Dark <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math> at the International Linear Collider
Yik Chuen San, Maxim Perelstein, Philip Tañedo
Abstract
A dark $Z$ is a massive Abelian gauge boson which is coupled to the Standard Model through both kinetic and mass mixing with the electroweak sector. We study the phenomenology of the dark $Z$ at an energy-frontier ${e}^{+}{e}^{\ensuremath{-}}$ collider, such as the proposed International Linear Collider (ILC). We show that precision electroweak constraints and the current bounds from the hadron colliders allow a dark $Z$ that is kinematically accessible at the ILC with 250 or 500 GeV center-of-mass energy. Further, the reach of the ILC searches for a dark $Z$ significantly exceeds the expected reach of the high luminosity LHC. If a signal consistent with a dark-$Z$ is discovered, it would motivate a dedicated run of the ILC at the center-of-mass energy matching the dark-$Z$ mass. We demonstrate that a short one month run at design luminosity could measure the dark-$Z$ chiral couplings to fermions with percent precision. This measurement can be used to discriminate between competing theoretical models of the resonance: for example, the dark $Z$ can be distinguished from a dark photon with purely kinetic mixing.