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Antiproton Collector (AC) and Antiproton Accumulator (AA)
CERN's Antiproton Collector (AC) and CERN's Antiproton Accumulator (AA).
photo: CERN
The Antiproton Decelerator

The Antiproton Decelerator (AD) is the only facility of its kind anywhere in the world. It is sort of an antimatter factory, uniquely capable of trapping antiprotons and slowing them to speeds where scientists can study them.

The researchers working in the AD are looking for differences in how matter and antimatter behave. Antimatter is something like a mirror image of matter. Researchers think that if there are differences between them, they might help us understand why matter won out against antimatter just after the Big Bang.

stochastic cooling pipe
The diagonal pipe is part of the stochastic cooling system, which takes measurements and transports them to the other side of the ring before the antiprotons arrive.
There are three experiments underway in the AD, and each of them will trap antimatter, see how it behaves, and compare this behavior the behavior of matter. Two of the experiments, ATHENA and ATRAP, will compare properties of hydrogen and antihydrogen. ASACUSA will create "atomcules," hybrid atoms containing a helium nucleus and orbited by an electron and an antiproton.

How it works

The AD is a relatively small ring (only several hundred meters in circumference, compared with 27 km in the LHC) used to slow down and focus a beam of antiprotons as they come speeding out of the Proton Synchrotron. The antiprotons arrive at the AD like a blast from a garden hose, scattered in various directions and moving at different speeds. The first task of the machine is to focus the antiprotons into a tidy, controllable beam. This is done with the help of a method called "stochastic cooling." With stochastic cooling, measurements of the beam are taken on one side of the ring, and sent across the middle of the ring in time to make adjustments to the AD before the beam comes around the bend from the other side.

antiproton pipe
The antiproton pipe turning a corner. Blue dipole magnets direct the beam, and red quadrupole magnets focus it.
Once the beam is refined enough, deceleration begins. This is a somewhat complicated process because, as the beam slows down, it has a tendency to expand. Physicists measure energy in units called electron volts (eV) and momentum in electron volts divided by the speed of light (eV/c). Because one eV is so small, scientists usually use giga-electron volts (GeV), which equals one billion electron volts. Scientists decelerate the beam in several steps, interspersing it with more cooling. First, the antiprotons are slowed from 3.57 GeV/c to 2 GeV/c by using radio frequency electric fields. Red dipole magnets (with two poles) keep the beam bent and flowing around the ring. As the beam is being slowed, blue quadropole magnets (with four poles) keep it focused. Stochastic cooling helps to streamline the beam as it travels through the pipe. Once it’s been slowed to 300 million eV/c, a second step begins, called "electron cooling." A thick, cool cloud of electrons travels in the same direction as the antiprotons, cooling them further. The process is similar to mixing flows of hot and cool water until they are lukewarm. It also helps line up the antiprotons in the direction they’re moving.

When the beam has been slowed down to 100 million eV/c, the antiprotons can be captured for the experiments. They are delivered in bunches, at about one bunch per minute, with about 10 million antiprotons per bunch. In order to conduct an experiment, scientists need to capture about 10 million antiprotons.

AD diagram
Inside the AD.

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