PASSAT at future neutrino experiments: Hybrid beam-dump-helioscope facilities to probe light axionlike particles

There are broadly three channels to probe axionlike particles (ALPs) produced in the laboratory: through their subsequent decay to Standard Model (SM) particles, their scattering with SM particles, or their subsequent conversion to photons. Decay and scattering are the most commonly explored channels in beam-dump type experiments, while conversion has typically been utilized by light-shining-through-wall (LSW) experiments. A new class of experiments, dubbed PASSAT (particle accelerator helioscopes for slim axionlike-particle detection), has been proposed to make use of the ALP-to-photon conversion in a novel way: ALPs, after being produced in a beam-dump setup, turn into photons in a magnetic field placed near the source. It has been shown that such hybrid beam-dump-helioscope experiments can probe regions of parameter space that have not been investigated by other laboratory-based experiments, hence providing complementary information; in particular, they probe a fundamentally different region than decay or LSW experiments. We propose the implementation of PASSAT in future neutrino experiments, taking a DUNE-like experiment as an example. We demonstrate that the magnetic field in the planned DUNE multipurpose detector is already capable of probing the ALP-photon coupling down to $g_{a\gamma \gamma }\sim \mathrm{few}\times {10}^{-5}{\mathrm{GeV}}^{-1}$ for ALP masses $m_a\lesssim 10\mathrm{eV}$. The implementation of a CAST or BabyIAXO-like magnet would improve the sensitivity down to $g_{a\gamma \gamma }\sim {10}^{-6}{\mathrm{GeV}}^{-1}$.

Figure 1

A schematic description of the main concept of PASSAT, which is a hybrid of the beam-dump experiment for production of ALP via the Primakoff effect (left) and the helioscope experiment for detection of ALP through the $a\to \gamma$ conversion (right).

Figure 2

A schematic layout of PASSAT implementation at a typical neutrino oscillation experiment. Scheme I of PASSAT consisting of a proton beam, a neutrino target, and a near detector is possible, if the near detector involves a magnetic field. Scheme II of PASSAT replaces the near detector part by a dedicated magnetic field exerted by external magnets. Here we show the possibility that the magnets are placed downstream from the decay pipe. The external magnets may accompany a photon detector (rear side) and/or shielding material (front side), depending on the detailed experimental design.

Figure 3

Energy spectra of 1-eV ALPs produced per year at the target. Only selected are ALPs which would travel toward the three benchmark detector systems, Scheme I with the MPD (green) and Scheme II with the magnets of CAST (blue) or BabyIAXO (red). The couplings in the legend are the values at which sensitivity arises (see Fig. 4). The fluctuations at higher $E_a$ are due to low statistics.

Figure 4

Expected sensitivity reaches that can be achieved by an implementation of the PASSAT idea in a DUNE-like experiment according to Scheme I with the MPD (green) and Scheme II with the magnets of CAST (blue) or BabyIAXO (red), in the plane of ALP mass $m_a$ and the associated photon coupling $g_{a\gamma \gamma }$. The limits are estimated at 90% C.L., under the assumptions of 100 (negligible) background events for Scheme I (Scheme II) with an exposure of $1.1\times {10}^{21}\mathrm{POT}/\mathrm{year}\times 7\mathrm{years}$. The shaded regions represent the existing constraints summarized in Refs. [71, 72]: the gray regions are excluded by existing laboratory-produced ALP searches, while those in light yellow show the (potentially avoidable) exclusion limits from existing astrophysical ALP searches. For comparison purposes, we also show the future sensitivity reaches expected at the gaseous argon near-detector of DUNE in the decay channel (orange dashed line) [37] and at reactor neutrino experiments MINER (purple dashed line), CONNIE (green dashed line), and CONUS (cyan dashed line), in the scattering (horizontal dashed lines) and decay channels [40]. The final-stage ALPS-II expectation (ALPS-IIc) [20] is also shown by the dashed gray line.

Figure 5

A configuration of an incoming photon, an outgoing ALP, and a detector (green shaded area). The photon is assumed to emerge at (0,0,0). The unprimed angles are defined with respect to the horizontal beam axis, while the primed angles are defined with respect to the incoming photon direction.