PARIS — Scientists at CERN said they had trapped dozens of hydrogen “antimatter” atoms, a technical feat that boosts research into one of the great puzzles of particle physics.

Under a theory expounded in 1931 by the eccentric British physicist Paul Dirac, when energy transforms into matter, it produces a particle and its mirror image — called an anti-particle — which holds the opposite electrical charge.

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When particles and anti-particles collide, they annihilate each other in a small flash of energy.

If everything were equal at the birth of the cosmos, matter and anti-matter would have existed in the same quantities. The observable Universe would have had no chance of coming into being, as these opposing particles would have wiped each other out.

In reality, though, matter came to be far more dominant, and antimatter is rare.

But understanding why there is this huge imbalance presents a daunting technical challenge.

Until now, experiments have produced anti-atoms, namely of hydrogen, but only in a free state. That means they instantly collide with ordinary matter and get annihilated, making it impossible to measure them or study their structure.

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In a paper published on Wednesday by the British journal Nature, a team at the European Organization for Nuclear Research (CERN) in Geneva explain a method of snaring these so-called antihydrogen atoms.

Experiments conducted in its ALPHA laboratory found a way of using strong, complex magnetic fields and a vacuum to capture and hold the mirror-image particles apart from ordinary matter.

Thousands of antihydrogen atoms have been made in the lab, but in the most successful experiment so far, 38 have been trapped long enough — one-tenth of a second — for them to be studied.

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“For reasons that no-one yet understands, nature ruled out antimatter,” CERN physicist Jeffrey Hangst said.

“It is thus very rewarding, and a bit overwhelming, to look at the ALPHA device and know that it contains stable, neutral atoms of antimatter. This inspires us to work that much harder to see if antimatter holds some secret.”