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November 25, 2010

Antihydrogen: Trapped at CERN


The concept of antimatter is not really new, it was predicted for the very first time in 1931 by Dirac. One year later, Carl D. Anderson discovered the positron. The antiproton was experimentally confirmed in 1955 by physicists Emilio Segrè and Owen Chamberlain, and the antineutron was discovered in proton–proton collisions at Bevatron by Bruce Cork in 1956. We must wait almost 40 years for produce 9 antihydrogen atoms at CERN through the LEAR (Low Energy Antiproton Ring), where antiprotons (produced in a particle accelerator), were shot at xenon clusters. Unfortunately this experiment produce highly energetic/warm antihydrogen atoms, which were unsuitable for detailed study. In addition the probability for producing antihydrogen from one antiproton was only about 10−19, so the method was forgetted as an alternative to produce large amount of antihydrogen atoms.
Following the primary goal of produce "cold" (a few thousand kelvin) antihydrogen atoms, in 2002 and 2004, experiments were successfully carried out by the ATRAP and ATHENA collaborations at CERN. The last need, trapping magnetically and for a non negligible time the anti-atoms, was achieved by the ALPHA collaboration (also at CERN). In November 17th, the ALPHA team announced that they had magnetically trapped 38 antihydrogen atoms for at least 172 ms.

Untrapped antihydrogen atoms annihilating on the inner surface of the ALPHA trap. These are measured by the ALPHA annihilation detector. The events are concentrated at the electrode radius of about 22.3 mm. The coordinates are defined in the Nature article, Figure 1b

Even if ALPHA experiment is considered only as a proof of principle, it's clear that it will lead to more robust trapping techniques and set the bases for a new era of fundamental physics tests. Two major subjects should be studied : the gravitational behaviour of antimatter (indications are that gravity should act on antihydrogen just as it acts on hydrogen) and the matter-antimatter symmetry (according to fundamental physics theories, antihydrogen should have the same spectrum as ordinary hydrogen). Subjecting these kind of anti-atoms to rigorous spectroscopic examination would constitute a compelling, model-independent test of the charge conjugation/parity/time reversal (CPT) theorem. In order to be able for doing these studies, the goal of 100 anti-atoms trapped on a timescale of seconds shall be reached.
What's about antimatter real-world applications ? Even if the techniques for trapping magnetically the antihydrogen atoms seems to be identified, we are far far away of concrete applications. Actually, the efficiency of these methods is very weak (only 38 anti-atoms were cold enough and slow enough to be confined from the interaction of about 107 antiprotons and 7 x 108 positrons) and the incarceration time is very short. As said Cliff Surko, a physicist at the University of California, the harnessing of antimatter as an energy source remains a far-fetched idea. "The problem is that ... it takes so much more energy to make than you get out that it's pretty inefficient," he said. "And you have to go to great lengths to confine it for a long time." "Even if the efficiency of the trapping process is increased, it is fundamentally limited by the amount of antiprotons that can be generated. Therefore I do not see applications in terms of new energy sources or weapons." So Star Trek-style propulsion system shall be wait.

Animation of how antihydrogen is trapped (voice of Prof. Joels Fajans from UC Berkeley):


Trapped antihydrogen, by G. B. Andresen, M. D. Ashkezari, M. Baquero-Ruiz, and others. Nature Advanced Online Publication, November 17 (2010) (doi:10.1038/nature09610)
Trapped Antihydrogen


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