Thesis_HC_04242023.pdf

  Present WIMP Dark Matter search strategies are mainly focused on possible direct detection through elastic or inelastic scatterings on atomic nuclei, or with electrons. This approach
becomes insensitive to M(DM) < 10 GeV. Indirect DM detection refers to the search for DM-DM or DM-M annihilation, decay debris from DM particles, or other particle production,
resulting in detectable species. 

   New physics processes, initiated by cosmic ray or dark matter interactions may be observable in underground indirect search experiments by excess high multiplicity neutron
production in nuclear targets. Even for M(DM) < 10 GeV, DM-M interaction is capable of
producing large signals, >200 neutrons if the energy is deposited in a Pb target.

  The NMDS-II detector, located at an underground laboratory within the Pyhasälmi
complex metal mine in central Finland, collated data for 6504 ± 1 hours at 583 m.w.e.
and for 1440 ± 1 hours at 1166 m.w.e.. The detector system consists of a 30 cm cube
Pb-target surrounded by 60 He-3 proportional tubes and a two layer Geiger Counter muon
detection system. The lead target is used to interact with potential dark matter particles, and
neutron numbers are measured with He-3 tubes. The neutron event multiplicity production is
compared to Geant4 simulations, starting with the well measured absolute muon momentum
and angular distribution flux rate at sea level, then propagating the muon flux through rock
while preserving the momentum-angular correlation to a depth 4m above the the detector at
the two depth locations. The muon flux modeling is compared to the uncorrelated Miyake
model at each depth as verification of the muon propagation simulation. Finally, the Geant4
fully simulates the passage of the muon and its induced showers through a model universe
10000 m^2 x 12 m depth, and the simulated response of the detector to the calculated muon
flux, is compared with the data.
 

  The Geant4 prediction and the observed data neutron event multiplicity distributions
have matching power law shapes, k × n^(-p), and do not have exponential shapes. For the
data collected at 583 m.w.e., p=2.36±0.10 with χ2/DoF = 0.76 and for the simulation
p=2.34±0.01 with χ2/DoF = 1.05. At 1166 m.w.e., p=2.29±0.007 for the simulation with  χ2/DoF = 1.16. And for the data the collection with only 6 detected events above multiplicity 5, yields p=2.50 ± 0.35 predicted by the Maximum Likelihood Estimatation method. 

  The DM acceptance as a function of mass is found using a proton-Pb spallation model.
The dark matter mass is assumed to be equal to the proton kinetic energy and to interact
uniformly over the volume of the lead target. The number of excess events is found to be
-6.1 ± 5.1, that is no excess events are observed. The upper limit with 90% confidence
level is then found assuming 2.3 events. The Poisson estimation then yielding search limits
1.1×10^(-44) cm^(-2) for 10 GeV deposited energy, 1.9×10^(-45) cm^(-2) at 1 GeV and 3.0×10^(-45) cm^(-2) for 500 MeV deposited energy and no acceptance at 100 MeV.

    An indirect dark matter search was conducted based on DM-M interactions depositing
energy in a Pb-target allowing DM masses to be probed in a region 100 MeV < M(DM) <
10 GeV not accessible to direct dark matter searches. Limits are placed on DM-M energy
deposition independent of the DM-M interaction.