Physics with low energy antiprotons (antihydrogen)

The first observations of antihydrogen (H) atoms [1, 2] in 1995 opened a new way of testing the fundamental symmetries of nature. These first experiments were not suited to precision comparison measurements having produced antihydrogen in small quantities and at high energy. A second generation of experiments was then built with the aim of creating antihydrogen in a controlled process inside an electromagnetic trap. ATHENA and ATRAP indeed succeeded in producing for the first time cold antihydrogen by mixing antiprotons (p’s) and positrons (e+’s) at low temperature (few K) in a nested Penning trap [3, 4]. This result was made possible thanks to the CERN Antiproton Decelerator (AD), the only facility in the world to deliver low energy antiprotons for antihydrogen physics. Since then, upgrades to the experiments were made to keep the antiatoms inside the traps, in order to perform atomic spectroscopy and thus test CPT. With such objective three experiments are at the moment on the floor: ALPHA, ASACUSA and ATRAP. Another experiment, AEgIS, was recently approved and will soon start construction in the AD hall. It aims at directly measuring the gravitational acceleration g on a beam of cold antihydrogen. An overview of past, present and future antihydrogen experimental activities will be given.