Antihydrogen experiments under way at CERN’s Antiproton Decelerator have so far aimed at making high-precision measurements of the frequency of optical transitions, such as that between antihydrogen’s 1S ground state and its first excited, 2S state, near 2466 THz. Comparing this with the same frequency for ordinary hydrogen constitutes a highly sensitive test of CPT symmetry, which involves simultaneous inversions of charge (C), parity (P) and time (T) (see “CPT ’07 goes in quest of Lorentz violation“).
Recently, the Japanese–European ASACUSA group made the first steps towards producing a low-velocity antihydrogen beam, which may be used to measure the hyperfine transition frequency between the two spin substates of antihydrogen’s ground state. Its value for ordinary hydrogen is near 1420 MHz.
Before this can be done antiprotons must be confined and cooled in an evacuated container in which magnetic and/or electric fields produce restoring forces that stop the antiprotons drifting to the container walls, where they would annihilate. To do this, the MUSASHI group of the ASACUSA collaboration has introduced a novel variant of the familiar Helmholtz coils. The MUSASHI coils differ from the usual configuration by having antiparallel rather than parallel excitation currents. This produces a magnetic quadrupole field rather than the normal constant one, and is symmetric about the coil axis. If a suitable electrostatic multipole field is added to this so-called “magnetic cusp” field, all of the restoring forces needed to confine both positive and negative charges are present within the container.
This “cusp trap” can thus also hold positrons, with which the antiprotons recombine to create the antihydrogen, as well as electrons. The latter can be used to cool the antiprotons to the extremely low temperature at which recombination occurs. In the recent tests, some 3 million antiprotons were stored in the trap and cooled with electrons.
A well known obstacle to CPT tests with antihydrogen is that both the hyperfine and the 1S–2S frequency measurements must be performed on ground-state atoms, while it appears that positron–antiproton recombination produces them in very highly excited states. One great advantage of the cusp trap is that if these neutral atoms are cold enough its quadrupole field pulls on their large magnetic moment, causing them to seek the field minimum at the trap centre. They remain confined there until they reach the ground state. However, since their magnetic moment falls as they de-excite, the pull weakens. This means that in the ground state, only antihydrogen atoms in one of the two possible spin states are pulled to the centre, while those in the other state are expelled along the trap axis, emerging as a spin-polarized, ground-state antihydrogen beam.
This is ideal for the classical type of slow atomic beam experiment in which a microwave cavity induces spin flips when tuned to the correct hyperfine frequency (see figure). The resonant frequency can then be detected using a sextupole magnet which focuses flipped atoms onto a detector but defocuses unflipped ones. Comparison with the well measured hydrogen frequency then gives a stringent test of CPT symmetry.
Although much of this remains to be done, the recent successes are so encouraging that further steps along the road to a slow antihydrogen beam are now planned.