60 years of Particle Physics Experiments

1. The ATHENA experiment at the CERN antiproton decelerator produced for the first time cold antihydrogen atoms in large quantities in a nested penning trap by overlapping antiproton and positron clouds. Once formed, antihydrogenis electrically neutral. Not confined anymore, it annihilates on the walls of the trap. The antihydrogen detector measured the annihilation vertex of the pionsand detected the two 511 keV from electron-positron annihilation. The angle between the two photons (180 degrees) was measured by using the annihilation vertex. • Sketch of the ATHENA antihydrogen detector (left). Photograph with one row of CsI crystals prior to installation (middle). Electronic reconstruction of an antihydrogen annihilation on the trap wall (right). The yellow lines show the tracks from three charged pions, the red lines the directions of the two 511 keV annihilation photons which are emitted in opposite directions. • Nuclear emulsions can measure the annihilation vertex of antihydrogen atoms with a precision of 1 micron. A substantial reduction of the data taking time needed to measure the gravitational sag of an antihydrogen beam can be achieved when combined with a time of flight measurement. 60 years of ParticlePhysicsExperiments Antiproton Phiysics at the Antiproton Decelerator Antihydrogen Production with ATHENA(2002) M. Amoretti et al, Nature 419 (2002) 456 Gravitational acceleration of antihydrogen (AEgIS), since 2008S. Aghion et al.,J. of Instrumentation 8 (2013) P08013 J. Storey et al., Nuclear Instruments and Methods in Physics Research A 732 (2013) 437 Antiproton annihilation into nuclear fragments and pions (MIP), observed at AEgIS (left). The scintillating fiber detector (right) detects the pions which are emitted when antiprotons annihilate on the surface of the vacuum vessel. The detector is used to measure antihydrogenproduction (right).

2. The Crystal Barrel experiment at the Low Energy Antiproton Ring (LEAR) at CERN studied the annihilation of antiprotons with protons in liquid hydrogen. The emphasis was on the detection of electrically neutral mesons. Several new light mesons were discovered. • Experimental Dalitz plot of the proton antiproton annihilation into three neutral pseudoscalarmesons decaying into 6 photons (a) - (c). Red and blue regions correspond to high and low event densities, respectively. • (a):1 neutral pion + 2 eta, 198 000 events; • (b) 2 neutral pions + 1 eta, 280 000 events; (c) 3 neutral pions, 712 000 events. Each event is entered six times for symmetry reasons; (d) 1 neutral pion and to long-lived neutral kaons, 37 358 events. • Note in particular the prominent scalar meson f0(1500) discovered here. 60 years of ParticlePhysicsExperiments THE CRYSTAL BARREL AT THE LOW ENERGY ANTIPROTON RING (LEAR), 1985-1996 Crystal Barrel apparatus annihilation into charged and neutral mesons: magnet yoke (1,2), coil (3), CsI-crystals (4), drift chamber (5), wire chamber (6), hydrogen target (7) For a review, see C. Amsler, Rev. Mod. Phys. 70 (1998) 1293