Scrutinizing a massless dark photon: Basis independence
Jun-Xing Pan (School of Physics and Information Engineering, Shanxi Normal University, Linfen, China); Min He (Tsung-Dao Lee Institute, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China); Xiao-Gang He (Physics Division, National Center for Theoretical Sciences, Hsinchu, Taiwan, Department of Physics, National Taiwan University, Taipei, Taiwan, School of Physics and Information Engineering, Shanxi Normal University, Linfen, China, Tsung-Dao Lee Institute, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China); Gang Li (Department of Physics, National Taiwan University, Taipei, Taiwan)
A new gauge boson field X can have renormalizable kinetic mixing with the standard model (SM) gauge boson field Y. This mixing induces interactions of X with SM particles even though X starts as a dark photon without such interactions. If the is not broken, both the dark photon field X and the photon field A are massless. One cannot determine which one of them is the physical dark photon or the photon by just looking at kinetic terms in the Lagrangian. We revisit this issue and show explicitly that when all contributions are included, all physical processes do not depend on which basis is used and the kinetic mixing effects do not show up in electromagnetic and weak interactions if only SM particles are involved in the calculations. On the other hand, the kinetic mixing provides a portal for probing the dark sector beyond the SM. We update constraints on the millicharged dark sector particles from the Lamb shift and lepton measurements.
