Physicists at the ALPHA experiment at CERN have taken an important first step towards measuring the gravitational mass of antihydrogen, which consists of a positron surrounding an antiproton. Although they did not see any evidence that anti-atoms respond to gravity differently than atoms, the possibility that antimatter responds much more strongly to gravity than matter was ruled out. The work involved measuring how long it takes atoms of antihydrogen to reach the edges of a magnetic trap after it was switched off.

While anti-atoms may sound like the stuff of science fiction, they were first detected at CERN in 1995 and the Geneva lab currently has several experiments investigating their properties.

Despite a large body of theoretical and experimental work suggesting that gravity acts in exactly the same way on antimatter as it does on matter, there has been no direct measurement of how gravity affects falling antimatter. But finding even the tiniest of differences between the behaviour of matter and antimatter is important as it could shed light on mysteries such as why there is so little antimatter in the universe – and the precise nature of dark matter and dark energy.

Such experiments are tricky, to say the least. As well as collecting enough anti-atoms to carry out an experiment, which has until recently been very hard, physicists also must ensure that the anti-atoms are moving very slowly when they are released. If they have large amounts of thermal motion when dropped, then any subtle differences in how anti-atoms fall would be washed out by this random motion. Therefore the atoms have to be massively cooled to reduce their random thermal motion.

Looking at old data

Physicists on the ALPHA antihydrogen experiment at CERN have now analysed data from the 2010–2011 run of the experiment to see if anything could be gleaned about the gravitational properties of antihydrogen. ALPHA was designed primarily to trap and hold large numbers of antihydrogen anti-atoms, which are made by combining positrons from a radioactive source with antiprotons produced by CERN's Antiproton Decelerator facility. The ultimate goal of the experiment is to carry out spectroscopy on antihydrogen and see whether its energy levels mirror that of hydrogen.

When ALPHA's magnetic trap is turned off, the cloud of antihydrogen starts to expand outward. When an anti-atom reaches the solid interior surface of the trap it annihilates, producing a flash of radiation that can be detected. The team recorded the position of each annihilation and the time when it occurred – telling the researchers the trajectory followed by each atom. The team began with data describing 434 anti-atoms but found that only 23 were moving slowly enough to make the gravity analysis significant.

Gravity or antigravity

As a result of its analysis, the team has been able to put the first direct limit on the ratio between the gravitational mass and the inertial mass of antihydrogen. Normal matter has a ratio of one – anything greater than one would mean that gravity acts more strongly on antihydrogen and that it would fall further than matter. More intriguing is a negative value of the ratio, which would indicate a force acting in the direction opposite to gravity – or in other words antigravity.

In its analysis, the team was able to rule out ratios less than about –65 and greater than about +110. While such extreme ratios were not expected, the spokesperson of the experiment, Jeffrey Hangst, told physicsworld.com that the work is a "proof of principle" that a magnetic trap could be used to measure the gravitational mass of antihydrogen.

What physicists really want to do is look at ratio values in the –1 to 1 range – in other words, to allow them first to rule out antigravity entirely and then look for a tiny deviation from 1. Hangst says that the team believes that it should be able to focus on the –1 to 1 range when experimental work starts up again at CERN in 2015. This will be possible, in part, thanks to improvements in how the measurement is done. In addition, the next generation of the experiment, ALPHA-2, is designed to trap larger amounts of cold antihydrogen, which should also improve the measurement.

Purpose-built trap?

While the ALPHA team gets its gravitational-mass data "free" because the other experiments also involve switching off the trap, it may not be possible to fully optimize the measurement because this could have a detrimental effect on other aspects of the experimental programme. Looking further into the future, Hangst says that the team could consider a purpose-built trap to study the gravitational mass of antihydrogen.

ALPHA researchers are not the only people at CERN looking to rule antigravity in or out. In particular, the AEGIS experiment seeks to measure the gravitational mass of antihydrogen. Instead of trapping anti-atoms, it will send a horizontal pulsed beam of cold antihydrogen through a "Moiré deflectomer", which will only pass anti-atoms within a very narrow range of velocities. As it travels, the beam is expected to fall 20 μm under the influence of gravity and the team will look for any tiny deviations from this drop. AEGIS spokesperson Michael Doser says his collaboration welcomes the "healthy competition" and adds that "maybe ALPHA can do it".

If either team manages to find a difference in the gravitational masses of hydrogen and antihydrogen it would come as an unexpected boon to physicists trying to work out why nearly all of the antimatter created in the Big Bang has since vanished. "There is something fishy about gravity," muses Doser, who points out that important mysteries such as the nature of dark matter and dark energy also seem to be related to the force.

The analysis is described in Nature Communications.

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