axion and the strong CP problem

Most physical processes look the same whether you run time forward or backward — a billiard collision filmed in reverse still obeys the rules. The strong force could, in principle, contain a built-in term that breaks this kind of symmetry, which would, for example, give the neutron a tiny lopsided electric charge distribution. Experiments have looked very hard and found that lopsidedness to be essentially zero. The strong CP problem is the puzzle of why that symmetry-breaking term, which nature was free to make sizeable, is instead fantastically close to nothing.

The leading solution promotes the offending fixed number into a dynamical field that can relax on its own to the value that switches the symmetry violation off. Quantum theory then says that field must have its own particle — light, electrically neutral, and barely interacting — christened the axion. Think of a ball settling to the bottom of a smooth bowl: wherever it starts, it rolls to the point where the strong CP term vanishes, and the small wobbles of the ball around that bottom are the axion.

The axion is doubly attractive because, besides curing the strong CP problem, a very light axion produced in the early universe could itself be the dark matter — a quite different candidate from the heavy WIMP. Experiments now hunt for axions by trying to convert them into faint microwave photons inside strong magnetic fields, or by looking for axions streaming from the Sun. None has been detected yet, but as WIMP searches come up empty, the axion has moved from a sideline idea to one of the most actively pursued candidates for physics beyond the Standard Model.