As far as crowning achievements in physics go, discovering a fundamental particle of nature is hard to beat. Why else, for example, would physicists be spending billions of Euros on the Large Hadron Collider at CERN to look for the famous Higgs boson and other new particles? But such discoveries are rare, which is why an anomalous signal reported by physicists at the National Institute for Nuclear Physics in Legnaro, Italy, last year is generating such excitement. As renowned theorist Andreas Ringwald put it recently in Science magazine: “If you believe the signal to be real, then the interpretation is a new particle.”
The signal in question is a very slight rotation in the plane of polarization of a laser beam that has passed through a strong magnetic field. It was spotted by Giovanni Cantatore and colleagues working on the PVLAS experiment, which was dreamed up several years ago by group leader Emilio Zavattini to study “quantum vacuum effects”. These effects arise because the vacuum is not empty space, as classical physics would have you believe, but is instead filled with fleeting particle–antiparticle pairs that can make it behave as if it is a dielectric. In the presence of a magnetic field, for instance, interactions between photons in a light beam and those in the vacuum can cause the beam to be refracted in a manner that depends on its polarization – an effect known as birefringence.
While the rotation seen by Cantatore and co-workers may yet turn out to have a mundane origin, it is best explained if a photon from the PVLAS laser combines with a photon in the vacuum to produce a light, weakly interacting neutral particle called an axion. Quickly decaying back into two photons, this intermediate particle would effectively remove photons with certain polarizations from the beam and therefore cause the overall polarization plane of the laser to be rotated by a measurable amount.
If this interpretation is correct, Cantatore’s team may have solved one of the biggest mysteries in science: determining the nature of “dark matter”, the mysterious substance thought to make up 25% of the universe yet which has escaped decades of dedicated searches. In an attempt to confirm this finding, physicists are planning further experiments. However, because the rotation seen by the PVLAS experiment is so miniscule, ideally we need to look beyond Earth-bound experiments.
In the November issue of Physics World, Giovanni Bignami and Arnaud Dupays discuss how our best bet for an independent crosscheck of the Legnaro result lies in the enormous magnetic fields generated by a pair of rotating neutron stars some 2000 light-years away.