Progress toward a direct experimental detection of γγ interactions

A fundamental quantum electrodynamics prediction which has so far not yet been confirmed experimentally by a direct observation in a laboratory experiment is γγ interactions. Such a direct observation requires a scenario where both the beam and the target are made of bosons, while so far experiments have exploited matter particles as beam and/or target. A consequence of the existence of γγ interactions is that vacuum features magnetic birefringence as a macroscopic property. Magnetic Birefringence of Vacuum (MBV) is due to interactions of beam photons with virtual photons of a magnetic field. These interactions are mediated by loops of electron-positron and (with extremely weaker effects) by loops of muons and loops of hadrons, and could possibly be mediated by hypothetical very light particles with a coupling to two photons. Experimentation to detect MBV not only has started much later than experiments that have provided magnificent validations of QED, like g-2 and Lamb shift, but has not yet matched the performances necessary to observe MBV. A summary of the main properties and performances of experiments aiming at MBV detection is given with focus on recent results of the PVLAS experiment. The time evolution of the missing factor which monitors the capability of an experiment to observe MBV is reported. This evolution points to MBV detection in a near future. MBV experimentation could evolve from detection to precision measurements modulo a change in scale of the experiments, if it will be possible to exploit together the peak performances achieved separately in components of different MBV experiments. Data collected with the aim of detecting MBV provide at present the best model independent limits on the coupling to two photons of (so far hypothetical) very light scalar and pseudoscalar particles.