Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field

We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is $(3.9\pm 0.5)\times {10}^{-12}\mathrm{rad}/\mathrm{\text{pass}}$, at 5 T with 44 000 passes through a 1 m long magnet, with $\lambda =1064\mathrm{nm}$. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.