While the CERN accelerator complex was being revamped in 2013, the ASACUSA experiment took time to carefully review the data taken in 2012 at the Antiproton Decelerator (AD) facility. This painstaking work paid off and they just announced in Nature having produced the first ever beam of antihydrogen atoms.
In laboratory experiments like the ones conducted at CERN, matter and antimatter are always created in equal amounts. The Big Bang theory predicts that the same quantities of matter and antimatter were also created at the origin of the Universe. However, nowadays, one sees absolutely no trace of this “primordial” antimatter. So what happened to all the antimatter that once was in the Universe?
To answer this question, CERN has a full antimatter program underway at the AD to check if antimatter has the same properties as matter. One of the best ways to do so is to compare antihydrogen atoms with hydrogen atoms. This is the simplest of all atoms, having only one electron orbiting around one single proton.
Antihydrogen atoms are replica of hydrogen atoms but with an anti-electron – called positron – and an antiproton replacing the electron and proton of regular atoms.
All matter emits light when excited just as a piece of metal shines when heated up. The light emitted gives a unique signature for each atom. For example, hydrogen emits and absorbs light of specific frequency when an electron jumps from one energy level to another. It also has a “hyperfine structure” corresponding to magnetic interactions between the nucleus and the electron.
The ASACUSA experiment aims to check the hyperfine structure of antihydrogen. This can be done by observing which frequencies antihydrogen atoms absorb.
The ASACUSA experiment at CERN (Image: Yasunori Yamakazi )
So here is what ASACUSA did: they produced antihydrogen atoms by first decelerating and cooling antiprotons down to very low temperature. Then they mixed antiprotons with positrons and combined them in a strong non-uniform magnetic field. These strong magnetic fields are necessary to keep antiprotons and positrons from touching any matter. That would cause their immediate annihilation and prevent the formation of antihydrogen atoms.
The next problem was to move the antihydrogen atoms away from this field to be able to study their hyperfine structure. Otherwise, the strong non-uniform magnetic field would mask the tiny effects generated by the magnetic interaction between the antiproton and the positron responsible for the hyperfine structure.
But atoms are neutral and cannot be controlled by electric fields. However, antihydrogen atoms are like tiny magnets. So by using non-uniform magnetic fields, the scientists were able to manipulate these tiny magnets and create a beam of antihydrogen atoms. It was directed towards a small detector located after a microwave cavity and a sextupole magnet.
The sextupole magnet focuses or defocuses antihydrogen atoms on the detector depending on the direction of the antihydrogen tiny magnets.
The ASACUSA setup. From left to right: the magnets (grey) used to produce antihydrogen atoms, the microwave cavity (green) to induce hyperfine transitions, the sextupole magnet (red and grey) and the antihydrogen detector (gold). Credit: Stefan Meyer Institute.
The detector reveals the number of antihydrogen atoms passing through the device after they go through a microwave cavity. It was turned off in 2012 but will be on in the future. The antihydrogen atoms will only absorb microwave photons having exactly the energy corresponding to its hyperfine transitions. This process will alter the trajectory of antihydrogen atom in the sextupole magnet, and eventually the number of antihydrogen atoms reaching the detector will be reduced.
By counting how many antihydrogen atoms reach the target when the microwave cavity is tuned to specific frequencies, the scientists will determine the frequencies of the hyperfine structure.
ASACUSA now has the proof that 80 antihydrogen atoms made it to their detector. The next step is to see if fewer are observed when the microwave cavity is turned on at the right frequency.
And then we will know if antihydrogen is the exact mirror image of hydrogen. This may reveal if antimatter differs from matter and explain why it has all vanished.
Pauline Gagnon
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