Antihydrogen formation mechanisms

New developments in the ATHENA data analysis are presented, with some insight into the mechanisms responsible for antihydrogen production under di erent ex- perimental conditions. Studies of antihydrogen, the bound state of a positron and an antiproton, can o er new insights into the fundamental symmetries of nature from comparisons of its properties with those of hydrogen. The production and trapping of antihydrogen at low energies has recently become possible, using specialised apparatus developed at CERN (1-5). There has been no detailed study of the processes involved in the formation of antihydrogen via antiproton-positron plasma mixing in a Penning trap environment, and much has been assumed from equilibrium studies at higher plasma temperatures and in the absence of applied fields. It is the nature of the mechanisms underlying antihydrogen formation that we address in this work. In particular, we show that a careful analysis of the correlated temporal and spatial dependencies of the antihydrogen on wall annihilation signals, available from a range of experiments performed by the ATHENA collaboration, can shed light on this problem. ATHENA aimed at mixing antiprotons (pbar) and positrons (e + ) and consisted of four main subsystems: the antiproton catching trap, the positron accumulator, the antiproton/positron mixing trap, and the antihydrogen annihilation detector (see Figure 1 and (6) for the datails). The positrons ( 10 8 e + ) and antiprotons ( 10 4 pbars) were held in devices known as nested Penning traps (see e.g. (7)) which provide confinement using a combination of applied fields. A series of cylindrical electrodes, aligned along the axis of a solenoid, were used to produce electrical wells which restrict the particles axially, while the 3 T magnetic field provides transverse confinement (see Figure 2). When an antiproton and a positron, after interacting, are bound together, they form an antihy- drogen atom (Hbar) which is neutral and which thus eventually impinges on the electrode walls and annihilates. The simultaneous annihilation of the pbar on a proton/neutron (producing charged pions) and of the e + on an electron (producing two -rays) is detected by a cylindrical detector which is also