Search for trapped antihydrogen

Abstract

We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ∼ 30 ms. After a three-week experimental run in 2009 involving mixing of 10 7 antiprotons with 1.3 × 10 9 positrons to produce 6 × 10 5 antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally.

Resonant quantum transitions in trapped antihydrogen atoms

“…In the strong trapping field, the coupling matrix element between the states is approximately topology is then used to discriminate between pion tracks and cosmic ray events, and ultimately to locate the spatial position ('vertex') of each annihilation event 16 . The detector and the 'default criteria' for the event discrimination procedure have been extensively described previously 5,16 .…”

“…For the much longer 'appearance mode' observation (180 s), we rely on an alternative set of acceptance criteria that, compared to the default criteria, reduces annihilations by 25% but lowers cosmic background by an order of magnitude. To avoid experimenter bias, the two sets of criteria are optimised and cross-checked using control samples 3,5 : cosmic ray events and annihilation events collected independently of the trapping experiments described here.…”

Observation of the 1S–2S transition in trapped antihydrogen

“…The particle manipulations that are necessary to produce trappable antihydrogen atoms have been described elsewhere 3,4,17 ; we note only that recent innovations (Methods) in these techniques have provided a large improvement in the number of trapped anti-atoms available per trial compared to the best previous result 12 . The antihydrogen production method involves a new technique in which we 'stack' anti-atoms resulting from two successive mixing cycles, which originate from independent shots of antiprotons from the antiproton decelerator and accumulations of positrons.…”

Detection of Trapped Antihydrogen

“…The competing ATRAP collaboration pursued a similar approach, but with a quadrupole-based Penning-Ioffe trap, and succeeded in demonstrating antiproton confinement [51] and antihydrogen production [52] in their magnetic trapping fields. More recently, ALPHA has been occupied with the systematic search for trapped antihydrogen in their apparatus [53]. This was followed shortly thereafter by the achievement of a long-awaited milestone: the observation of magnetically confined antihydrogen atoms in the ALPHA apparatus [54] (and of long survival times of those trapped atoms [55]).…”

“…However, the ALPHA traps are coupled to the outside environment at both ends of the electrode stack, as well as through the electrode cables. Although an extensive effort has be made to heat-sink the cables to 57 the cryogenic bath, and minimize the heating sources at the trap ends, the thermalized antiproton temperature is measured to be 358 ± 55 K [53], which is well above the cryogenic temperature. The complementary technique of evaporative cooling of antiprotons can be used to reduce the antiproton temperature, and will be discussed in Sec.…”