Probing Axionlike Particles and the Axiverse with Superconducting Radio-Frequency Cavities

Abstract

Axion-like particles (ALPs) with couplings to electromagnetism have long been postulated as extensions to the Standard Model. String theory predicts an "axiverse" of many light axions, some of which may make up the dark matter in the universe and/or solve the strong CP problem. We propose a new experiment using superconducting radiofrequency (SRF) cavities which is sensitive to light ALPs independent of their contribution to the cosmic dark matter density. Off-shell ALPs will source cubic nonlinearities in Maxwell's equations, such that if a SRF cavity is pumped at frequencies ω1 and ω2, in the presence of ALPs there will be power in modes with frequencies 2ω1 ± ω2. Our setup is similar in spirit to light-shining-through-walls (LSW) experiments, but because the pump field itself effectively converts the ALP back to photons inside a single cavity, our sensitivity scales differently with the strength of the external fields, allowing for superior reach as compared to experiments like OSQAR while utilizing current technology. Furthermore, a welldefined program of increasing sensitivity has a guaranteed physics result: the first observation of the Euler-Heisenberg term of low-energy QED at energies below the electron mass. We discuss how the ALP contribution may be separated from the QED contribution by a suitable choice of pump modes and cavity geometry, and conclude by describing the ultimate sensitivity of our proposed program of experiments to ALPs.

Dark matter targets for axionlike particle searches

“…However, since the BEAST experiment is a topic of current discussion in the literature [118][119][120][121] we do not show them on our plot. We also do not show other limits from other as-yetunpublished proposals, such as [122][123][124][125][126].…”

Towards string theory expectations for photon couplings to axionlike particles

“…Both of the techniques mentioned above should see improvements in future years. The International Axion Observatory (IAXO), a next-generation helioscope, should reach g aγγ = 2 × 10 −12 GeV −1 [3], with a prototype taking data within the next few years, while next-generation superconducting radiofrequency cavities hope to reach a similar limit on the same timescale [4].…”

“…This applies not only to the usual parity even operator F µν F µν , but also the parity odd operator, requiring the existence of a coupling L ⊃ − 1 4 g aγγ aF µνF µν (1) in the effective Lagrangian, whereF µν = µνρσ F ρσ ., The pseudoscalar a is an axion-like particle (ALP), which is not necessarily the QCD axion, and it may be a nontrivial linear combination of the hundreds or thousands of ALPs expected from studies of string vacua. Numerous ground-based experiments [1][2][3][4] and satellite observations [5][6][7][8][9][10][11] place constraints on ALP-photon interactions, probing widely different regimes for the axion mass m a and coupling strength g aγγ . Existing limits are already remarkable, within a few orders of magnitude of GUT scale decay constants f aγγ ≡ g −1 aγγ .…”

Axion dark matter detection by superconducting resonant frequency conversion

“…Different SRF setups have also been considered for production and detection of light, non-DM axions[37,38]. Another, distinct idea is the proposal of refs [39,40].…”