Foraging for dark matter in large volume liquid scintillator neutrino detectors with multiscatter events

We show that dark matter with a per-nucleon scattering cross section $\gtrsim {10}^{-28}{\mathrm{cm}}^2$ could be discovered by liquid scintillator neutrino detectors like Borexino, $\mathrm{SNO}+$, and JUNO. Due to the large dark matter fluxes admitted, these detectors could find dark matter with masses up to $10^{21}\mathrm{GeV}$, surpassing the mass sensitivity of current direct detection experiments (such as XENON1T and PICO) by over 2 orders of magnitude. We derive the spin-independent and spin-dependent cross section sensitivity of these detectors using existing selection triggers, and we propose an improved trigger program that enhances this sensitivity by 2 orders of magnitude. We interpret these sensitivities in terms of three dark matter scenarios: (1) effective contact operators for scattering, (2) QCD-charged dark matter, and (3) a recently proposed model of Planck-mass baryon-charged dark matter. We calculate the flux attenuation of dark matter at these detectors due to the earth overburden, taking into account the earth’s density profile and elemental composition, as well as nuclear spins.

Figure 1

Sensitivities, as derived in Sec. 2, of liquid scintillator detectors to multiscatter-inducing dark matter undergoing spin-independent (top) or spin-dependent (bottom) scattering, illustrated with Borexino. The mass reach is determined by the total flux of dark matter transiting the detector, while the detection threshold cross section is set by the minimum rate of energy deposition required for the photomultiplier tubes to tell significantly between signal scintillation and dark counts. Due to similar dimensions and detection technology, $\mathrm{SNO}+$ would have very similar sensitivities; due to larger dimensions, JUNO would reach greater dark matter masses. The vertical green line is the maximum mass reach of XENON1T after 1 year [1]. The black lines indicate upper bounds (derived in Sec. 4) on the cross section from the slowing down of dark matter by Earth’s atmosphere and rock. The dotted-dashed (dashed) lines indicate the halving (tenthing) of total dark matter flux by this earth overburden. Also shown are spin-independent scattering bounds from molecular gas cloud cooling [6], Skylab [7, 8, 9], and searches for dark matter tracks in ancient mica [10, 11]. Similar bounds should apply to spin-dependent scattering; this would require a reanalysis of these constraints accounting for the fraction of spin-containing nuclei in gas clouds, Skylab detectors, and ancient mica. The horizontal brown line denotes the maximum cross section imposed by unitarity at zero angular momentum; this cross section could be higher for higher $\ell$ modes.

Figure 2

Left: Geometric schematic of a dark matter particle traveling along a straight path to a dark matter detector through the Earth’s atmosphere and rock. This diagram is not to scale. Right: Contours in the $\cos \theta \text{−}{m}_{\chi }$ plane, of the spin-independent per-nucleon cross sections (in ${\mathrm{cm}}^2$) required for the Earth overburden to slow down dark matter below detector thresholds. This illustrative calculation assumes an isotropic dark matter distribution with a uniform $\text{speed}=220\mathrm{km}/\mathrm{s}$.