Searching for exotic scalars at fusion reactors

TL;DR

This work proposes using high-neutron-flux fusion reactors to produce weakly coupled light spin-0 particles (a scalar φ and a pseudoscalar a) via exotic nuclear transitions and neutron interactions within breeding blankets. It develops two benchmark models with explicit couplings to photons, nucleons, and electrons, and derives production mechanisms through resonant neutron capture and NDA-based processes, followed by detection via deuteron dissociation in a SNO-like detector. The study provides conservative flux estimates, includes realistic reactor materials and neutron spectra, and shows that Year-long reactor-based searches can set leading constraints on dark scalar and pseudoscalar nucleon couplings, often surpassing bounds from SNO, SN1987A, and kaon decays in relevant parameter regions. It also discusses complementary methods such as magnetic conversion and outlines practical considerations for reactor design and experimental implementation. Overall, fusion reactors emerge as promising laboratories for exploring MeV-scale light NP with potential substantial impact on beyond-Standard-Model searches.

Abstract

The energy created in deuterium-tritium fusion reactors originates from a high-intensity neutron flux interacting with the reactor's inner walls. The neutron flux can also be used to produce a self-sustaining reaction by lining the walls with lithium-rich `breeding blankets', in which a fraction of neutrons interacts with lithium, creating the tritium fuel. The high-intensity neutron flux can also result in the production of dark sector particles, feebly interacting light scalars or pseudoscalars, via nuclear transitions within the breeding blanket. We estimate the potential size of such dark sector flux outside the reactor, taking into account all current constraints, and consider possible detection methods at current and future thermonuclear fusion reactors. As a by-product, we also recast the SNO axion bound for a CP even scalar. We find that year-long searches at current and future reactors can set leading constraints on dark scalar -- and dark pseudoscalar -- nucleon couplings.

Figures (9)

• Figure 1: Schematic depiction of new physics production and detection in nuclear fusion facilities.
• Figure 2: Spin-0 particle production (left) and detection (right) mechanisms.
• Figure 3: The $^7$Li, $^6$Li and $^{56}$Fe neutron capture cross sections extracted from Ref. kopecky1997atlas, with E1 (resonant M1) transition dominated rates denoted with dashed (solid) lines. See Eq. \ref{['eq:M1:nonres']} and the accompanying discussion regarding non-resonant M1 transitions. For $^7$Li the analysis regarding the separation of the E1, M1 contributions is taken from Refs. heil1998nfernando2012leadinglynn1991direct. For the Fe cross section we assume that the resonance at $E_n=14.4\keV$ is due to an M1 transition. We have no clear M1 resonance for ${}^6$Li.
• Figure 4: The cross sections for neutron scattering on $^7$Li, $^6$Li and $^{56}$Fe, taken from the EXFOR database OTUKA2014272. The neutron energy range shown is the one relevant for detection via deuterium dissociation, i.e., $E_n\geq 2.2 \eV[M]$.
• Figure 5: The incident neutron flux per energy at the first wall of the reactor as a function of the incident neutron energy, $E_n$. We show the expected neutron flux for two sample cases, taken from Ref. fleming2018fispact: ITER DT and WCLL DT, with their respective power outputs normalized to $P\simeq 2000$ MW. In the numerical analysis, we use ITER DT as the benchmark value.