Six-Lepton $Z^{\prime }$ Resonance at the Large Hadron Collider

New physics models admit the interesting possibility of a $Z^{\prime }$ weak boson associated with an extra $U(1)$ gauge symmetry and a Higgs boson that is heavy enough to decay into a pair of $Z$ bosons. Then $Z^{\prime }$ production and decay via $Z^{\prime }\to ZH\to ZZZ$ has a distinctive LHC signal that is nearly background-free and reconstructs the $H$ and $Z^{\prime }$ masses and widths. The $Z^{\prime }$ decay to 3 pairs of leptons is especially distinctive. The $ZH$ decay mode exists even if the $Z^{\prime }$ is decoupled from leptons, which motivates an independent 6-lepton resonance search regardless of the dilepton search results.

Figure 1

6-lepton resonance without direct $Z^{\prime }$ coupling to leptons.

Figure 2

(a) $Z^{\prime }\to ZH\to ZZZ$ cross section at the LHC (with $\sqrt{s}=14\mathrm{TeV}$). (b) $Z^{\prime }\to ZH$ branching ratio. (c) Required luminosity for five 6-lepton resonance events at the LHC (with $\sqrt{s}=14\mathrm{TeV}$). SM-like couplings (dashed) and leptophobic couplings (solid) are assumed with $m_H=200\mathrm{GeV}$ (thin blue line) and 300 GeV (thick red line).

Figure 3

Reconstruction of the $Z^{\prime }$ and Higgs boson for $M_{Z^{\prime }}=800\mathrm{GeV}$, ${\Gamma }_{Z^{\prime }}=16.6\mathrm{GeV}$, $m_H=300\mathrm{GeV}$ and leptophobic couplings. (a) 6 leptons around the $Z^{\prime }$ mass. (b) 4 leptons around the Higgs mass. (c) Dalitz plot revealing the Higgs mass. The axes are 2 possible reconstructions for the $m_{\mathrm{inv}}(4\ell )$. There are 3 combinations of 2 pairs of $ZZ$. These 3 combinations are sorted and plotted together in {lightest, middle} (blue circle), {middle, heaviest} (red box), and {heaviest, lightest} (yellow diamond).