A Very Narrow Shadow Extra Z-boson at Colliders

TL;DR

The paper introduces a minimal SM extension with a hidden U(1)_s (

Abstract

We consider the phenomenological consequences of a hidden Higgs sector extending the Standard Model (SM), in which the ``shadow Higgs'' are uncharged under the SM gauge groups. We consider a simple U(1) model with one Higgs singlet. One mechanism which sheds light on the shadow sector is the mixing between the neutral gauge boson of the SM and the additional U(1) gauge group. The mixing happens through the usual mass-mixing and also kinetic-mixing, and is the only way the ``shadow $Z$'' couples to the SM. We study in detail modifications to the electroweak precision tests (EWPTs) that the presence of such a shadow sector would bring, which in turn provide constraints on the kinetic-mixing parameter, $s_ε$, left free in our model. The shadow $Z$ production rate at the LHC and ILC depends on $s_ε$. We find that observable event rate at both facilities is possible for a reasonable range of $s_ε$ allowed by EWPTs.

Figures (7)

• Figure 1: The triple and quartic gauge vertices and the momentum labelling.
• Figure 2: The bound on $s_\epsilon$ and $M_3$ from EWPT. The upper band is the excluding region by too large a deviation from SM, $\chi_2> 2 \chi_2^{SM}$. The lower band in the parameter space gives comparable to SM results in the global fit. And the middle one is the allowed region where $(\triangle\chi_2/\chi_2^{SM})<0.01$.
• Figure 3: Branching ratio for the $Z_s$ decays as functions of $M_{Z_s}$. The curves shown here are generated with $M_{h_1^0}=120$ GeV and $c_\alpha$, the Higgs mixing angle as defined in Eq. (\ref{['Eq:HiggsMix']}), set to one.
• Figure 4: The maximal expected number of $Z_s$ events at the LHC for integrated luminosity of $100 fb^{-1}$ For a fixed $M_{Z_s}$, we have used the largest allowed $s_\epsilon$ comes from the global fit studied in previous section. The left and right panes are for $0\%$EWPT and $0\%$EWPT respectively.
• Figure 5: The cross section for $e^+e^-\rightarrow f\bar{f}$ with a $500$ GeV $Z_s$ and $s_\epsilon=0.066$. (In the main frame, the spike tips have been chopped to avoid overlapping among curves.)