Sensitivity to sub-GeV dark matter in forthcoming spallation-source neutrino experiments

Abstract

Sub-GeV thermal dark matter weakly interacting with the Standard Model through vector-portal mediators provides a well-motivated and predictive framework that remains challenging to probe with conventional direct detection experiments. Motivated by the rapid development of neutrino facilities based on spallation neutron sources, we study the sensitivity of future coherent elastic neutrino-nucleus scattering experiments to light dark matter produced in neutral pion decays. We consider scalar dark matter interactions mediated by two different vector portals, a generic dark photon and a baryophilic vector mediator. The neutral pion yield is calculated through a GEANT4 simulation and the results are compared with those obtained with the Sandford-Wang parametrization. We show that predictions based on either approach do not produce significant differences. Our results demonstrate that upcoming low-threshold neutrino detectors at the European Spallation Source (ESS), the Japan Proton Accelerator Research Complex (J-PARC) and the China Spallation Neutron Source (CSNS) will test regions in parameter space not yet explored. We point out that these facilities will strengthen the global experimental program searching for secluded sectors.

Figures (13)

• Figure 1: Left graph: Two-dimensional normalized neutral pion PDF in the $T_{\pi^0}-\cos\theta_{\pi^0}$ plane derived from the $\pi^0$ four momentum sample obtained through a GEANT4 simulation. The sample accounts for about $3\times 10^5$ pion events and applies for the ESS forecasted experimental configuration (see Tab. \ref{['tab:dim_Geant4']}) Baxter:2019mcx. Right graph: Two-dimensional normalized neutral pion PDF in the $T_{\pi^0}-\cos\theta_{\pi^0}$ plane obtained from the S & W parametrization. The result has been derived by constructing a four-momentum sample of $5\times 10^5$ events by rejection sampling based on Eq. \ref{['eq:pi0_DDCS_T_costh']}. The difference in the samples size does not affect the conclusion. It has been chosen that way only to improve resolution.
• Figure 2: Left panel: One-dimensional, normalized $\pi^0$ kinetic-energy PDFs obtained from the GEANT4 simulation and from the Sanford–Wang analytical parametrization. Right panel: Same comparison, but for the $\cos\theta_{\pi^0}$ distribution. In both cases, visible differences arise from subleading physics processes included in GEANT4 but not captured by the analytical parametrization.
• Figure 3: Schematic illustration of the sub-GeV DM production (through a vector portal), and detection pipeline.
• Figure 4: Left graph: Normalized $\pi^0$ kinetic energy PDFs for the CSNS, ESS and J-PARC. Right graph: Same as the left panel, but for the $\pi^0$ polar-angle PDFs, defined with respect to the proton beam direction (see Fig. \ref{['fig:DM:dist']}). Results were obtained from GEANT4 simulations of proton collisions against a fixed target of tungsten (ESS and CSNS) and mercury (J-PARC) and with proton kinetic energies as given in Tab. \ref{['tab:dim_Geant4']}. For the simulation the FTFP_BERT physics list has been used Allison:2016lfl.
• Figure 5: Left (right) graphs: Expected DM total energy (angular) normalized PDFs at production. Results are for ESS (top row), J-PARC (middle row), and CSNS (bottom row). Each PDF is calculated for different dark photon masses, from lighter to heavier (lighter to darker color) assuming $m_{A'} = 3m_{\phi}$. Results follow from GEANT4 simulations with $10^6$ POT (see text for further details).