Cold Anti-Hydrogen Atoms

Cold anti-hydrogen atoms have been made and detected for the first time in an experiment at CERN. The ATHENA collaboration makes the anti-H atoms when a swarm of antiprotons is loosed upon a cloud of positrons held within the same 16-cm-long cylindrical trap. Anti-H atoms announce their presence when they drift out of the trap region and annihilate with ordinary atoms in a sort of double suicide. The antiproton perishes when it meets a regular proton, resulting in the creation of a few pi mesons detected in silicon microstrips, a process which points to the annihilation vertex with a precision of 4 mm. Meanwhile the positron partner from each anti-H meets its separate fate when it collides with the nearest electron, producing a telltale pair of 511-keV gamma rays which show up in adjoining CsI crystals. The next step for ATHENA will be to shine laser light upon its captive sample and determine from the re-emitted spectrum whether anti-hydrogen behaves like regular hydrogen.

This is not the first time anti-H atoms have been made. Positron-antiproton pairs, engendered on the fly amid high energy collisions at CERN and Fermilab, were observed several years ago (Updates 253, 297). But these anti-atoms could not be stored or studied since they immediately annihilated with regular atoms. Hence the need to slow down antiprotons (made at extremely high energies) and to store them in a dedicated facility such as CERN's Antiproton Decelerator (AD), where several experiments are underway to study anti-atoms. In February 2002, one of those experiments, conducted by the ATRAP collaboration had attained many of the conditions needed for storing anti-H atoms but were not yet in a position to detect them directly (see Update 577). ATHENA estimates that they make about 50,000 anti-hydrogen atoms, having used a contingent of about 1.5 million antiprotons. (Amoretti et al., Nature, posted online, 18 Sept)