New bounds on light millicharged particles from the tip of the red-giant branch

Stellar energy loss is a sensitive probe of light, weakly coupled dark sectors, including ones containing millicharged particles (MCPs). The emission of MCPs can affect stellar evolution, and therefore can alter the observed properties of stellar populations. In this work, we improve upon the accuracy of existing stellar limits on MCPs by self-consistently modelling (1) the MCP emission rate, accounting for all relevant in-medium effects and production channels, and (2) the evolution of stellar interiors (including backreactions from MCP emission) using the MESA stellar evolution code. We find MCP emission leads to significant brightening of the tip of the red-giant branch. Based on photometric observations of 15 globular clusters whose bolometric magnitudes are inferred using parallaxes from Gaia astrometry, we obtain robust bounds on the existence of MCPs with masses below 100 keV.Comment: 12 pages, 8 figure

Electron recoils from terrestrial upscattering of inelastic dark matter

The growing interest in the interactions between dark matter particles and electrons has received a further boost by the observation of an excess in electron recoil events in the XENON1T experiment. Of particular interest are dark matter models in which the scattering process is inelastic, such that the ground state can upscatter into an excited state. The subsequent exothermic downscattering of such excited states on electrons can lead to observable signals in direct detection experiments and gives a good fit to the XENON1T excess. In this work, we study terrestrial upscattering, i.e., inelastic scattering of dark matter particles on nuclei in the Earth, as a plausible origin of such excited states. Using both analytical and Monte Carlo methods, we obtain detailed predictions of their density and velocity distribution. These results enable us to explore the time dependence of the flux of excited states resulting from the rotation of the Earth. For the case of XENON1T, we find the resulting daily modulation of the electron recoil signal to be at the level of 10% with a strong dependence on the dark matter mass

Early-Universe Model Building

Theoretical investigations into the evolution of the early universe are an essential part of particle physics that allow us to identify viable extensions to the Standard Model as well as motivated parameter space that can be probed by various experiments and observations. In this white paper, we review particle physics models of the early universe. First, we outline various models that explain two essential ingredients of the early universe (dark matter and baryon asymmetry) and those that seek to address current observational anomalies. We then discuss dynamics of the early universe in models of neutrino masses, axions, and several solutions to the electroweak hierarchy problem. Finally, we review solutions to naturalness problems of the Standard Model that employ cosmological dynamics.Comment: contribution to Snowmass 202