matter–antimatter asymmetries. Proposed and experimental applications include studies of nuclear interactions, diagnostic tools, and exploratory medical research into antiproton radiotherapy, though clinical use remains unestablished. The study of antiprotons continues to play a role in particle physics, astrophysics and applied research.