Search for WIMP Dark-Matter Inelastic Interactions using a Lead Target

A 270 live-day search for inelastic (indirect) scattering of WIMP dark matter with baryonic matter using a 300 kg Pb target and 64-element neutron multiplicity detector system (NMDS) at 583 m.w.e was undertaken at the CUPP underground facility in Finland. The study resulted in a detection limit of 3 neutral candidate events each producing ~140 neutrons in a single point interaction yielding a nucleon cross-section limit of <5.5E(-43)cm^-2 . The complete disintegration of the Pb nucleus in a central weak interaction annihilation with WIMP DM is the key signature of the indirect inelastic event. The equivalent direct (elastic) cross-section limit of <7.6E(-45)cm^-2 , based on the Compton wavelength of 8 GeV DM, is near the current detection limit for massive recoil detectors (LUX, XENON1T, PandaX-II, 2017). NMDS events induced by muons in the target were found to be within +/-15% agreement with measurements and predictions of previous underground studies at CUPP (2005) and Mei and Hime (2006) at several underground laboratories.