ALPHA-g clocks the freefall of antihydrogen

Ever since the discovery of antimatter 90 years ago, physicists have striven to measure its properties in new and more precise ways. Experiments at CERN’s Antimatter Factory represent the state of the art. In addition to enabling measurements of properties such as the antiproton charge-to-mass ratio with exquisite precision (recently shown by the BASE experiment to be equal to that of the proton within a remarkable 16 parts per trillion), the ability to trap and store large numbers of antihydrogen atoms for long periods by the ALPHA experiment has opened the era of antihydrogen spectroscopy. Such studies allow precise tests of fundamental symmetries such as CPT. Until now, however, the gravitational behaviour of antimatter has remained largely unknown.

Equivalence principle

Using a modified setup, the ALPHA collaboration recently clocked the freefall of antihydrogen, paving the way for precision studies of the magnitude of the gravitational acceleration between antiatoms and Earth. The goal is to test the weak equivalence principle of general relativity, which requires that all test masses must react identically to Earth’s gravity. While models have been built that suggest differences could exist between the freefall rates of matter and antimatter (for example due to the existence of new, long-range forces), the theoretical consensus is clear: they should fall to Earth at the same rate. In physics, however, you don’t really know something until you observe it, emphasises ALPHA spokesperson Jeffrey Hangst: “This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the universe.”

The ALPHA collaboration creates antihydrogen by binding antiprotons produced and slowed down in the Antiproton Decelerator and ELENA rings with positrons accumulated from a sodium-22 source. It then confines the neutral, but slightly magnetic, antimatter atoms in a magnetic trap to prevent them from coming into contact with matter and annihilating. Until now, the team has concentrated on spectroscopic studies with the ALPHA-2 device. But it has also built an apparatus called ALPHA-g, which makes it possible to measure the vertical positions at which antihydrogen atoms annihilate with matter once the trap’s magnetic field is switched off, allowing the antiatoms to escape.

The ALPHA team trapped groups of about 100 antihydrogen atoms and then slowly released them over a period of 20 seconds by gradually ramping down the top and bottom magnets of the trap. Numerical simulations indicate that, for matter, this operation would result in about 20% of the atoms exiting through the top of the trap and 80% through the bottom – a difference caused by the downward force of gravity. By averaging the results of seven release trials, the ALPHA team found that the fractions of antiatoms exiting through the top and bottom were in line with simulations. Since vertical gradients in the magnetic field magnitude can mimic the effect of gravity, the team repeated the experiment several times for different values of an additional bias magnetic field, which could either enhance or counteract the force of gravity. By analysing the data from this bias scan, the team found that the local gravitational acceleration of antihydrogen is directed towards Earth and has magnitude a_g = [0.75 ± 0.13 (stat. + syst.) ± 0.16 (sim.)]g, which is consistent with the attractive gravitational force between matter and Earth.

This is the start of a new avenue of experimental exploration that pushes the development of trapping and other techniques

The next step, says Hangst, is to increase the precision of the measurements via laser-cooling of the antiatoms, which was first demonstrated in ALPHA-2 and will be implemented in ALPHA-g in 2024. Two other experiments at CERN’s Antimatter Factory, AEgIS and GBAR, are poised to measure a_g  using complementary methods. AEgIS will measure the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms in an approximately 1 m-long flight tube, while GBAR will take advantage of new ion-cooling techniques to measure ultra-slow antihydrogen atoms as they fall from a height of 20 cm. All three experiments are targeting a measurement of  a_g  at the 1% level in the coming years.

Even higher levels of precision will be needed to test models of new physics, say theorists. “The role of antimatter in the ‘weight’ of antihydrogen is very little, since practically all the mass of a nucleon or antinucleon comes from binding gluons, not antiquarks,” says Diego Blas of Institut de Física d’Altes Energies and Universitat Autònoma de Barcelona. “Any new force that couples differently to matter and antimatter would therefore need to have a huge effect in antiquarks, which makes it difficult to build models that are consistent with existing observations and where the current measurements by ALPHA-g would be different.” Things start to get interesting when the precision reaches about one part in 10 million, he says. “This is the start of a new avenue of experimental exploration that pushes the development of trapping and other techniques. If you compare the situation with the sensitivity of the first prototypes of gravitational-wave detectors 50 years ago, which had to be improved by six or seven orders of magnitude before a detection could be made, anything is possible in principle.”