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Science

Physicists Trap Antimatter for First Time

Every known particle or atom of “normal” matter in the measurable universe has its own corresponding antiparticle which is in every way like the normal particle except for its charge (opposite of the normal particle) and parity (its left or right orientation).

Mathematically, such antimatter particles can be described as moving backward in time.* When matter and antimatter meet they annihilate each other, often resulting in high energy photons (gamma rays). Given this tendency, and given that we live in a universe dominated by “normal” matter, it has been a huge technological challenge to generate antimatter in a laboratory setting.

But this Promethean feat of physics was achieved in 1994 with the creation of the first antihydrogen atom by scientists at CERN (the Switzerland-based high-energy particle physics lab, also in charge of the LHC) and again by physicists here in the U.S. at Fermi Lab. But there was just one problem: the generated anti-atom only lasted for 1/40 billionth of a second before it made contact with normal matter and annihilated itself. It was too “hot” (high energy) to be contained — leading some particle physicists to experiment with “cold” antimatter. It seemed that holding on to such an exotic form of (anti) matter for any significant length of time was virtually impossible.

But recently, scientists working with the ALPHA research group at CERN have demonstrated a “proof of principle” that antimatter can be temporarily confined, if only for a few tenths of a second (which is a vastly longer time than previous experiments). Publishing their results in the November 17 edition of Nature, the team was able to generate and contain a “few dozen” antihydrogen atoms. This was determined by analyzing a shower of secondary particles, known as pions, that result when such antimatter makes contact with normal matter. This (relatively) short capture time is still long enough to afford more detailed study of the antimatter.

Over head view of the ALPHA experiment at CERN, where anti-atoms have been created and trapped (photo by Maximilien Brice)

An antihydrogen atom is composed of one antiproton (a negatively charged proton) and one antielectron (also known as a positron, which is a positively charged electron). Such positrons have been generated for many years in particle accelerators and are used in PET (positron emission tomography) scan machines. While this bottom-up building of an antihydrogen atom was achieved in the 1990’s, there remained a major obstacle to furthering research:  the anti-atom of hydrogen, which is electrically neutral, cannot be confined in the conventional manner used to contain other charged particles.

But the newly formed antimatter atom still has a magnetic moment, and this means that it can be influenced by a sufficiently powerful magnetic field. The physicists had previously developed special “superconducting magnets” that can be switched on and off almost immediately, allowing for very precise control parameters.

This breakthrough will open up a new avenue of research in high energy physics with applications that can only be imagined at this stage. It is conceivable that some advanced form of the technology, if deployed in space, could be used to protect the Earth from high energy radiation (thus becoming a form of geoengineering).

In terms of pure research, physicists hope to answer the question of why the universe is dominated by normal matter (known as baryon asymmetry), when according to some theories, there should be much more antimatter in the visible universe. Recently, massive clouds of antimatter were discovered near the galactic center of our Milky Way.

The main reference for this post: Upping the Anti: CERN Physicists Trap Antimatter Atoms for the First Time, on the Scientific American website.

* This description of matter “moving backward in time” is a consequence of the principle of time reflection symmetry.

top image: Cloud chamber photograph of the first positron ever observed (1933); Carl Anderson, American Physical Society; Public Domain

second image:  Maximilien Brice/CERN




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