In the current study, Xu considered a scenario where the dark matter sector is composed of two particle species in the Universe. One is a non-relativistic particle species called heavy dark matter (HDM φ) , with mass of O(TeV), that is between 1 TeV and 100 TeV and the other is much lighter particle species called light dark matter (LDM), which is a relativistic product due to the decay of HDM. HDM comprises the bulk of present-day dark matter (DM) .
The lifetime for the decay of HDMs to standard model (SM) particles is strongly constrained (τ ≥ O(1026 − 1029)s) by diffuse gamma and neutrino observations. However, in Xu’s scenario, since HDM does not decay to standard model (SM) particles, constraints relevant in his case are only those based on cosmology. Then, in the present work, Xu considered an assumption that HDMs only decay to LDMs. So, he took τ_HDM to be between 1017 s (the age of the Universe) and 1026 s.
According to him, when the HDM wind sweeps through the Earth, those non-relativistic particles could collide with the matter in the Earth and lose their kinetic energy. Then they can be captured by the Earth’s gravity and enter the earth core. After a long period of accumulation, the HDMs inside the Earth can begin to decay into LDMs at an appreciable rate and these particles from the earth core pass through the Earth and ice. Meanwhile they interact with matter of the Earth and ice. Cherenkov Photons are produced by cascades due to LDM interaction with nuclei within the IceCube (see Fig. 1). A small part of these photons will be detected by the IceCube detector. Since these LDMs interact with the nuclei in the ice and this is very similar to DIS of neutrino interaction with nuclei via a neutral current, its secondary particles develop into a cascade at IceCube. In his paper, Xu assumed that all LDMs and neutrinos, detected by IceCube, interact with the ice within its volume.
He have taken Z’ portal dark matter model for LDMs to interact with nuclei via a neutral current interaction mediated by a heavy gauge boson Z’. With the different lifetimes of decay of HDMs and Z’ masses, the event rates of LDMs, measured by IceCube, are evaluated in the energy range between 1 TeV and 100 TeV.
According to the IceCube data and results, the upper limit for LDM fluxes is estimated at 90% C.L. at IceCube (shown in Fig 4 below) and it is possible that LDMs are directly detected in the energy range between O(1TeV) and O(10TeV) at IceCube with mZ′ ≲ 500GeV and τφ ≲ 1021 s. Thus, this might prove whether there exist HDMs in the Universe.
“Certainly, the results are based on the assumption that the efficiencies for measuring LDMs and neutrinos are set to be 100%. Besides, sufficient exposure will be used to determine whether it is possible that LDMs are detected at a km³ neutrino telescope.”— told Xu, author of the study
Featured image: The IceCube Neutrino Observatory at the South Pole. Credit: IceCube/National Science Foundation
Reference: Ye Xu, “Measurement of TeV dark particles due to decay of heavy dark matter in the earth core at IceCube”, pp. 1-10, ArXiv, 2021. https://arxiv.org/abs/2004.09497
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