CompEx II: A Pathway in Search of BSM Physics using Compton Scattering

Constancy and anisotropy of vacuum refractive index serves as a strong way to probe the predictions of theories beyond the standard model (BSM), especially those that predict breaking of local Lorentz and CPT symmetries. For photons of energies in the ranges $9-46$ MeV using a $1.16$ GeV electron beam, a constraint on vacuum refractive index, $(n-1) < 1.4 \times 10^{-8}$ was imposed using the Compton polarimeter in Hall - C of Jefferson Lab (JLab). Absence of sidereal modulation of the vacuum refractive index was then used to constrain the Minimal Standard Model Extension (MSME) parameters of $\sqrt{\kappa_X^2 + \kappa_Y^2} < 8.6 \times 10^{-10}$. These preliminary set of measurements will be followed up by measurements using the $11$ GeV electron beam at JLab with a sensitivity better than current leading constraints by a factor of $4-8$. Furthermore, quantum gravity models predict crystalline nature of space at Planck scales which may manifest as vacuum birefringence that can be probed by Compton scattering using circularly polarized light. We show that future facilities such as the ILC provide tantalizingly interesting possibilities.