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CERN | Geneva | Switzerland

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Measuring the gravitational constant with antimatter

This might not be the most intuitive way to measure g, the gravitational acceleration constant. Nevertheless, this is what a team of about 50 scientists from the AEgIS collaboration (Antihydrogen Experiment – Gravity, Interferometry, Spectroscopy) is trying to achieve. This might soon become even easier, thanks to a recently approved new project to build ELENA, a new decelerator for antiprotons.

Antimatter is not new at CERN. Strictly speaking, we have been producing particles and antiparticles for decades. But producing full atoms is another story. What is much more recent, is that a small group of about thirty physicists from the ALPHA experiment managed to produce anti-hydrogen atoms and keep them for about 1000 seconds.

Hydrogen is the simplest atom. It is made of one proton and one electron. Anti-hydrogen atoms are identical to hydrogen atoms except that they have a positron (the anti-electron) orbiting around an antiproton. In antimatter, the properties of the antiparticles, such as their mass, are identical to their particle counterpart, except that some other charges, like the electric charge, are inverted. Even electrically neutral particles have their antiparticle. An antineutron is made of three antiquarks (one anti-u quark of charge of -2/3, and two anti-d quarks, each one with a charge of +1/3) while the neutron is made of three quarks: u,d and d.

Producing antiprotons is a piece of cake when you have powerful accelerators like those at CERN. Accelerate protons, send them on to a target and a slew of particles will come out, including antiprotons. These are then channeled into a beam line and directed to the Antiproton Decelerator facility. This is just a large rectangular “ring” where the antiprotons are decelerated using radio frequency cavities and clouds of electrons that slow down the antiprotons by means of small successive collisions. In this way, the antiprotons see their energy reduced by a factor of 35 between their arrival at the decelerator and their delivery to the experiments two minutes later, going from 3.5 GeV down to about 100 MeV. At this point, five different teams, ACE, AEgIS, ALPHA (the ATHENA successor), ASACUSA and ATRAP extract the antiprotons to conduct various experiments.

The ACE experiment studied antiprotons to see if they could be used for cancer therapy. The goal would be to direct them onto the nucleus of cancer cells, where the antiprotons would annihilate when meeting protons from these cells. The energy released would in turn kill nearby cancerous cells. The hope is to provide a treatment that would cause less damage to healthy tissue than in current cancer therapy techniques.

The other four experiments create antihydrogen. For this, they need positrons, which they create from their own radioactive source, namely sodium 22.

The tricky part is to arrange for these two ingredients to meet and bind into anti-atoms. The name of the game here is to decelerate them enough to allow the antiproton to capture a positron. So far, different techniques have been used. The Japanese teams from ASACUSA use electric fields; others use thin aluminum foil to further decelerate the antiproton but at a high cost: many are lost in the process.

This is where ELENA will soon bring a new era in antimatter production at CERN. Recently approved, this new facility, which will be operational in 2015, will allow antiprotons to be decelerated further by another factor of 1000, bringing them to only 100 keV and in much greater numbers: two key ingredients for success.

All this must happen in a very pure vacuum to avoid interactions with regular matter. Otherwise, as soon as matter and antimatter meet, the two annihilate, leaving pure energy behind. Magnetic or electric fields are used in various places of the experimental setups to confine the antimatter and keep it away from any surface.

And what can you do with antimatter once you manage to produce some? Basically, reproduce all the physics measurements done last century with hydrogen atoms. One plan is to study their spectroscopy to see if antihydrogen behaves like hydrogen and emits the same characteristic spectral lines.

But AEgIS has an even more ambitious program. They hope not only to create antihydrogen, but then to direct a beam of antihydrogen onto a series of slits to create fringes by interferometry and measure by how much these fringes move as the antihydrogen atoms fall under the effect of gravity while going across a chamber of known length. The goal of course is to see if gravity (or g) is the same for anti-matter.

This reminds me of this classic joke about a physics student who is given a barometer to measure the height of a building. Her professor expects her to measure the pressure difference and from that, infer the height of the building. But she thinks this is so dumb that instead she suggests to use it as a pendulum and measure its period, or to drop it from the roof and measure the time it takes to fall. But each time, the professor refuses her suggestions. Eventually, she proposes to knock at the door of the building janitor and offer him a barometer as a gift if he tells her the height of the building!

But as Michael Doser, the head of the AEgIS team, put it: “Knocking on nature’s door to ask for the answer is not allowed…”

Pauline Gagnon

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A view of the ASACUSA experimental setup (photo credit: Michael Doser)