The Dark Z and Charged Higgs Decay

If there is an additional U(1) symmetry under which Standard Model particles are singlets, then there can be mixing between the additional gauge boson and the hypercharge gauge boson. This can lead to a very light, but weakly coupled "dark" gauge boson, the $Z_d$, with a mass of O(1) GeV. If the $Z_d$ gets its mass entirely from a Higgs singlet, it is called a dark photon; whereas if there is a second Higgs doublet, it is called a dark Z. We look at charged Higgs boson decays in the dark Z model. If the charged Higgs mass is between 90 GeV and 170 GeV, then its dominant two-body decays are $H^+ \rightarrow \tau\ \nu\ $ and $H^+ \rightarrow W^+ Z_d$. The former is suppressed by the small tau mass-squared and by $1/\tan^2 \beta$, whereas the latter is suppressed by a loop and a small mixing. We find that for much of the allowed parameter-space $H^+ \rightarrow W^+ Z_d$ will dominate. Since $Z_d$ decays to a lepton pair much of the time, this leads (using Drell-Yan pair production) to $pp \rightarrow H^+H^- \rightarrow W^+ W^-$ plus two highly collimated "lepton jets". Current bounds on lepton jets from ATLAS come close to constraining some of parameter space, and a dedicated search using the 2012 data would substantially improve the coverage.