Title: Measurement of Upward-going Milli-charged particles at the Pierre Auger Observatory

Abstract: It is assumed that superheavy dark matter particles with O(EeV) mass may decay to relativistic milli-charged particles (MCPs). The upward-going MCPs passing through the Earth could be measured by the fluorescence detectors (FD) of the Pierre Auger observatory (Auger). The massless hidden photon model was taken for MCPs to interact with nuclei, so that the numbers and fluxes of expected MCPs and neutrinos may be evaluated at the FD of Auger. Based on the assumption that no events are observed at the FD of Auger in 14 years, the corresponding upper limits on MCP fluxes were calculated at 90\% C. L.. These results indicated that MCPs could be directly detected in the secondaries' energy range O(1EeV)-O(10EeV) at the FD of Auger, when $3\times10^{-13}\lesssim\epsilon^2\lesssim 10^{-3}$. And a new region of 10 MeV < $m_{MCP}$ < 1 PeV and $10^{-7}$ $\lesssim$ $\epsilon$ $\lesssim$ $3\times10^{-2}$ is ruled out in the $m_{MCP}$-$\epsilon$ plane with 14 years of Auger data.