Now coming into action for physics is CERN’s new Antiproton Decelerator (AD), opening another chapter of CERN’s tradition of physics with antiprotons. With the AD, the focus switches from exploiting beams of antiprotons to capturing the precious nuclear antiparticles.
When CERN’s low-energy antiproton ring (LEAR) was closed in 1996 after more than 10 years of operation, it had supplied 1.3 x 1014 antiprotons – enough to supply about 10 000 particles to everyone on the planet, but representing a theoretical accumulation of only 0.2 ng of antimatter.
Although LEAR slowed down the particle beams supplied by CERN’s antiproton factory from 26 GeV by a factor of about 10 (itself no mean feat), its antiprotons were nevertheless still moving extremely fast. A particle with 100 MeV momentum corresponds to a temperature of billions of degrees.
Of all LEAR’s antiprotons, just a few were privileged to be selected and eventually cooled down to temperatures approaching absolute zero. The techniques learned in this work opened up substantial economies for antiparticles – probably one of the rarest, and therefore most expensive, commodities in the world.
Cooling antiprotons is a tricky business. They quickly annihilate with ordinary matter such as liquid helium, the conventional ultra-refrigeration medium. Instead, antiprotons have to be supercooled by a gas of electrons (negatively charged antiprotons can peacefully coexist with electrons).
In this way the TRAP Bonn-Harvard-Seoul collaboration was able to stack several thousand ultracold antiprotons at a time. Antiprotons cooled to such a low energy by the electrons were locked in a shallow trap using electric and magnetic fields to contain the valuable antiparticles. Meanwhile a large electromagnetic well was opened alongside to receive a fresh batch of antiprotons, which were then similarly cooled. The energies of the individual antiparticles were then just one ten-millionth of what they were in LEAR.
Interesting antiproton physics thus became feasible using a less ambitious antiproton source. This is the motivation behind CERN’s new AD, which supplies antiprotons to several experiments – ATRAP (son of TRAP at LEAR), ATHENA and ASACUSA.
One ultimate physics objective at LEAR was to isolate a lone antiproton and study it carefully. Gradually reducing the electromagnetic “depth” of its snare, the TRAP team spilled out excess antiparticles until just a single antiproton survived.
Like any other captive electrically charged particle, an antiproton orbits in a magnetic field – the principle of the cyclotron. Comparing the frequencies of this rotation for an antiproton and a proton gives a direct comparison of the masses of the particle and its antiparticle.
The TRAP team at LEAR was able to ascertain that the proton and antiproton masses are equal with increasing precision, eventually to just one part in 10 billion. Making a measurement to such astonishing accuracy is equivalent to fixing the position of an obect on the surface of the Earth to within a few millimetres.
This is by far (a factor of a million) the most incisive comparison yet of proton and antiproton properties. According to the fundamental theorems of physics, a particle and an antiparticle should be exactly equal and opposite so that their scalar quantities, like mass, are the same, but quantum numbers, like electron charge, should have opposite signs.
The major objective of ATRAP and ATHENA at the new AD is to synthesize and study antihydrogen – the simplest electrically neutral atoms of antimatter, each made up of a positron orbiting a lone antiproton.
Antihydrogen was first produced by experiment PS210 at LEAR in 1995. Synthesizing atomic antimatter was a major achievement, but no measurements were made – the antihydrogen was too hot and dissociated quickly into its component positrons and antiprotons.
Using electromagnetic traps, ATRAP and ATHENA aim to collect supercold antihydrogen that can be stored for further study. Comparing the properties of this antihydrogen with hydrogen under the same conditions will provide a much more stringent test of whether matter and antimatter behave in exactly the same way.
ASACUSA uses antiprotons for collision and annihilation studies, particularly to form exotic atoms, in which the negatively charged antiproton is captured in a target atom, replacing the electron of everyday atoms.
