AEgIS on track to test free-fall of antimatter

The AEgIS collaboration at CERN’s Antiproton Decelerator (AD) has reported a milestone in its bid to measure the gravitational free-fall of antimatter – a fundamental test of the weak equivalence principle. Using a series of techniques developed in 2018, the team demonstrated the first pulsed production of antihydrogen, which allows the time at which the antiatoms are formed to be known with high accuracy. This is a key step in determining “g” for antimatter.

“This is the first time that pulsed formation of antihydrogen has been established on timescales that open the door to simultaneous manipulation, by lasers or external fields, of the formed atoms, as well as to the possibility of applying the same method to pulsed formation of other antiprotonic atoms,” says AEgIS spokesperson Michael Doser of CERN. “Knowing the moment of antihydrogen formation is a powerful tool.”

General relativity’s weak equivalence principle holds that all particles with the same initial position and velocity should follow the same trajectories in a gravitational field. It has been verified for matter with an accuracy approaching 10^–14. Since theories beyond the Standard Model such as supersymmetry, or the existence of Lorentz-symmetry violating terms, do not necessarily lead to an equivalent force on matter and antimatter, finding even the slightest difference in g would suggest the presence of quantum effects in the gravitational arena. Indirect arguments constrain possible differences to below 10^–6g, but no direct measurement for antimatter has yet been performed due to the difficulty in producing and containing large quantities of it.

ALPHA, AEgIS and GBAR are all targeting a measurement of g at the 1% level in the coming years.

Antihydrogen’s neutrality and long lifetime make it an ideal system in which to test this and other fundamental laws, such as CPT invariance. The first production of low-energy antihydrogen, reported in 2002 by the ATHENA and ATRAP collaborations at the AD, involved a three-body recombination reaction (e⁺+e⁺+p → H+e⁺) involving clouds of antiprotons and positrons. Since then, steady progress by the AD’s ALPHA collaboration in producing, manipulating and trapping ever larger quantities of antihydrogen has enabled spectroscopic and other properties of antimatter to be determined in exquisite detail.

Whereas three-body recombination results in an almost continuous antihydrogen source, in which it is not possible to tag the time of the antiatom formation, AEgIS has employed an alternative charge-exchange process between trapped and cooled antiprotons and positronium (e⁺e^– bound system). Bursts of positrons are accelerated and then implanted into a nano-channelled silicon target above an electromagnetic trap containing cold antiprotons, where, with the aid of laser pulses, they produce a cloud of excited positronium a few millimetres across. This can lead to the formation of antihydrogen within sub-μs timescales, the moment of production being defined by the wellknown laser firing time and the transit time of positronium toward the antiproton cloud. Since the antihydrogen is not trapped in the apparatus, it drifts in all directions until it annihilates on the surrounding material, producing pions and photons that are detected by a scintillating array read out by photomultipliers. The scheme allows the time at which 90% of the atoms are produced to be determined with an uncertainty of around 100 ns.

Further steps are required before the measurement of g can begin, explains Doser. These include the formation of a pulsed beam, greater quantities of antihydrogen, and the ability to make it colder. “With only three months of beam time this year, and lots of new equipment to commission, most likely 2022 will be the year in which we establish pulsed beam formation, which is a prerequisite for us to perform a gravity measurement.”

Targeted approach

Following a proof-of-principle measurement of g for antihydrogen by the ALPHA collaboration in 2013, ALPHA, AEgIS and a third AD experiment, GBAR, are all targeting a measurement of g at the 1% level in the coming years. In contrast to AEgIS’s approach, whereby the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms will be measured in an approximately 1 m-long flight tube, GBAR will take advantage of advances in ion-cooling techniques to measure ultraslow antihydrogen atoms as they fall from a height of 20 cm. ALPHA, meanwhile, will release antihydrogen atoms from a vertical magnetic trap and measure the distribution of annihilation positions when they hit the wall – ramping the trap down slowly so that the coldest atoms, which are most sensitive to gravity, come out last. All three experiments have recently been hooked up to the AD’s ELENA synchrotron, which enables the production of very low-energy antiprotons.

Given that most of the mass of antinuclei comes from massless gluons that bind their constituent quarks, physicists think it unlikely that antimatter experiences an opposite gravitational force to matter and therefore “falls up”. Nevertheless, precise measurements of the free fall of antiatoms could reveal subtle differences that would open an important crack in current understanding.