Antimatter consists of particles that have the same mass as ordinary particles but opposite electric charge and other quantum numbers. The antiproton—the proton’s antiparticle—had been predicted by theory but not confirmed experimentally until the 1950s. Using the Bevatron, a large synchro-cyclotron at the University of California, Berkeley, Segrè and Chamberlain accelerated protons to about 6 GeV and smashed them into a gold target to create antiprotons. They bent the outgoing particles in a magnetic field and used detectors to show that a negatively charged particle with the same mass as the proton had been produced. The discovery supported the idea that matter and antimatter are symmetric, influencing studies of the early universe and high-energy physics. It also drove rapid advances in accelerator and detector technology, paving the way for today’s Large Hadron Collider (LHC). Research on antiprotons later found applications in PET medical imaging and materials analysis.

The discovery exploited pair-production in high-energy collisions, p + p → p + p + p̄ + p, with careful kinematic calculations to choose the beam energy where the production cross-section peaks. Segrè and colleagues used a 2 m bending-magnet spectrometer and a magnetic analysis system to determine the charge-to-mass ratio q/m from track curvature, while time-of-flight measurements gave mass identification. The observed antiproton count exceeded background by more than 5σ, providing compelling evidence. CPT symmetry demands that the antiproton’s mass and lifetime equal the proton’s; the experimental data confirmed these expectations. The result accelerated work on strange particles and later the quark model. Development of high-intensity beams and hadron detection technologies fed directly into experiments on CP violation in neutral kaons and the quest for quark–gluon plasma in heavy-ion collisions. Even today, antiprotons are studied for precision annihilation cross-sections and as potential beams for oncological therapy.

The Bevatron (magnetic radius 6.5 m, B ≈ 1.5 T) delivered 6.2 GeV/c protons enabling baryon-pair production on a Au target. Antiproton identification was two-step: curvature-derived q/p combined with β from time-of-flight to reconstruct m = p/(γβc), selecting m ≈ 938 MeV/c² with q = −e. Background dominated by π⁻, μ⁻, K⁻ was suppressed via threshold Cherenkov counters and multi-fold coincidence logic. The measured production cross-section σ ≈ 1 µb agreed with contemporary Pomeranchuk estimates. The extracted magnetic moment |μp̄| = 2.79 μN supported CPT invariance. Subsequent p̄-p storage rings enabled the J/ψ discovery and later W/Z production experiments. Modern p̄ facilities (AD, ELENA) have established trapping and cooling techniques, leading to gravitational free-fall tests (AEGIS, ALPHA-g). Future FAIR/FLAIR programs will employ medium-energy p̄ beams for hadron spectroscopy and ultra-precise CPT tests.