How different are matter and antimatter? This is a question that gets at the heart of modern particle physics and early-universe cosmology. The objects of everyday experience are made of ordinary matter. While some natural process produce limited amounts of antimatter, every known star and galaxy appears to be matter-based.

If matter and antimatter are physical mirrors of each other, then an atom of antihydrogen will behave in the same way as an atom of normal hydrogen. However, if there is an asymmetry between matter and antimatter, then the forces of nature may act differently on matter and antimatter.

A recent experiment by C. Amole et al. has used trapped antihydrogen atoms to measure a quantum transition in antihydrogen. The spin-flip transition involves placing antihydrogen in a microwave resonant cavity to manipulate the relative orientations of the antiproton and positron spins. While the experiment lacked the precision needed to distinguish any differences between matter and antimatter, it highlights how to proceed if we're going to make a successful measurement in the future.

If there are differences between matter and antimatter, they could make their presence felt through differences in the behavior of antihydrogen, which consists of a positron (the antimatter version of an electron) and an antiproton. However, to measure this, we need to trap antimatter atoms long enough to study them, which has proven difficult. But CERN is now keeping antihydrogen atoms around long enough to do some detailed measurements on them.

So, what can we measure that will help us determine if atoms and antiatoms behave differently? Protons and electrons interact through electrical forces that hold the atoms together. But they also interact magnetically through the relative orientation of their spins. In hydrogen, altering one or both spin directions changes the internal energy of the atom very slightly, producing a photon with a wavelength of 21 centimeters (in the microwave portion of the spectrum). The transition is known as the spin-flip transition, and the tiny relative change in energy is known as the hyperfine structure of the hydrogen atom.

The precise energy of the spin-flip transition can be measured by placing neutral hydrogen in a cavity and bombarding it with microwaves. The resonant frequency corresponds to the precise energy of the spin flip.

Because of the relative simplicity of the interactions involved, the hyperfine structure of hydrogen is one of the most precisely determined numbers in all of science. Thus, it makes a lot of sense to test whether antihydrogen's spin-flip transition occurs at the same energy as in hydrogen. From a theoretical point of view, if the spin-flip transition occurs even at a slightly different energy for antihydrogen than it does for hydrogen, it means a fundamental asymmetry in nature.

The Standard Model of particles and interactions does not distinguish between matter and antimatter as far as the fundamental forces are concerned; the energies of interactions are unchanged upon swapping a particle with its antimatter partner. This particular symmetry is known as CPT, for charge, parity (swapping directions, as in a mirror), and time-reversal. If antihydrogen has a different hyperfine spectrum than hydrogen, then CPT symmetry is violated for electromagnetism, at distinct odds with the predictions of the Standard Model.

The first hurdle is preparing and trapping enough antihydrogen atoms to perform any measurement at all. Using the Antiproton Decelerator apparatus at CERN, the researchers culled low-speed antiprotons (with an equivalent temperature of 0.5 degrees above absolute zero) and mixed them with cold positrons. Each trial combined about 2 million positrons with roughly 20,000 antiprotons; from these ingredients, they managed to obtain on the order of 6,000 antihydrogen atoms. Using magnetic fields to confine the antiatoms (a standard technique used in many experiments), Amole et al. managed to trap an average of one antihydrogen atom in each run.

With such small numbers of antiatoms to work with, the experiment split the problem into two pieces: determining if the antihydrogen has a resonant frequency roughly corresponding to the hyperfine structure, and measuring if that transition occurs at the same place as for hydrogen. Due to the small number of trapped atoms, the researchers were unable to perform the second measurement, but they identified 23 antiatoms (out of 110 attempts) that survived in an off-resonant frequency, and 2 (out of 103) that survived at the resonant frequency. (They also measured the annihilation of the antiatoms that got away when they impinged on a surrounding sheath of silicon.)

As the authors acknowledge, this is a proof-of-principle experiment: it shows how later tests may be done to measure the actual hyperfine spectrum for antihydrogen, but they don't actually perform said measurement. By increasing the number of antiatoms trapped in the cavity and narrowing the range of frequencies to determine the resonant frequency, later experiments should be able to determine whether CPT symmetry is violated or not

Nature, 2012. DOI: 10.1038/nature10942 (About DOIs).