Antihydrogen Production and Precision Experiments The ATHENA Collaboration

The ATHENA (AnTiHydrogEN Apparatus) experiment has the goal of producing slow antihydrogen atoms and comparing their physical properties to those of hydrogen. For example, the measurement of the 1S-2S transition offers the possibility for comparing hydrogen and antihydrogen to a precision of 1 part in 10$^{15}$, owing to the long lifetime of the metastable 2S state (122 msec). This would be the most precise test of CPT invariance in the lepton and baryon sector. The first phase of the experiment, which has been installed and commissioned at the Antiproton Decelerator (AD) - is devoted to the study of antihydrogen production. For this purpose, about 10$^{5}$ antiprotons (p=100 MeV/c ) are trapped and cooled to milli-eV energies. Independently, about 10$^{8}$ low energy positrons per minute are collected in the positron accumulator, using a Na-22 source (2 GBq). Formation of antihydrogen will take place in the recombination region, where the antiproton cloud is made to interact with the dense positron plasma transferred previously from the positron accumulator. The experimental apparatus of Phase 1 (see Fig.1) consists of four major parts: the superconducting solenoid (3T) with a cold bore at T = 77K and an UHV copper tube at T = 4 K, containing the antiproton capture trap, the recombination trap, and the transfer electrodes; the transfer region between the superconducting solenoid and the positron accumulator; the positron accumulator (10$^{8}$) positrons per minute); the antihydrogen detector, which surrounds the recombination region. It consists of 2 layers of 16 Si-strip detectors, surrounded by a cylinder made of 192 small CsI crystals, which are read out by photodiodes. The detector measures the annihilation vertex using the charged pion tracks from the antiproton-nucleon annihilation, and also both 511 keV gammas from positron-electron annihilation, thus allowing a complete reconstruction of the antihydrogen annihilation in space and time.