RECODE: Reactor Neutrino CEνNS & ALP Experiment

• The paper introduces RECODE as a reactor-based experiment employing low-threshold germanium detectors for precise CEνNS measurements and ALP searches.
• Its dual-site configuration—with near and far detectors—enhances signal detection while rigorously controlling systematic uncertainties.
• Projected exposures of 10 kg·year and 100 kg·year target competitive limits in the cosmological triangle, advancing both Standard Model tests and new physics exploration.

The REactor Neutrino COherent scattering Detection Experiment (RECODE) is a reactor-based experimental program aiming to detect and exploit the process of coherent elastic neutrino–nucleus scattering (CEνNS) using low-threshold germanium detectors. RECODE's configuration, low background, and proximity to a high-power nuclear reactor are optimized both for precision Standard Model studies and for probing new physics scenarios, specifically focusing on axion and axion-like particle (ALP) searches in addition to a CEνNS physics program (Dai et al., 1 Sep 2025).

1. Experimental Configuration and Detector Design

RECODE employs two arrays of low-threshold, high-purity germanium (Ge) detectors, each array comprising 5 kg Ge mass. The "near" detector is installed at 11 m from the core of a 3.4 GW thermal power reactor at the Sanmen Nuclear Power Plant and is housed inside the reactor containment vessel. The "far" detector sits at 22 m outside the containment. Both arrays feature sub-keV thresholds, advanced shielding, and a dual-site layout. The near–far arrangement allows for both an enhanced signal flux (in the near array) and strong control of systematics (relative far–near measurement).

Two exposure scenarios are considered:

• Baseline: 10 kg·year (with background ≈ 2 counts/(keV kg day), cpkkd)
• Upgrade: 100 kg·year (background reduced to 0.2 cpkkd)

This dual-exposure approach enables exploration of physics reach as a function of detector mass, run time, and background rejection capability.

2. Axion and ALP Search Strategy

RECODE is explicitly designed to search for sub-MeV axions and ALPs, motivated by their hypothetical role as solutions to the strong CP problem and as dark matter candidates. In a reactor environment, the intense gamma-ray flux facilitates ALP production through Primakoff processes (γ+N→a+N, where a is the ALP, N a nucleon) and via Compton-like electron couplings. The focus is on two couplings:

• ALP-photon coupling: $g_{a\gamma}$
• ALP-electron coupling: $g_{ae}$

The relevant interaction Lagrangian is

$$\mathcal{L}_\text{int} = -\frac{1}{4}g_{a\gamma} a F_{\mu\nu} \tilde{F}^{\mu\nu} - g_{ae} a\, \bar{\psi}_e \gamma_5 \psi_e$$

where $F_{\mu\nu}$ and $\tilde{F}^{\mu\nu}$ are the electromagnetic field tensor and its dual, and $\psi_e$ is the electron field.

RECODE's near and far detector configuration enables the suppression of systematics and backgrounds through relative comparison of measured event rates, enhancing robustness of new physics searches, especially for rare ALP signals.

3. Sensitivity and Projected Limits in the Cosmological Triangle

The experiment is optimized to probe the "cosmological triangle" region, defined as a region in the ALP parameter space (ALP mass $m_a$ between 0.3 and 0.9 MeV, and ALP-photon coupling $g_{a\gamma}$ between $1.3 \times 10^{-5}$ and $9.1 \times 10^{-5}$ GeV$^{-1}$), previously inaccessible to laboratory searches. Sensitivity analyses demonstrate that with a 10 kg·year exposure at 2 cpkkd background, RECODE attains projected limits on $g_{a\gamma}$ that are competitive with the best beam-dump results, and the upgraded 100 kg·year, 0.2 cpkkd configuration will fully cover the cosmological triangle.

Projected exclusion plots (e.g., Fig. 4 in (Dai et al., 1 Sep 2025)) show RECODE achieving world-leading bounds in both $g_{a\gamma}$ and $g_{ae}$ in the targeted mass range, surpassing both stellar cooling and beam-dump experiment exclusions in the sub-MeV ALP window.

Exposure	Background (cpkkd)	Key Sensitivity Achievements
10 kg·year	2	Competitive to best $g_{a\gamma}$ limits
100 kg·year	0.2	Full coverage of cosmological triangle

4. Production Mechanisms and Signal Modeling

ALP production arises dominantly via the Primakoff process given the high photon flux of the reactor, sensitive to $g_{a\gamma}$, and by Compton-like processes (sensitive to $g_{ae}$). Production and detection cross sections are computed for photo-conversion and Compton processes, and sensitivity calculations integrate over the expected energy spectra from these processes at both near and far sites.

Each detected event must exceed the energy threshold governed by detector response and relevant background, while selection criteria are tuned to isolate a signal compatible with theoretical ALP (or axion) spectra and spatial profiles. Both prompt ALP decay signatures (e.g., $a \to \gamma\gamma$) and secondary processes (inverse Primakoff, interaction with electrons) are considered in modeling the experimental response.

5. Systematic Control and Background Mitigation

The experimental design leverages the near–far comparison to control for correlated systematic uncertainties and reactor-sourced backgrounds. By placing one detector inside the containment (shielded by reactor infrastructure) and another outside at twice the distance, environmental and uncorrelated backgrounds (e.g., cosmic rays, ambient neutrons/gammas) are subtracted by comparing the two sets of data.

A background rate of 2 cpkkd is assumed for baseline, with prospects to lower this to 0.2 cpkkd using advanced shielding and potentially through pulse-shape discrimination or event topology cuts in the detector arrays.

6. Broader Theoretical Implications

A discovery or exclusion in the cosmological triangle by RECODE would have significant consequences for models of dark matter, early-universe cosmology, and extensions of the Standard Model invoking light pseudoscalars. Detection of ALPs in the targeted mass-coupling domain would have implications for the strong CP problem (Peccei–Quinn axion hypothesis), string axion models, and would affect interpretations of astrophysical gamma-ray propagation.

Simultaneous sensitivity to both $g_{a\gamma}$ and $g_{ae}$ allows discrimination between different ALP and axion scenarios. The dual-detector geometry provides redundancy and robustness for potential discoveries.

7. Prospects and Outlook for RECODE and Reactor Physics

The RECODE experiment demonstrates the feasibility and physics return of reactor-based, low-threshold Ge array experiments for beyond–Standard Model searches, complementing and surpassing accelerator-, beam-dump-, and astrophysically-based approaches in the relevant parameter spaces. Its approach in deploying a modest-mass, low-background, and spatially resolved detector array is broadly applicable to a range of precision and new-physics studies in the reactor environment.

The planned upgrade to 100 kg·year exposure with 0.2 cpkkd background will enable world-leading coverage, with the potential to exclude (or discover) ALPs in the cosmological triangle, directly testing previously unconstrained fundamental physics scenarios (Dai et al., 1 Sep 2025).