Making long-lived positronium atoms for antimatter gravity experiments

March 6, 2019 by Ana Lopes, CERN
The experimental set-up used by the AEgIS collaboration to make long-lived positronium. Credit: Ruggero Caravita/CER

The universe is almost devoid of antimatter, and physicists haven't yet figured out why. Discovering any slight difference between the behaviour of antimatter and matter in Earth's gravitational field could shed light on this question. Positronium atoms, which consist of an electron and a positron, are one type of antimatter atoms being considered to test whether antimatter falls at the same rate as matter in Earth's gravitational field. But they are short-lived, lasting a mere 142 nanoseconds – too little to perform an antimatter gravity experiment. Researchers are therefore actively seeking tricks to make sources of positronium atoms that live longer. In a paper published today in the journal Physical Review A, the AEgIS collaboration at CERN describes a new way of making long-lived positronium.

To be useful for gravity experiments, a source of atoms needs to produce long-lived atoms in , and with known velocities that can be controlled and are unaffected by disturbances such as electric and magnetic fields. The new AEgIS source ticks all of these boxes, producing some 80 000 positronium atoms per minute that last 1140 nanoseconds each and have a known velocity (between 70 and 120 kilometres per second) that can be controlled with a high precision (10 kilometres per second).

The trick? Using a special positron-to-positronium converter to produce the atoms and a single flash of ultraviolet laser light that kills two birds with one stone. The laser brings the atoms from the lowest-energy electronic state to a long-lived higher-energy state and can select among all of the atoms only those with a certain velocity.

This is not the first time that researchers have produced a source of long-lived positronium atoms. There are other techniques that do so, including one that involves bringing the atoms to electronic states called Rydberg states, and which could also be used to perform gravity experiments with positronium. But all of these are very sensitive to electric and magnetic fields, which influence the atoms' velocity and would need to be factored into future gravity measurements. The new method devised by AEgIS is "cleaner," in that it is almost insensitive to these fields.

The next step on the long path to measuring the effect of on positronium with the new AEgIS source (the AEgIS team and other CERN collaborations mainly plan to take measurements with antihydrogen atoms) will be to confirm that the produced are electrically neutral. CERN's accelerator complex is currently shut down for a major two-year upgrade programme, so most experiments at the Laboratory, which require a beam of protons, have ceased to operate during this period. An advantage of this positronium experiment is that it doesn't require protons, so it can continue to be operated during the shutdown.

Explore further: Does antimatter weigh more than matter? Lab experiment to find out the answer

More information: C. Amsler et al. Velocity-selected production of 2S3 metastable positronium, Physical Review A (2019). DOI: 10.1103/PhysRevA.99.033405

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2 / 5 (1) Mar 06, 2019
Anti-gravity concept using accelerated bi-directional positrons, 9 years ago.
Da Schneib
3.7 / 5 (3) Mar 06, 2019
This is cool, and a very necessary experiment. We'll have to get to antihydrogen too, but we need to test both the lepton sector and the baryon sector, so if this one's easier it makes sense to do it first.
1 / 5 (1) Mar 06, 2019
if we can ever figure out the higgs boson, then lookout universe.
1 / 5 (2) Mar 06, 2019
Positronium atoms, which consist of an electron and a positron
They make an atom out of an electron and its anti-matter counterpart?? Why doesn't it annihilate itself "immediately"?
5 / 5 (2) Mar 07, 2019
@carbon_unit: because they orbit each other.
4 / 5 (4) Mar 07, 2019
produce the atoms and a single flash of ultraviolet laser light that kills two birds with one stone.

Dammit. You got a UV laser. Why do you need to resort to stones to kill birds?
5 / 5 (1) Mar 07, 2019
They make an atom out of an electron and its anti-matter counterpart?? Why doesn't it annihilate itself "immediately"?
They excite the atom before it can annihilate into a long-lived higher-energy state. Similar to the big bang where virtual particle pairs were excited enough to become real particles except gamma rays were used in the big bang instead of lasers.
5 / 5 (1) Mar 07, 2019
Note - most of the science world would be thrilled if they could make a billion plus one electrona for every billion positrons. That way they could then explain the excess of matter over antimatter. But that would destroy my opinion about a parallel universe of antimatter created at the big bang as evidenced by the area around the cold spot on the WMAP map.
5 / 5 (2) Mar 07, 2019
Particle Antiparticle is Chiral

The universe is almost devoid of antimatter, and physicists haven't yet figured out why!
The reason is as simple as antimatter its self
the antiparticle of the particle
the difference
in the electric charge
electrons have - charge
positrons have + charge
protons have + charge
antiprotons have – charge
the reason lies in the proton
the proton repels all positrons
and attracts all electrons
the reason lies deep on the proton
in a similar way right hander's dominate
as in nature this has its roots
in the rotational spirals of atomic structures
The difference in particle antiparticle is chiral
5 / 5 (1) Mar 07, 2019
the reason lies in the proton
the proton repels all positrons
Yes and antiprotons don't like electrons so the antiprotons and positrons packed their bags and went to a parallel universe at the big bang..

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