Thesis_HC_04242023.pdf
Creators: name: Cao, Haichuan
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Particle physics

description: 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<br> becomes insensitive to M(DM) &lt; 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,<br> resulting in detectable species. <br> New physics processes, initiated by cosmic ray or dark matter interactions may be observable in underground indirect search experiments by excess high multiplicity neutron<br> production in nuclear targets. Even for M(DM) &lt; 10 GeV, DM-M interaction is capable of<br> producing large signals, &gt;200 neutrons if the energy is deposited in a Pb target. <br> The NMDS-II detector, located at an underground laboratory within the Pyhasälmi<br> complex metal mine in central Finland, collated data for 6504 ± 1 hours at 583 m.w.e.<br> and for 1440 ± 1 hours at 1166 m.w.e.. The detector system consists of a 30 cm cube<br> Pb-target surrounded by 60 He-3 proportional tubes and a two layer Geiger Counter muon<br> detection system. The lead target is used to interact with potential dark matter particles, and<br> neutron numbers are measured with He-3 tubes. The neutron event multiplicity production is<br> compared to Geant4 simulations, starting with the well measured absolute muon momentum<br> and angular distribution flux rate at sea level, then propagating the muon flux through rock<br> while preserving the momentum-angular correlation to a depth 4m above the the detector at<br> the two depth locations. The muon flux modeling is compared to the uncorrelated Miyake<br> model at each depth as verification of the muon propagation simulation. Finally, the Geant4<br> fully simulates the passage of the muon and its induced showers through a model universe<br> 10000 m^2 x 12 m depth, and the simulated response of the detector to the calculated muon<br> flux, is compared with the data. <br> The Geant4 prediction and the observed data neutron event multiplicity distributions<br> have matching power law shapes, k × n^(-p), and do not have exponential shapes. For the<br> data collected at 583 m.w.e., p=2.36±0.10 with χ2/DoF = 0.76 and for the simulation<br> 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. <br> The DM acceptance as a function of mass is found using a proton-Pb spallation model.<br> The dark matter mass is assumed to be equal to the proton kinetic energy and to interact<br> uniformly over the volume of the lead target. The number of excess events is found to be<br> -6.1 ± 5.1, that is no excess events are observed. The upper limit with 90% confidence<br> level is then found assuming 2.3 events. The Poisson estimation then yielding search limits<br> 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.<br> An indirect dark matter search was conducted based on DM-M interactions depositing<br> energy in a Pb-target allowing DM masses to be probed in a region 100 MeV &lt; M(DM) &lt;<br> 10 GeV not accessible to direct dark matter searches. Limits are placed on DM-M energy<br> deposition independent of the DM-M interaction. <br> <br> <br> <br>