Multiple Axions in Laboratory Experiments

TL;DR

This work develops a general formalism for axion-photon mixing in the presence of an arbitrary number N of axions, capturing coherence and interference across a spectrum of masses m_n and couplings c_n. It analyzes how such multi-axion dynamics modify laboratory searches, including light-shining-through-a-wall, helioscope, and haloscope experiments, revealing regimes where signals are enhanced (∝ N^2) or suppressed due to decoherence or destructive interference. The study introduces benchmark models (Stringy Axions, KK Maxions, KK ALPs) to illustrate the phenomenology and demonstrates that parameter variation (magnetic-region lengths, wall thickness, buffer gas) can reveal the underlying axion multiplicity, enabling spectral reconstruction through Fourier analysis in LSW setups and resonant/medium effects in helioscopes. The results establish that multi-axion scenarios can qualitatively alter experimental signatures and provide diagnostic strategies to probe axion multiplicity, thereby expanding the reach and interpretive power of existing and upcoming laboratory searches for beyond-Standard-Model pseudoscalars.

Abstract

Axions and axion-like particles generically appear in extensions of the Standard Model. While many searches assume only a single axion species, there may exist a whole spectrum of multiple such fields. We develop general formulas for axion-photon oscillations in the presence of multiple axions and analyze the implications for experimental searches, including light-shining-through-a-wall experiments, helioscopes and haloscopes. We demonstrate that axion multiplicity can qualitatively alter observational signatures, particularly through coherence and interference effects. Multiple axions can not only enhance signals compared to single axion scenarios, but also suppress them. We show that variations of experimental parameters and searches allow identifying contributions of multiple axions and obtaining information about their properties.

Figures (8)

• Figure 1: Schematic illustration of an LSW experiment. Photons from a laser beam (red) propagate through a magnetic field region of length $L_1$, where they can convert into a superposition of axion states through the axion-photon interaction. The axion state traverses an opaque to photons wall of thickness $L_w$. Subsequently, in a second magnetic field region of length $L_2$, axions can reconvert into photons that are detectable.
• Figure 2: Representative spectra of axion masses and associated scales in the benchmark scenarios considered in this work, including an axiverse-inspired log-normal distribution ("Stringy Axions", blue circles), a KK-Maxion-type spectrum ("KK Maxions", orange squares) and a KK-type spectrum with a tree-level mass ("KK ALPs", green diamonds), considering $N = 100$ axions. See Tab. \ref{['table-parameters']} for parameter details.
• Figure 3: Expected number of regenerated photon events $n_{\rm ev}$ for an observation time of $\Delta t=40$ hr as a function of the length of the symmetric magnetic field region $L$ for the benchmark models introduced in Sec. \ref{['sec:models']}, assuming the ALPS II experimental configuration. The nominal ALPS II design baseline $L=106$ m is highlighted by a red dashed line.
• Figure 4: Comparison between the photon signal obtained using Eqs. \ref{['eq:signal']} and \ref{['eq:probability-formula']} for a measurement time of $40\, {\rm{hours}}$ per wall-length point scanned and its FFT for different sampling rates (SR) of the wall length in the KK Maxion benchmark of Tab. \ref{['table-parameters']}, scanning wall thicknesses up to $L_w = 40 \,\mathrm{m}$ (all other parameters as in the standard ALPS II setting). For clarity, only the three heaviest (out of a maximum of 100) axion modes are shown. The expected positions of the Dirac delta peaks are indicated by dashed black lines in the lower panel. Owing to the specific structure of the mass spectrum, the feature at $k \simeq 0.2~\mathrm{m}^{-1}$ consists of two overlapping peaks, represented by a thicker dashed line.
• Figure 5: Differential photon flux at a terrestrial detector for the stringy axion benchmark model of Tab. \ref{['table-parameters']}, comparing different treatments of coherence effects. Neglect of coherence entirely (blue), inclusion of coherence for axion pairs with $\Delta m^2 < 0.1 \times 2\pi(\omega/L_{\rm solar})$ (orange), and inclusion of coherence for axion pairs with $\Delta m^2 < 2\pi (\omega/L_{\rm solar})$ (green) are considered. The results are shown for a regeneration length $L_\text{mag} = 20~\mathrm{m}$ and a magnetic field strength $B = 9~\mathrm{T}$. We have taken $L_{\rm solar} =R_{90\%}= 2 \times 10^{8}~\mathrm{m}$ as a representative value. The orange and the green dashed line are essentially superimposed on top of each other.