We review the physics of low-energy antiprotons, and its link with the nuclear forces. This includes: antinucleon scattering on nucleons and nuclei, antiprotonic atoms and antinucleon-nucleon annihilation into mesons.
Modern perspectives 39 Outlook 39 1 A brief historyIn 1932, the positron, the antiparticle of the electron, was discovered in cosmic rays and confirmed in the β + decay of some radioactive nuclei. See, e.g., the Nobel lecture by Anderson [1]. It was then reasonably anticipated that the proton also has an antiparticle, where A denotes the target nucleus. For A = p, this is a standard exercise in relativistic kinematics to demonstrate that the kinetic energy of the incoming proton should be higher than 6 m, where m is the proton mass, and c = 1. This threshold decreases if the target is more massive. The Bevatron was completed in 1954, and the antiproton was discovered in 1955 by a team lead by Chamberlain and Segrè, who were awarded the Nobel prize in 1959. 2 Shortly after the antiproton, the antineutron,n, was also discovered at Berkeley, and up to now, for any new elementary particle, the corresponding antiparticle has also been found. The discovery of the first anti-atom was well advertised [2], but this was not the case for the earlier observation of the first antinucleus, antideuterium, because of a controversy between an European team [3] and its US competitors [4]. In experiments at very high energy, in particular collisions of heavy ions at STAR (Brookhaven) and ALICE (CERN), one routinely produces light antinuclei and even anti-hypernuclei (in which an antinucleon is replaced by an antihyperonΛ) [5][6][7].The matter-antimatter symmetry is almost perfect, except for a slight violation in the sector of weak interactions, which is nearly exactly compensated by a simultaneous violation of the left-right symmetry, i.e., the product P C of parity P and charge-conjugation C is only very marginally violated. Up to now, there is no indication of any violation of the product CP T , where T is the time-reversal operator: this implies that the proton and antiproton have the same mass, a property now checked to less than 10 −9 [8].Many experiments have been carried out with low energy antiprotons, in particular at Brookhaven and CERN in the 60s and 70s, with interesting results, in particular for the physics of the mesons produced by annihilation. However, in these early experiments, the antiprotons were part of secondary beams containing many negativelycharged pions and kaons, and with a wide momentum spread.In the 70s, Simon van der Meer, and his colleagues at CERN and elsewhere imagined and developed the method of stochastic cooling [9], which produces antiproton beams of high purity, sharp momentum resolution, and much higher intensity than in the previous devices. CERN transformed the fixed-target accelerator SpS into a protonantiproton collider, SppS, with the striking achievement of the discovery of the W ± and Z 0 , the intermediate bosons of the electro-weak interaction. A similar scheme was lat...