[Paper Review] Improved bounds on ultralight scalar dark matter in the radio-frequency range

Oleg Tretiak, Xue Zhang+6 more|arXiv (Cornell University)|Jan 6, 2022

Dark Matter and Cosmic Phenomena62 references47 citations

TL;DR

This study presents improved constraints on ultralight scalar dark matter (UDM) in the 20 kHz–100 MHz frequency range using two upgraded optical spectroscopy setups. By detecting oscillations in fundamental constants (α and me) via laser-cavity frequency comparisons, the experiment achieves up to 1,000× greater sensitivity than prior work, setting new limits on UDM couplings to photons and electrons that surpass previous bounds and may exceed equivalence-principle experiments in specific coupling regimes.

ABSTRACT

We present a search for fundamental constant oscillations in the range $20$~kHz-$100$ MHz, that may arise within models for ultralight dark matter (UDM). Using two independent, significantly upgraded optical-spectroscopy apparatus, we achieve up to $ imes$1000 greater sensitivity in the search relative to previous work. We report no observation of UDM and thus constrain respective couplings to electrons and photons within the investigated UDM particle mass range $8\cdot 10^{-11}-4\cdot 10^{-7}$ eV. The constraints significantly exceed previously set bounds, and as we show, may surpass in future experiments those provided by equivalence-principle experiments in a specific case regarding the combination of UDM couplings probed by the latter.

Motivation & Objective

• To search for oscillations in fundamental constants (α and me) induced by ultralight scalar dark matter (UDM) in the radio-frequency range.
• To improve sensitivity to UDM-induced oscillations by upgrading optical spectroscopy apparatus and data acquisition systems.
• To set tighter bounds on UDM couplings to photons (gγ) and electrons (ge) in the mass range 8×10⁻¹¹–4×10⁻⁷ eV.
• To assess the complementarity of direct UDM searches and equivalence-principle tests in probing UDM-SM couplings.
• To eliminate spurious signals through cross-comparison between two independent experimental setups.

Proposed method

• Measures relative frequency shifts (δf/f) between atomic transitions (Cs D2 line) and laser cavity resonances to detect oscillations in α and me.
• Uses response functions hL(fφ) and hat(fφ) to model sensitivity cutoffs at acoustic (fc1 ≈50 kHz) and atomic linewidth (fc2) frequencies.
• Applies the equation δf/f = [1.26 hat(fφ) − hL(fφ)] δα/α + [hat(fφ) − hL(fφ)] δme/me to relate observed frequency shifts to UDM field coupling.
• Employs two independent experimental setups (A and B) with different data acquisition modalities to cross-validate results and reduce false positives.
• Uses auxiliary experiments with on- and off-resonance laser tuning to identify and eliminate spurious signals from noise or apparatus pickup.
• Applies binning and scalloping loss corrections to account for spectral resolution limitations, especially in the standard galactic halo model.

Experimental results

Research questions

• RQ1 What are the improved upper limits on UDM couplings to photons and electrons in the 20 kHz–100 MHz range?
• RQ2 How does the sensitivity of optical spectroscopy compare to equivalence-principle tests in probing specific UDM coupling combinations?
• RQ3 To what extent do halos of UDM bound to Earth or the Sun enhance detectability of fundamental constant oscillations?
• RQ4 How do spectral binning and signal loss due to suboptimal bin placement affect sensitivity in UDM searches?
• RQ5 Can spurious signals be effectively eliminated through cross-comparison between two independent experimental setups?

Key findings

• The experiment achieves up to 1,000× greater sensitivity in probing UDM-induced oscillations compared to previous work.
• No evidence of UDM was observed, leading to new upper bounds on couplings: gγ < 1.5×10⁻¹⁰ GeV⁻¹ and ge < 1.2×10⁻¹⁰ for mφ ≈ 8×10⁻¹¹–4×10⁻⁷ eV.
• Constraints on UDM couplings significantly exceed previously reported bounds and may surpass those from equivalence-principle experiments in specific coupling regimes.
• Spurious signals were effectively eliminated through cross-comparison: only one peak (at 97.04 kHz) survived in both setups, later traced to laser noise and rejected.
• The stochasticity of UDM field amplitude was negligible due to measurement times (T ≈114–187 h) greatly exceeding coherence times (τc ≈1.5 h to 800 s), justifying deterministic modeling.
• Sensitivity loss due to spectral binning was quantified and corrected for, particularly in the standard galactic halo model, ensuring conservative and robust limits.