Probing the vacuum birefringence with the blazar optical polarization

Although the Standard Model (SM) of particle physics provides a satisfactory theoretical fundament able to explain all known processes acting on elementary particles, it cannot be considered as the ultimate theory of fundamental interactions, as it does not really unify electroweak and strong interactions, does not include gravity and does not describe dark matter or dark energy. Therefore, the SM could only be considered as a low energy approximation of a more fundamental theory, describing and unifying all four fundamental interactions at the quantum level. There are several proposed extensions, aiming to complete the SM. Some of these extensions predict the existence of vacuum birefringence, which causes rotation of the polarization plane of photons of different energy. We use blazar polarization angle measurements, performed simultaneously in two optical bands (V and R) at different epochs to constrain the possible influence of the vacuum birefringence. Among the objects we monitored were S5 0716+714, S4 0954+65, B2 1308+32 (OP 313) and BL Lacertae. Our preliminary results do not show detectable vacuum birefringence but the work is ongoing.