Trapped antihydrogen

Abstract

Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies since 2002 at CERN, Europe's particle-physics lab near Geneva, Switzerland. Antihydrogen is of fundamental interest for testing the standard model of elementary particles and interactions. However, experiments to date have produced antihydrogen that is not confined, precluding detailed study of its structure. Now the trapping and controlled release of atoms of antihydrogen has been achieved, paving the way for precision measurements on anti-atoms. In this historic experiment, an interaction between about 107 antiprotons and 7 × 108 positrons generated an observed 38 annihilation events corresponding to 38 atoms of antihydrogen briefly confined in ultra-cold superconducting traps. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN since 2002. It is of fundamental interest for testing the standard model of elementary particles and interactions. However, experiments so far have produced antihydrogen that is not confined, precluding detailed study of its structure. Here, trapping of antihydrogen atoms is demonstrated, opening the door to precision measurements on anti atoms. Antimatter was first predicted1 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 produced2,3 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 1014 for the frequency of the 1s-to-2s transition4), 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 antimatter5. 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 107 antiprotons and 7 × 108 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.