Resonantly Enhanced Axion-Photon Regeneration

Abstract

We point out that photon regeneration-experiments that search for the axion, or axion-like particles, may be resonantly enhanced by employing matched Fabry-Perot optical cavities encompassing both the axion production and conversion magnetic field regions. Compared to a simple photon regeneration experiment, which uses the laser in a single-pass geometry, this technique can result in a gain in rate of order ${\cal F}^2$, where ${\cal F}$ is the finesse of the cavities. This gain could feasibly be $10^{(10-12)}$, corresponding to an improvement in sensitivity in the axion-photon coupling, $g_{aγγ}$ , of order ${\cal F}^{1/2} \sim 10^{(2.5-3)}$, permitting a practical purely laboratory search to probe axion-photon couplings not previously excluded by stellar evolution limits, or solar axion searches.

Figures (2)

• Figure 1: (color) (a) Simple photon regeneration. (b) Resonant photon regeneration, employing matched Fabry-Perot cavities. The overall envelope schematically shown by the thin dashed lines indicates the important condition that the axion wave, and thus the Fabry-Perot mode, in the conversion magnet must follow that of the hypothetically unimpeded photon wave from the Fabry-Perot mode in the production magnet. Between the laser and the cavity is the injection optics (IO) which manages mode matching of the laser to the cavity, imposes RF sidebands for reflection locking of the laser to the cavity, and provides isolation for the laser. The photon detectors are also preceded by matching and beam-steering optics. Not shown at all is the electro-optical system required to lock the two cavities together in frequency.
• Figure 2: (color) Current exclusion plot of mass and photon coupling $(m_a, g_{a\gamma\gamma})$ for the axion, and the 5 $\sigma$ discovery potential for the resonantly enhanced photon regeneration experiment calculated for a configuration of 4 + 4 LHC dipole magnets. The existing exclusion limits indicated on the plot include the cavity microwave experiments assuming axions saturate the dark matter halo density Brad03, the best direct solar axion search (CAST collaboration) Ziou05, the Horizontal Branch Star limit Raff96, and previous laser experiments Ruos92. The red error ellipse indicates the positive result of the PVLAS collaboration, if interpreted as a light pseudoscalar, based on measurements of magnetically-induced dichroism of the vacuum Zava06. For the estimated limits of resonantly enhanced photon regeneration presented here, the solid curve corresponds to the $\uparrow\uparrow\uparrow\uparrow$ configuration of the individual LHC dipole magnets in both the production and regeneration strings; the dotted curve indicates the extension of the mass reach by additionally running in the $\uparrow\uparrow\downarrow\downarrow$, and $\uparrow\downarrow\uparrow\downarrow$ configurations.