Trapped antihydrogen

Abstract

Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7 × 10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4 ± 1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.

Observation of the hyperfine spectrum of antihydrogen

“…The observation of hyperfine structure in atomic hydrogen by Rabi and co-workers 1-3 and the measurement 4 of the zero-field groundstate splitting at the level of seven parts in 10 13 are important achievements of mid-twentieth-century physics. The work that led to these achievements also provided the first evidence for the anomalous magnetic moment of the electron [5][6][7][8] , inspired Schwinger's relativistic theory of quantum electrodynamics 9,10 and gave rise to the hydrogen maser 11 , which is a critical component of modern navigation, geo-positioning and very-long-baseline interferometry systems.…”

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

“…Historical examples of its importance include the wavelength measurements of absorption lines in the solar spectrum by Fraunhofer, the identification of transition lines by Balmer, Lyman and others, the empirical description of allowed wavelengths by Rydberg, the quantum model of Bohr, the capability of quantum electrodynamics to precisely predict transition frequencies, and modern measurements of the 1S-2S transition by Hänsch 1 to a precision of a few parts in 10 15 . Recent technological advances have allowed us to focus on antihydrogen-the antimatter equivalent of hydrogen [2][3][4] . The Standard Model predicts that there should have been equal amounts of matter and antimatter in the primordial Universe after the Big Bang, but today's Universe is observed to consist almost entirely of ordinary matter.…”

Resonant quantum transitions in trapped antihydrogen atoms

“…Working at the Antiproton Decelerator 8 facility at CERN, we recently demonstrated magnetic confinement of cold antihydrogen atoms 3 and showed that -once trapped -these atoms end up in their ground state, where they can be held 4 for up to 1000 s. Here we use the same apparatus, modified to enable injection of microwaves into the trapping volume ( trapped, on average. The trapping field currents can be ramped down with a time constant of 9 ms, releasing trapped atoms in a well-defined time window 3 .…”

“…The trapping field currents can be ramped down with a time constant of 9 ms, releasing trapped atoms in a well-defined time window 3 . The trapping volume is surrounded by a three-layer, 30,720-channel imaging silicon detector 9 , which can locate the spatial positions -'vertices' -of antiproton annihilations.…”

“…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.…”