[Paper Review] Illuminating WISPs with Photons

Paola Arias, Andreas Ringwald|DESY Publication Database (PUBDB) (Deutsches Elektronen-Synchrotron)|Oct 10, 2011

Dark Matter and Cosmic Phenomena5 references3 citations

TL;DR

This paper reviews the theoretical motivation for Weakly Interacting Slim Particles (WISPs), including axions, axion-like particles (ALPs), and hidden photons, and evaluates photon regeneration experiments—specifically light-shining-through-walls (LSW) setups—as a leading method to detect them. It demonstrates that next-generation LSW experiments, optimized with resonant cavities and improved laser power, could achieve sensitivities surpassing astrophysical probes and access previously unexplored parameter space for ALPs and hidden photons.

ABSTRACT

Physics beyond the Standard Model naturally gives rise to very light and weakly interacting particles, dubbed WISPs (Weakly Interacting Slim Particles). A prime example is the axion, that has eluded experimental detection for more than thirty years. In this talk we will review some of the strongly motivated candidates for such particles, the observational hints for them and the present status of searches with photon regeneration experiments, as well as possible future improvements.

Motivation & Objective

• To review the theoretical motivation for WISPs, including axions solving the strong CP problem and axion-like particles from string theory.
• To summarize astrophysical and cosmological hints suggesting the existence of WISPs, such as anomalies in X-ray emissions and CMB data.
• To analyze the current status and limitations of light-shining-through-walls (LSW) experiments as a primary laboratory probe for WISPs.
• To propose and evaluate technical improvements—such as resonant cavity enhancement and buffer gas use—for increasing LSW sensitivity.
• To project the discovery potential of next-generation LSW experiments, showing they could surpass existing bounds and probe new regions of parameter space for ALPs and hidden photons.

Proposed method

• Uses quantum field theory to model WISP-photon mixing via effective Lagrangians, particularly the axion-photon coupling $\mathcal{L} \propto a F_{\mu u} \tilde{F}^{\mu\nu}$.
• Applies the effective Lagrangian to derive the photon-to-WISP conversion probability in external magnetic fields, using the formula $P_{\gamma \to \chi} \propto g_{\gamma\chi}^2 B^2 L^2 \sin^2(\Delta m^2 L / 4E)$.
• Optimizes LSW experiment sensitivity by maximizing power build-up in optical cavities using mirrors, with optimal cavity length $Z_{\text{opt}} \approx 89.2\,\text{m}$ for $a=20\,\text{mm}$, $\lambda=1064\,\text{nm}$.
• Incorporates buffer gas to tune refractive index and shift sensitivity dips caused by oscillatory behavior in conversion probability.
• Evaluates performance using existing dipole magnets (HERA, LHC, Tevatron), accounting for gaps between magnets and their impact on sensitivity.
• Projects future sensitivity using benchmark parameters from Table 1, assuming improved laser power, cavity locking, and detector efficiency.

Experimental results

Research questions

• RQ1 What are the strongest theoretical motivations for the existence of WISPs, particularly axions and axion-like particles?
• RQ2 How do astrophysical and cosmological observations suggest the presence of WISPs, such as in X-ray emissions or CMB anomalies?
• RQ3 What are the key technical limitations of current light-shining-through-walls experiments in detecting WISPs?
• RQ4 How can resonant cavity enhancement and buffer gas techniques improve the sensitivity of photon regeneration experiments?
• RQ5 To what extent can next-generation LSW experiments surpass existing astrophysical and terrestrial bounds in probing WISP parameter space?

Key findings

• Next-generation LSW experiments, with optimized cavity length and power build-up, could achieve a sensitivity limit of $g_{\gamma\chi} \sim 10^{-11}\,\text{GeV}^{-1}$ for ALPs, surpassing current astrophysical bounds.
• The optimal cavity length for maximum conversion probability is $Z_{\text{opt}} = 89.2\,\text{m}$ for a 20 mm beam radius and 1064 nm laser, corresponding to $\delta_0^{\text{clip}} / (\delta_0 + \delta_2) = 0.177$.
• The use of buffer gas shifts sensitivity dips in the conversion probability, effectively filling gaps in the sensitivity landscape caused by oscillatory behavior.
• With improved laser power, cavity locking, and detector performance, the next phase of ALPS could probe parameter space previously inaccessible to solar and astrophysical searches.
• For hidden photons, next-gen LSW experiments are expected to surpass previous limits and test the hidden CMB hypothesis, which suggests hidden photons could contribute to the CMB energy density.
• Laboratory-based LSW experiments are now the most sensitive purely laboratory probes for WISPs, offering model-independent constraints due to their controlled environment.