SCIENCE

In a forthcoming issue of Nature now online, the ALPHA team reports the results of 335 experimental trials, each lasting one second, during which the anti-atoms were created and stored. The trials were repeated at intervals never shorter than 15 minutes. To form antihydrogen during these sessions, antiprotons were mixed with positrons inside the trap. As soon as the trap’s magnet was “quenched,” any trapped anti-atoms were released, and their subsequent annihilation was recorded by silicon detectors. In this way the researchers recorded 38 antihydrogen atoms, which had been held in the trap for almost two-tenths of a second.

The positions of the 38 real anti-atom annihilations (circles and triangles) match predicted antihydrogen distribution (gray dots in upper panel) but not the simulated distribution of bare antiprotons (colored dots in lower panel). Charged bare antiprotons would be steered to different clusters by different electric fields (red right bias, blue left bias, green no bias), but anti-atoms are neutral so their distribution is unaffected. (The violet star is an energetic positron.) (Click on image for best resolution.)

“Proof that we trapped antihydrogen rests on establishing that our signal is not due to a background,” says Fajans. While many more than 38 antihydrogen atoms are likely to have been captured during the 335 trials, the researchers were careful to confirm that each candidate event was in fact an anti-atom annihilation and was not the passage of a cosmic ray or, more difficult to rule out, the annihilation of a bare antiproton.

To discriminate among real events and background, the ALPHA team used supercomputer simulations based on theoretical calculations to show how background events would be distributed in the detector versus how real antihydrogen annihilations would appear. Fajans and Francis Robicheaux of Auburn University contributed simulations of how mirror-trapped antiprotons (those confined by magnet coils around the ends of the octupole magnet) might mimic anti-atom annihilations, and how actual antihydrogen would behave in the trap.

Learning from antimatter

Before 1928, when anti-electrons were predicted on theoretical grounds by Paul Dirac, the existence of antimatter was unsuspected. In 1932 anti-electrons (positrons) were found in cosmic ray debris by Carl Anderson. The first antiprotons were deliberately created in 1955 at Berkeley Lab’s Bevatron, the highest-energy particle accelerator of its day.

At first physicists saw no reason why antimatter and matter shouldn’t behave symmetrically, that is, obey the laws of physics in the same way. But if so, equal amounts of each would have been made in the big bang—in which case they should have mutually annihilated, leaving nothing behind. And if somehow that fate were avoided, equal amounts of matter and antimatter should remain today, which is clearly not the case.

In the 1960s, physicists discovered subatomic particles that decayed in a way only possible if the symmetry known as charge conjugation and parity (CP) had been violated in the process. As a result, the researchers realized, antimatter must behave slightly differently from ordinary matter. Still, even though some antiparticles violate CP, antiparticles moving backward in time ought to obey the same laws of physics as do ordinary particles moving forward in time. CPT symmetry (T is for time) should not be violated.

One way to test this assumption would be to compare the energy levels of ordinary electrons orbiting an ordinary proton to the energy levels of positrons orbiting an antiproton, that is, compare the spectra of ordinary hydrogen and antihydrogen atoms. Testing CPT symmetry with antihydrogen atoms is a major goal of the ALPHA experiment.

How to make and store antihydrogen

To make antihydrogen, the accelerators that feed protons to the Large Hadron Collider (LHC) at CERN divert some of these to make antiprotons by slamming them into a metal target; the antiprotons that result are held in CERN’s Antimatter Decelerator ring, which delivers bunches of antiprotons to ALPHA and another antimatter experiment.

Wurtele says, “It’s hard to catch p-bars”—the symbol for antiproton is a small letter p with a bar over it—“because you have to cool them all the way down from a hundred million electron volts to fifty millionths of an electron volt.”

In the ALPHA experiment the antiprotons are passed through a series of physical barriers, magnetic and electric fields, and clouds of cold electrons, to further cool them. Finally the low-energy antiprotons are introduced into ALPHA’s trapping region.

Meanwhile low-energy positrons, originating from decays in a radioactive sodium source, are brought into the trap from the opposite end. Being charged particles, both positrons and antiprotons can be held in separate sections of the trap by a combination of electric and magnetic fields—a cloud of positrons in an “up well” in the center and the antiprotons in a “down well” toward the ends of the trap.