Do atoms and anti-atoms obey the same laws of physics?

ALPHA physicists have recently succeeded in trapping anti-atoms for the first time. Being able to hold on to the simplest atoms of antimatter is an important step towards the collaboration’s ultimate goal: precision spectroscopic comparison of hydrogen and antihydrogen. The question they are seeking to answer: do atoms and anti-atoms obey the same laws of physics? The Standard Model says that they must.


The ALPHA Collaboration celebrates the successful results.

The ALPHA collaboration has taken it up a gear and trapped 38 atoms of antihydrogen for the first time. Antihydrogen atoms have been mass-produced at the Antiproton Decelerator (AD) since 2002, when ATHENA (ALPHA’s predecessor) and ATRAP learned how to mix clouds of antiprotons and positrons at cryogenic temperatures. However, these anti-atoms were not confined, and flew off in a few microseconds to meet their fate: annihilation with matter in the walls of the experiment.

ALPHA uses antiprotons produced at the (AD) facility, trapping and then cooling them, and making them interact with positrons accumulated from a radioactive source (22Na). Cold antihydrogen atoms are formed inside a special magnetic “bottle” that prevents them from coming into contact with surrounding matter and annihilating. From the interaction of about 107 antiprotons and 7x108 positrons, the ALPHA collaboration observed 38 annihilation events that followed the controlled release of trapped antihydrogen. The antihydrogen atoms must be produced with an energy equivalent to less than 0.5 degrees above absolute zero or else they escape the trap and annihilate. The technical challenges were to produce these very cold atoms and to be able to detect rare annihilation events against background sources such as cosmic rays or stubborn antiprotons that linger in the atom trap after the charged particles should have been kicked out. Accelerator buffs will appreciate the fact that ALPHA empties its magnetic trap of antihydrogen in 30 ms by “quenching” the trap’s superconducting magnets – and has done so several thousand times since the experiment began operation in 2006. Annihilations are detected by a three-layer silicon vertex detector similar to those used in many high energy physics experiments.

The charge-conjugation, parity, time-reversal (CPT) theorem, a crucial part of the theoretical foundation of the Standard Model, demands that hydrogen and antihydrogen have the same spectrum. Subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of this fundamental symmetry of nature. This latest breakthrough puts ALPHA a major step closer to being able to perform this test. The announcement in Nature on 17 November resulted in worldwide press coverage for ALPHA – from CNN to Al Jazeera and beyond. In the words of one reviewer: "The very fact of a proof-of-principle demonstration of wall-free confinement of even a small number of antimatter atoms has an intrinsic philosophical value".




by Jeffrey Hangst