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To study antimatter, you must first slow it down

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In July 2018, physicists at CERN attempted to slow down antimatter particles moving at breakneck speeds as part of an international collaboration called ALPHA. The goal was then to give oneself the time to analyze them. The results of this work were published in the journal Nature.

The mystery of matter / antimatter asymmetry

Antimatter is a mirror of matter. Imagine the same particles, but with opposite charges. While matter electrons have a negative charge, antimatter positrons (or antielectrons) are positively charged. Or while protons are positively charged, their antiprotons are negatively charged, and so on.

Matter and antimatter were produced in the same quantity at the time of the Big Bang. However, they did not behave in the same way. If the two materials meet, they annihilate each other. However, in the first second after the Big Bang 13.77 billion years ago, we know that matter has won its match against antimatter.

Thus, at the end of this process of annihilation, instead of the disappearance of all this little world, there thus remained a surplus of matter. That’s why we’re still here to talk about it. Without this initial imbalance, the universe would not exist. The question is : why did such an imbalance occur in the first place?

For now, cosmologists still ignore it. To give us the means to understand this enigma, we must therefore study antimatter. This is obviously not an easy task. As said above, the antiparticles annihilate in the presence of matter. In addition, they move at breakneck speeds.

The idea is therefore to produce or capture them, then slow them down sufficiently in order to be able to analyze their properties.

In everyday life, you will have less time to appreciate a beautiful car speeding at full speed than if the same car starts to brake when approaching a red light. It’s the same in quantum physics: the more time you have to observe a certain property, the more precise your measurement.

Big Bang artist illustration. Credit: ESA

Slow down an antiparticle at the speed of a cheetah

Over the past few decades, our ability to experiment with this antimatter has increased dramatically. The latest project, ALPHA, “tinkers” at CERN with atoms fromantihydrogen (a positron orbiting an antiproton, just like an electron orbiting a proton in regular hydrogen).

Captured for the first time in 2011, antihydrogen indeed presents itself as a candidate of choice for such an experiment, because it is thesimplest atomic atom. Don’t panic, the quantities produced are tiny.

As part of this project, CERN physicists trapped these antiatoms in a magnetic field before bombarding them with photons (particles of light) fired from a laser. “Once the photons hit the atoms, they excite them and change their movements“, Explains Takamasa Momose, one of the main players in this work.

When trapped, antihydrogens travel at just over 300 km / h. By controlling the light, the researchers here have given themselves the means to slow them down to about the speed of a cheetah, thus reducing their temperature.

Physicists have thus demonstrated their ability to manipulate the movement of antimatter atoms by laser light. This work will pave the way for potentially revolutionary future experiments. One of them will be to throw anti-atoms into empty space and slow them down to analyze how they fall under the effect of gravity.


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