Millicharged Cosmology

  • May 6, 2024, 2:00 am US/Central
  • Curia II
  • Xucheng Gan, New York University

In the standard model, the minimal electric charge an elementary particle can have is 1/3. This 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. This 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, I would briefly discuss the situation when there are no mCPs but merely the dark photons in the dark sector, including the updated COBE-FIRAS constraint on the dark photon from the CMB spectral distortion and the cosmologically varying kinetic mixing scenario for the production of the dark photon dark matter.