The Coolest Antiprotons

Phys. Rev. Focus 26, 1
The coldest antiprotons ever produced–which reached 9 kelvin–were cooled with a tried-and-true method for neutral atoms. The result is an essential step on the road to studying antihydrogen.
Antiproton steam. A standard method for cooling atoms is a lot like letting hot coffee cool–the hottest atoms evaporate away, like steam from a cup of coffee. Now a team has used this technique with charged particles–antiprotons–possibly getting them cold enough to make antihydrogen that can be trapped and studied.

A new record low temperature for a cloud of antiprotons was measured at CERN in Geneva, announces a report in the 2 July Physical Review Letters. Researchers cooled a cloud of about 4,000 antiprotons down to 9 kelvin using a standard approach for cooling atoms that has never been used with charged particles or ions. The technique could provide a new way to create and trap antihydrogen, which could help researchers probe a basic symmetry of nature.

Antihydrogen, the antimatter counterpart of hydrogen, is composed of one antiproton and one positron (anti-electron). According to the CPT (charge-parity-time) theorem, a fundamental pillar of the standard model of particle physics, hydrogen and antihydrogen should share many basic traits, like mass, magnetic moment, and emission spectrum. If antihydrogen and hydrogen have even slightly different spectra, it indicates some new physics principles beyond the standard model, a very big deal.

“We’d like to trap some antihydrogen and shine a laser on it and see if it looks just like hydrogen,” says Jeffrey Hangst of Aarhus University in Denmark and the ALPHA collaboration at CERN. In 2002, the ATHENA collaboration at CERN, ALPHA’s predecessor, created about one hundred antihydrogen atoms per second by bringing antiprotons and positrons together in a trap of electric and magnetic fields [1]. The trap could only hold charged particles, so the neutral antihydrogen atoms escaped as soon as they formed. To trap antihydrogen in a neutral atom trap and measure its properties, researchers need it to be as cold as half a kelvin.

That calls for extremely cold antiprotons, which are responsible for most of the thermal energy in antihydrogen. Earlier techniques cooled antiprotons with cold electrons, but the coldest antiproton temperature recorded with this method was about 100 kelvin. To turn down the heat, Hangst and colleagues used a technique called evaporative cooling, which had previously been used only for neutral atoms. “It’s exactly how your coffee cools itself,” Hangst says. “The steam above your coffee, those molecules are the hottest ones. They can escape from the coffee and carry away energy, so the coffee is absolutely colder.”

Cold, charged particles are especially sensitive to stray electromagnetic fields, so the ALPHA team had to design electronics that were unusually “noise free.” They also took advantage of new tricks they had discovered for increasing the density of their antiproton plasma [2] since the ATHENA densities were too low for evaporative cooling.

Hangst and colleagues put about 40,000 antiprotons at a temperature of about 1000 kelvin into their electromagnetic trap. A particle trap is something like a bowl, and the team slowly lowered one side of the bowl, making it shallower and allowing the hottest antiprotons to escape over the side. At the end of the experiment, only 10 percent of the original antiprotons remained. But they were at a temperature of only 9 kelvin, ten times colder than previous antiproton cooling experiments. “These are the coldest anti-protons ever measured,” Hangst says and notes that some of the particles are probably cold enough to make trappable antihydrogen.

Typically, physicists use lasers to cool ions down to extremely low temperatures, but that only works if the ions contain the right set of energy states. Hangst says the ALPHA experiment shows that evaporative cooling could work with almost any charged particles or ions–even molecules–and need not be limited to neutral atoms.

The result is “a real step forward,” says Cliff Surko of the University of California, San Diego. “In the longer view, it could really be very significant to the overall effort” to trap antihydrogen.

–Lisa Grossman

Lisa Grossman is a freelance science writer.


  1. M. Amoretti et al., “Production and Detection of Cold Antihydrogen Atoms,” Nature (London) 419, 456 (2002)
  2. G. Andresen et al., “Compression of Antiproton Clouds for Antihydrogen Trapping,” Phys. Rev. Lett. 100, 203401 (2008)

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Atomic and Molecular Physics

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