CERN Group Detects More than 100 Antihydrogens

Does an atom of antihydrogen ( $\overline{\text{H}}$ ) have the same energy levels as its charge conjugate, the hydrogen atom? Yes, according to the venerable CPT theorem, which proclaims the invariance of the laws of physics under the simultaneous operation of charge conjugation (C), parity inversion (P), and time reversal (T). Does antimatter fall at the same rate as ordinary matter in a gravitational field? It should, if the equivalence principle of general relativity holds. Although CPT invariance has been tested to high precision in several systems, its importance impels us to explore its limits: Comparing H to $\overline{\text{H}}$ should provide the most sensitive test yet of a system involving both a baryon and a lepton. As for gravitational properties, they have not yet been measured on antimatter. To conduct either test, though, one needs large quantities of $\overline{\text{H}}$ atoms at low enough temperatures to make high-precision measurements.

That’s been the dream of many researchers, but the task requires great persistence. The first hurdles were to cool and trap quantities of antiprotons and of positrons. Those barriers were surmounted in the 1990s by the TRAP collaboration at CERN, which has now evolved into ATRAP. ¹ The collaborators, led by Gerald Gabrielse of Harvard University, also introduced a technique to use positrons for the final cooling of antiprotons. ²

The next challenge was to bring together the two clouds of these oppositely charged particles and induce the formation of slowly moving $\overline{\text{H}}$ atoms. In 1999, the ATRAP team confined antiprotons and positrons in the same trap, ³ at temperatures as low as 4 K, but has not yet shown that antihydrogen is formed. ATHENA, a second antihydrogen collaboration at CERN, has now accomplished that feat, detecting more than 100 $\overline{\text{H}}$ atoms. ⁴ Rolf Landua of CERN is the spokesman for ATHENA, which involves 39 scientists from nine institutions worldwide. Jeffrey Hangst of the University of Århus in Denmark coordinated the recent experiment.

It’s not the first time that $\overline{\text{H}}$ has been formed in the lab. Six years ago, yet another team at CERN created nine such atoms of antimatter ⁵ (see Physics Today, March 1996, page 17 ), and Fermilab scientists produced several dozen more not long after. ⁶ But those atoms were moving at nearly the speed of light; they were formed when an antiproton ( $\overline{\text{p}}$ ) beam hit a target and created some positrons (and electrons). Occasionally, one of the newly produced positrons (e⁺) moved in the same direction and with the same speed as the “mother” $\overline{\text{p}}$ , which then picked it up to form antihydrogen. Such experiments proved that $\overline{\text{H}}$ could be formed, but the antiatoms were moving far too fast to allow high-precision spectroscopic tests of CPT.

Assembling the pieces

CERN’s Antiproton Decelerator (AD) facility, created largely to support antihydrogen research (see Physics Today, October 2000, page 22 ), supplies antiprotons traveling at about one-tenth the speed of light. To further slow and capture the antiprotons, ATHENA and ATRAP researchers channel them through a thin aluminum foil into a Penning trap, where magnetic fields confine them radially and electric fields trap them axially. ATHENA’s $\overline{\text{p}}$ trap captures about 10 000 of the slower moving antiprotons among the roughly 20 million in each bunch sent from the AD. Within the ATHENA trap, they are cooled to 15 K by collisions with a cloud of cold electrons.

The ATHENA group produces positrons in the radioactive decay of sodium-22. The researchers let these positrons pass through a thin foil of solid neon, which reemits them with a smaller energy spread. The positrons are then cooled by interaction with gas molecules ⁷ and held in another Penning trap. The ATHENA experimenters trap about 150 million positrons in every five-minute cycle.

ATHENA team members accumulate antiprotons and positrons in separate traps on opposite sides of a mixing trap that’s kept at 15 K. Once these $\overline{\text{H}}$ constituents are in place, the experimenters release them into the mixing trap—a nested Penning trap that has separate potentials for the two oppositely charged particles. ⁸ Although the positrons are largely trapped in a potential well at the center of the trap, the antiprotons bounce back and forth through the positron cloud. Every now and then, a $\overline{\text{p}}$ is able to pull an e⁺ into orbit to form antihydrogen.

Coincidence detection

Once formed, the neutral $\overline{\text{H}}$ atoms are free to escape the hold of the electric and magnetic fields. They travel no farther than the nearest wall, however, wherein their antimatter constituents are annihilated by collision with ordinary electrons and nucleons. To prove that they had formed antihydrogen, the ATHENA collaborators had to detect the nearly simultaneous annihilations of a $\overline{\text{p}}$ and an e⁺.

The annihilation of a $\overline{\text{p}}$ by collision with a nucleon typically produces 3–4 charged pions. The encounter of an e⁺ with an e⁻ destroys them both and creates two photons. The electrons and positrons of this experiment are moving sufficiently slowly that the annihilation photons are emitted in opposite directions and with telltale energies of 511 keV each.

To observe signatures of these annihilations, the ATHENA team built a high-resolution detector (see the figure on this page). The detector is mounted outside the electrodes of the nested Penning trap but inside the bore of the solenoidal magnet. As seen for the sample event, the double-sided, position-sensitive silicon strip detectors register the points where charged pions crossed the detectors; from these points, one can extrapolate the pion trajectories to pinpoint the $\overline{\text{p}}$ decay. The scintillating cesium iodide crystals measure the energies and positions of photons passing through them; the simultaneous arrival of 511-keV photons in two such crystals signals a possible e⁺ annihilation.

After eliminating possible spurious signals, ATHENA researchers looked for events in which candidates for $\overline{\text{p}}$ and e⁺ annihilations were seen within 5 μs of each another. Of these coincident events, true $\overline{\text{H}}$ events are among those in which the 511-keV photons emerge in opposite directions—that is, those for which the cosine of the two-photon opening angle is − 1. The histogram on page 19 shows a peak in the number of events in the − 1 bin. The peak was consistent with the detection of 131 ± 22 $\overline{\text{H}}$ events, sitting on a much larger background of events for which the opening angle is random. As expected, no comparable peak showed up when the positron plasma was heated with radiofrequency signals, because $\overline{\text{H}}$ production should be suppressed at higher temperatures (see the black triangles). The experimenters also confirmed that the $\overline{\text{H}}$ peak was absent when only antiprotons (no positrons) were loaded into the trap. Because of the low detection efficiency, ATHENA collaborators estimated that the 131 events seen corresponded to 50 000 $\overline{\text{H}}$ atoms actually produced.

Gabrielse described ATHENA’s accomplishment as “an important milestone. The experiment is so hard that forming antihydrogen is a big deal.”

He and his ATRAP collaborators are also attempting to produce and detect antihydrogen. Although they have built a detector with resolution comparable to ATHENA’s, they are still waiting for the arrival of a new solenoidal magnet that will be compatible with it. In the meantime, during the run that ended in October, the ATRAP team was trying some techniques to enhance the $\overline{\text{H}}$ signal, said Gabrielse.

The next step

Having formed antiatoms, researchers now need to plan how they will do measurements on them. Unfortunately, as Dan Kleppner of MIT observed, “doing spectroscopy on antihydrogens is as daunting as creating them in the first place.” Yet, he added, the recent production has generated new enthusiasm for the task.

The ATHENA collaboration has not settled on an approach for comparing the H and $\overline{\text{H}}$ spectra, Hangst said. One option is to make the measurements on an $\overline{\text{H}}$ beam, much like the high-precision spectroscopy on hydrogen beams done by Theodor Hänsch and his coworkers at the University of Munich and at the Max Planck Institute for Quantum Optics in Garching, Germany. ⁹ Channeling enough $\overline{\text{H}}$ atoms into a directed beam, however, will be a difficult task. The other option, which the ATRAP group has chosen, is to perform the measurements on antihydrogen held in a trap. Hangst commented that neutral traps for antihydrogen are not very deep and thus require the atoms to be as cold as a few degrees kelvin.

So far the experiments have given no detailed information on how the $\overline{\text{H}}$ atoms were formed nor on what quantum state they occupy. The two main candidates are a three-body reaction (with an extra e⁺ to carry off the binding energy) and radiative recombination. Atoms formed during three-body recombination tend to be in a high Rydberg state, and they must be de-excited before measurements can be made on their ground-state transitions.

There’s another reason why it’s important to understand the $\overline{\text{H}}$ formation process. ATHENA collaborator Claudio Lenz Cesar (Federal University of Rio de Janeiro, Brazil) says that he and his colleagues now have some preliminary ideas about how the antiprotons and positrons are interacting in the mixing trap. These ideas, he explains, may enable them to engineer the way in which $\overline{\text{H}}$ atoms emerge and hence encourage the formation of a beam-like flux.