Axionlike particle searches in short-baseline liquid scintillator neutrino detectors

TL;DR

This work investigates axionlike particle (ALP) searches in short-baseline, large liquid scintillator detectors such as JUNO-TAO and CLOUD, focusing on couplings to photons ($g_{a\gamma\gamma}$) and electrons ($g_{aee}$). By analyzing ALP production in reactors via Primakoff-like and Compton-like processes and their detection through inverse processes and decays, the authors identify distinctive prompt-delayed photon and recoil signatures that could enable strong background discrimination. Sensitivity projections show that, with efficient background suppression, these detectors can probe previously unexplored regions, including the cosmological triangle and MeV-mass ALPs, with particularly notable gains for electron-coupled ALPs. The study highlights the potential of enhanced time-resolution (e.g., in CLOUD) to substantially broaden discovery reach and complements existing ALP searches across astrophysical and laboratory constraints.

Abstract

Short-baseline reactor neutrino experiments using large organic liquid scintillator detectors provide an experimentally rich environment for precise neutrino physics. Neutrino detection is done through inverse beta decay and relies on prompt and delayed signals, which enable powerful background discrimination. In addition to their neutrino program, they offer an ideal experimental environment for other physics searches. Here we discuss the case of axionlike particles (ALPs) produced by either Primakoff-like or Compton-like processes. Their detection relies on the corresponding inverse processes, axio-electric absorption and ALP decays to photon or electron pairs. Assuming experimental parameter values broadly representative of JUNO-TAO or CLOUD we show that scattering ALP processes involve a prompt photon signal component followed by a delayed photon signal about 5 ns after. We point out that if these signals can be resolved, this might allow for efficient ALP signal discrimination against radioactive background. Likewise, coincident events from ALP decays and scintillation light followed by Auger electrons from axio-electric absorption might as well allow for background discrimination. We determine sensitivities in both the nuclear and electron channels using as a benchmark case an experimental setup resembling that of the CLOUD or JUNO-TAO detectors. Our findings demonstrate that with efficient background discrimination these type of technology has the capability to test regions in parameter space not yet explored. In particular, the cosmological triangle can be fully tested and regions of MeV ALP masses with ALP-electron couplings of the order of $10^{-8}$ can be entirely explored.

Figures (13)

• Figure 1: Photon energy probability distribution functions (PDFs) in pair annihilation for representative electron antineutrino energies (all of them substantially above threshold) for $\gamma_1$ (left graph) and $\gamma_2$ (right graph). These results are based on random sampling of the kinematic relations shown in Eqs. (\ref{['eq:Egamma_IBD']}) and (\ref{['eq:Egamma_prime_IBD']}). For the PDFs $10^4$ scattering events and 20 energy bins have been assumed. The latter assuming a $\Delta E_\gamma\simeq 8\,$keV photon energy resolution. Back-to-back photons cluster towards low and high energy bins.
• Figure 2: Estimated light yield---produced in a single nuclear recoil---as a function of nuclear recoil energy. We use 4500 photoelectrons per MeV (9 photons per keV) as a proxy and a PDE of 50% JUNO:2020ijm.
• Figure 3: Event rate per minute as well as number of PEs in detection through inverse Primakoff scattering as a function of nuclear recoil energy. A point in parameter space within the cosmological triangle has been used (see text for more details): $m_a=0.3\,\text{MeV}$ and $g_{a\gamma\gamma}=9.5\times 10^{-3}\,\text{GeV}^{-1}$.
• Figure 4: Normalized Gaussian function (by setting $I_0$ = 1 in Eq. \ref{['eq:normalized_intensity']}) with $\mu=430\,$nm as a representative photon emission spectrum for the JUNO-TAO cocktail.
• Figure 5: Left graph: Probability distribution function as a function of the delayed photon wavelength for nuclear recoil scintillation light emission spectrum at different recoil energies (delayed photon signal). Right graph: Photon kinematic spectrum probability distribution function as a function of the prompt photon wavelength for light emission in inverse Primakoff processes $a + N\to \gamma + N$ (prompt photon signal).