When matter and antimatter collide

December 24, 2010
Figure 1: A schematic diagram of the antiproton decelerator at CERN that is used to smash antiprotons and molecular hydrogen molecules to together so that the remaining particles can be analyzed to provide insight to their interactions. Credit: 2010 Helge Knudsen

Antimatter, a substance that often features in science fiction, is routinely created at the CERN particle physics laboratory in Geneva, Switzerland, to provide us with a better understanding of atoms and molecules. Now, Japanese scientists at RIKEN, as part of a collaborative team with researchers from Denmark, Japan, the United Kingdom and Hungary, have shown that antiprotons—particles with the same mass as a proton but negatively charged—collide with molecules in a very different way from their interaction with atoms. The result sets an important benchmark for testing future atomic-collision theories.

RIKEN scientist Yasunori Yamazaki explains that to assess such collisions: “We shot the simplest negatively charged particles, slow antiprotons, at the simplest molecular target, molecular hydrogen.” Slow antiprotons are a unique probe of and because their negative charge does not attract electrons—thereby simplifying theoretical modelling. Further, slower projectile speeds mean longer-lasting, stronger interactions and avoid the need for complicated relativistic calculations.

The scientists at created antiprotons by firing a beam of high-speed protons into a block of the metal iridium. Then, in a facility known as the Antiproton Decelerator, they used magnets to focus the antiprotons before applying strong electric fields to slow them down to approximately 10% of the speed of light. Yamazaki and his colleagues trapped and cooled these antiprotons to 0.01% of the velocity of light before accelerating them one by one to the desired velocity (Fig. 1). They then slammed antiprotons into a gas of molecular deuterium—a pair of bound hydrogen atoms each with a nucleus comprising one proton and one neutron—and used sensitive equipment to detect the remnants of the collision.

Yamazaki and the team found that the likelihood of the ionization of the deuterium molecules scales linearly with the antiproton velocity. This is contrary to what is expected for the atomic target, hydrogen. “This was a big surprise, and it infers that our understanding of atomic collision dynamics, even at a qualitative level, is still in its infancy,” says Yamazaki. The team suggests that molecular targets provide a mechanism for suppressing the ionization process. As an antiproton approaches one of the protons in the molecule, the presence of the second proton shifts the orbiting electron cloud. The slower the antiproton, the more time the electron has to adjust, and hence the smaller the chance of ionization.

The team now hopes to investigate how ionization depends on the antiproton–target distance and the orientation at the moment of collision.

Explore further: Fermilab's Recycler beams take electron cooling to new heights

More information: Knudsen, H., et al. Target structure induced suppression of the ionization cross section for very low energy antiproton–hydrogen collisions. Physical Review Letters 105, 213201 (2010). Read the article here: prl.aps.org/abstract/PRL/v105/i21/e213201

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not rated yet Dec 24, 2010
The team suggests that molecular targets provide a mechanism for suppressing the ionization process. As an antiproton approaches one of the protons in the molecule, the presence of the second proton shifts the orbiting electron cloud.

Maybe the ionization process is more difficult because in molecule there are two electrons and fermions like to couple (Cooper pairs, electron orbitals, nucleus..) ?
It might be useful that there was recently also another surprising scattering result using anti-particles: http://physicswor...ws/44265
1 / 5 (1) Dec 25, 2010
I can't believe we are still here....
not rated yet Dec 25, 2010
Yes, it's kind of sad that after a century such looking basic atomic physics still brings surprises ... maybe it's because quantum mechanics we use simplifies picture too much - for example it recently occurred that photoemission isn't really instant: http://www.scienc...986/1658
I've recently found that there was models with corpuscular(localized) particles which gives much better agreement than e.g. Bohr's (especially scatterings!) - in which we remember that electron has also magnetic moment ( http://en.wikiped...ic_model ) - maybe it's time to look at them closer ...
1 / 5 (1) Dec 25, 2010
Proton and antiproton (or positron and electron) - particles with various symmetry. The antimatter can have other existential (spatially - time) basis. We can not see him. Energy of an antimatter can not be defined. That can we not we search?
not rated yet Dec 29, 2010
I am still trying to understand how matter and
antimatter can collide. I can understand when
matter collides with matter, and even when
antimatter collide with antimatter. But how can
matter collide with antimatter? It would be like
a big bang theory.

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