title: Millicharged dark matter in quantum gravity and string theory.
abstract: We examine the millicharged dark matter scenario from a string theory perspective. In this scenario, kinetic and mass mixings of the photon with extra U(1) bosons are claimed to give rise to small electric charges, carried by dark matter particles, whose values are determined by continuous parameters of the theory. This seems to contradict folk theorems of quantum gravity that forbid the existence of irrational charges in theories with a single massless gauge field. By considering the underlying structure of the U(1) mass matrix that appears in type II string compactifications, we show that millicharges arise exclusively through kinetic mixing, and require the existence of at least two exactly massless gauge bosons.
authors.affiliation: Department of Physics, University of Wisconsin, 1150 University Avenue, Madison, Wisconsin 53706, USA and Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Gary
authors.initials: G
authors.lastname: Shiu
authors.affiliation: Department of Physics, University of Wisconsin, 1150 University Avenue, Madison, Wisconsin 53706, USA and Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Pablo
authors.initials: P
authors.lastname: Soler
authors.affiliation: Department of Physics, University of Wisconsin, 1150 University Avenue, Madison, Wisconsin 53706, USA and Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Fang
authors.initials: F
authors.lastname: Ye