ALPHA freezes antiprotons

Laboratories like CERN can routinely produce many different types of antiparticles. In 1995, the PS210 experiment formed the first antihydrogen atoms and a few years later, in 2002, ATRAP and ATHENA were already able to produce several thousand of them. However, no experiment in the world has succeeded in ‘trapping’ these anti-atoms in order to study them. This is the goal of the ALPHA experiment, which has recently managed to cool down the antiprotons to just a few Kelvin. This represents a major step towards trapping the anti-atom, thus opening a new avenue into the investigation of antimatter properties.


Members of the ALPHA collaboration working on the apparatus in the Antiproton Decelerator experimental hall at CERN.

Just like the atom, the anti-atom is neutral. Unlike the atom, the anti-atom is made up of antiprotons (as opposed to protons in the atom) and positrons (as opposed to electrons). In order to thoroughly study the properties of the anti-atoms, scientists need to have them trapped for a reasonable amount of time (up to one second). The ALPHA experiment, installed at CERN’s Antiproton Decelerator (AD), has recently reached an important milestone by cooling antiprotons to just a few Kelvin. “This is the first time that a cloud of antiprotons has been cooled to such low temperatures”, explains Jeffrey Hangst, ALPHA’s spokesperson. “The next step will be to mix the cool antiprotons with positrons and form cold antihydrogen atoms that can stay trapped and can then be studied”.

The technique used by the ALPHA collaboration to cool down the antiprotons is borrowed from the neutral atom physics field and is called ‘evaporative cooling’. “You have to imagine your antiprotons contained in a bowl – in our case this is an electrostatic well. Initially, antiprotons move about quite a lot because they have quite a high energy. If you lower one side of the bowl, the hot ones will come out while the others will continue to interact. They end up with a lower temperature than they had before the hot ones escaped. You keep doing it: you let the hot ones escape while those remaining in the bowl come to a lower temperature thermal equilibrium”, says Jeffrey. Although this process leads to a heavy loss of antiprotons in the sample, the overall probability of forming antihydrogen atoms that can be trapped increases drastically.

The AD machine produces antiprotons at an energy of 5.3 MeV that corresponds to about 6x1010 Kelvin. Using the ‘evaporative cooling’ technique, the ALPHA collaboration managed to cool the antiprotons down to a temperature of 9 Kelvin, increasing the probability of trapping the anti-atoms by a factor of 100 with respect to other cooling techniques. “In order to achieve this temperature of just a few Kelvin, we need to carefully control the voltage in our trap with a precision of the order of a millivolt. Indeed, a potential change of 1 V is equivalent to a temperature change of more than 11000 K! The electric noise in the system must be kept very low to avoid heating up the antiprotons. The whole cooling process takes tens of seconds to complete”, explains Jeffrey.

The ALPHA collaboration is now working to integrate this technique into the main experiment. Trapping antihydrogen will be the next step, which will hopefully happen by the end of the year.

by CERN Bulletin