Photon probes of the Low Energy Frontier

Giovanni Cantatore (INFN Triest)
DESY auditorium, 17:00h

Interest in low energy interactions photon-photon interactions has recently expanded after the realization that the low energy landscape is richer than previously thought. Phenomena such as photon-photon scattering, as predicted by QED, and sub-eV particle production (Axion-Like Particles, Mini-Charged Particles, chameleons, to mention a few) can be investigated using low energy (1-2 eV) photons as probes of a target vacuum region where electromagnetic energy is present, a magnetic field for instance. The properties of the emerging photon beam, typically its intensity and polarization, carry information on the physical processes which took place, much in the same fashion as scattering experiments at particle accelerators. From an experimental point of view, one carries out polarimetric measurements to detect acquired ellipticities and/or rotations of the polarization plane, or one tries to detect the faint signature of "regenerated photons". These are produced as photons oscillate into ALPs in a first magnetic field, which then propagate into a second magnetic field zone, barred to the original photons, where they oscillate back into photons. The polarimetric approach has been historically pioneered by BFRT and PVLAS, and is now being actively pursued by BMV, OSQAR and PVLAS-Phase II. The fascinating photon-regeneration scheme, poetically known as "shining light through a wall", has been proposed in 1987, and it has first been tried experimentally by BFRT in 1990. More recently, several experimental efforts have started (ALPS, GammeV, LIPPS, OSQAR and BMV) bringing innovative and clever techniques to bear in the search for ALP production. Polarization measurements will soon probably be able to directly detect QED processes. Reaching an interesting unexplored zone in the ALP parameter space is, however, currently beyond their power, even in very optimistic scenarios. Regeneration experiments, on the other hand, have the potential to extend the reach of laser experiments beyond what is possible with polarization detection schemes. With the novel "resonant regeneration" scheme one can exploit the coherence properties of the ALP and photon fields to enhance the ALP-photon conversion probability by a factor which can be as large as 1e10, or more, by using two frequency-locked Fabry-Perot optical resonators. The PVLAS-Phase II group in Trieste has just put forth a proposal to build a table-top resonant regeneration pilot apparatus. At the moment, resonant regeneration appears as the sole purely laboratory-based method capable of investigating a region of the ALP parameter space now accessible, in part, only to astrophysical observations, such as those from the CAST magnetic helioscope for ALPs. In the optimistic, though not a priori excluded, case of a positive signal one would obtain a discovery of great scientific value.

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