Axion detection through resonant photon-photon collisions

We investigate the prospect of an alternative laboratory-based search for the coupling of axions and axionlike particles to photons. Here, the collision of two laser beams resonantly produces axions, and a signal photon is detected after magnetic reconversion, as in light-shining-through-walls (LSW) experiments. Conventional searches, such as LSW or anomalous birefringence measurements, are most sensitive to axion masses for which substantial coherence can be achieved; this is usually well below optical energies. We find that using currently available high-power laser facilities, the bounds that can be achieved by our approach outperform traditional LSW at axion masses between 0.5–6 eV, set by the optical laser frequencies and collision angle. These bounds can be further improved through coherent scattering off laser substructures, probing axion-photon couplings down to $g_{a\gamma \gamma }\sim {10}^{-8}{\mathrm{GeV}}^{-1}$, comparable with existing CAST bounds. Assuming a day long measurement per angular step, the QCD axion band can be reached.

Figure 1

A diagram of the experimental setup. Photons from two lasers collide in an interaction region, producing any hypothetical axions (either scalar or pseudoscalar). These pass through an intervening wall, and then reconvert into photons in the presence of a magnetic field. These photons are the signal detected.

Figure 2

A standing wave is formed in the circled region by passing a single laser pulse through a 50-50 beam splitter, each part of which then reflects off an off-axis parabolic mirror, as outlined in [26].

Figure 3

The orange curve plots the angular step size ${\delta }_{12}$ against the chosen angle ${\theta }_{12}$ for each shot. Here the spectral width is $10/\tau$. On the left axis, the black curve indicates the central mass probed for a given ${\theta }_{12}$ and ${\omega }_1^0={\omega }_2^0=1.55\mathrm{eV}$, the shaded region indicates the width $\pm \delta m_a$. Assuming a minimum possible step size ${\delta }_{12}\gtrsim 1°$, the full mass range can be scanned in $\sim 30$ shots. This step size imposes a lower bound on ${\theta }_{12}\gtrsim 0.4\mathrm{rad}$ corresponding to $m_a\gtrsim 0.6\mathrm{eV}$.

Figure 4

Exclusion plot for axion parameter space. The light blue region shows existing bounds from the OSQAR experiment [32]; the orange region is excluded by PVLAS [19]; the dashed blue line depicts CAST constraints [14]; the lower horizontal dashed line comes from stellar cooling lifetimes [33] and the upper from solar Bragg diffraction experiments [16]. The dotted black and dark blue lines correspond to our proposal performed at $\omega =1.55\mathrm{eV}$ and the first harmonic, respectively, with ELI parameters and no substructure coherence. The black line shows the possible bounds using standing wave coherence and the red line indicates the same parameters but 1 day of shots per angular step instead of a single shot. The region above the line is excluded in each case. The QCD axion region indicates particular theoretical predictions for where the axion might be, given dark matter abundances [34].