Millicharged Cosmology

by Dr Xucheng Gan (DESY)

Friday 27 Dec 2024, 11:00 → 12:00 Asia/Shanghai

S423

In the standard model, an elementary particle's minimal electric charge is 1/3. This fact raises a fundamental question: Do particles with smaller charges exist? Termed as the ‘millicharged particle’ (mCP), this new type of particle is divided into two categories: the Pure mCP and the mCP resulting from the kinetic mixing. Even though both types of mCPs are inevitably produced during the early universe and affect the cosmological evolution, their specific behaviors differ qualitatively: The pure mCPs are easily overproduced. The mCPs from the kinetic mixing annihilate into dark photons and contribute to Neff. Moreover, the mCPs from the kinetic mixing contribute to the dark plasmon mass. Such a phenomenon induces the resonant oscillation from the CMB photons to the dark photons and causes the CMB spectral distortion, which enables us to test the mCPs. Additionally, the existence of nonrelativistic mCPs also alleviates the dark photon’s COBE-FIRAS bound, which motivates the ongoing dark SRF experiment. Apart from testing the mCPs, we can also detect the low reheating temperature that suppresses mCP production. In the end, we will briefly discuss the situation when there are no mCPs but merely the dark photons in the dark sector, focusing on the updated COBE-FIRAS constraint on the dark photon from the CMB spectral distortion.

Bio:
Xucheng Gan is a postdoctoral researcher in the DESY theory group. He earned his bachelor's degree from Fudan University, followed by a master's degree at the University of Chicago under the guidance of Professor Lian-Tao Wang. He then completed his PhD at New York University under the mentorship of Professor Joshua T. Ruderman. His research focuses on light dark sectors, with particular interest in dark photons, millicharged particles, axions, ultralight dark matter, and their signatures in cosmology, astroparticle physics, and terrestrial experiments.