Testing helicity-dependent γγ → γγ scattering in the region of MeV

Light-by-light scatterings contain rich information on photon coupling to virtual and real particle states. In the context of quantum electrodynamics (QED), photons can couple to a virtual e(+)e(-) pair. Photons may also couple to known resonance states in the context of quantum chromodyanmics and electroweak dynamics in higher energy domains and possibly couple to unknown resonance states beyond the standard model. The perturbative QED calculations manifestly predict a maximized cross section at the MeV scale; however, no example of exact real-photon-real-photon scattering has yet been observed. Hence, we propose direct measurement with the maximized cross section at the center-of-mass system energy of 1-2MeV to establish a firm footing at the MeV scale. Given current state of the art high power lasers, helicity-dependent elastic scattering may be observed at a reasonable rate, if a photon-photon collider exploiting gamma-rays generated by the inverse nonlinear Compton process with electrons delivered from laserplasma accelerators (LPA) are properly designed. We show that such verification is feasible in a table-top scale collider, which may be an unprecedented breakthrough in particle accelerators for basic physics research in contrast to energy frontier colliders.