Axions are posited to have exceedingly low mass--less than a millionth that of an electron--and are electrically neutral. They interact only very weakly with other particles, making detection difficult. But physicists predict that a tiny fraction of any photons passing through a magnetic field will change into axions. (That is how the sun is predicted to produce them.) Indeed, the Italian experiment, based at the National Laboratories of Legnaro and led by Emilio Zavattini and Giovanni Cantatore of the INFN Trieste, saw evidence for axions in the behavior of a laser beam. The beam's polarization was rotated by 10 millionths of a degree after transiting 44,000 times back and forth through an extremely strong magnetic field. Such rotation is just the fingerprint expected if some photons converted to invisible axions, or more precisely, what physicists call axionlike particles.
From its data, the PVLAS group infers the mass of the putative axions and how strongly they interact. Puzzlingly, however, the results contradict other observations and do not fit with constraints deduced from astrophysics. In particular, the CERN Axion Solar Telescope (CAST) ran for six months in 2003 and failed to detect any axions arriving from the sun. That result would seem to rule out a large swath of possible masses and interaction strengths, including the values seen by PVLAS. Furthermore, if axions interact as strongly as PVLAS indicates, they should be produced in large quantities in stars, causing stars to grow old much faster than they are known to.
Such considerations "put the bar pretty high before one can accept the PVLAS results," says axion expert Pierre Sikivie of the University of Florida and CERN. On the other hand, he adds, "these people are very competent, and they have worked on it a long time." By all accounts, the PVLAS researchers have been careful to exclude effects that could be confounding the data; moreover, in work that is not yet published, the group has also obtained consistent results with a different laser. Some theorists have already proposed ways to reconcile the PVLAS results with those of CAST and other astrophysical limits.
Only further experiments will determine the truth. If the PVLAS results are correct, then axions should appear in an experiment known as "shining a light through a wall." The idea is this: A laser beam is sent through a strong magnetic field at an opaque wall. Some of the photons in the beam are converted to axions, which pass through the wall. On the other side, another magnetic field induces a small fraction of the axions to convert back to photons, which can be detected. Such an experiment, using a large, strong magnet and sensitive photon detectors, would convincingly confirm (or refute) the PVLAS results in a matter of minutes. Research groups, including the PVLAS team, are gearing up to perform that experiment. By the end of the year the axion could be a firm addition to the particle menagerie--or back on physicists' most-wanted list.