Exotic seesaw-motivated heavy leptons at the LHC

We study the LHC potential for discovering TeV-scale $SU(2{)}_L$ 5-plet fermions introduced recently to explain small neutrino masses. We show that the Drell-Yan production and the decays of new exotic $\Sigma$ leptons are testable at the LHC. Their production is abundant due to nontrivial electroweak gauge charges. For $1{\mathrm{fb}}^{-1}$ of integrated luminosity at the present LHC $\sqrt{s}=7\mathrm{TeV}$, there can be 270 $\Sigma -\overline{\Sigma }$ pairs produced for $M_{\Sigma }=400\mathrm{GeV}$. Besides producing same-sign dilepton events, they could lead, due to a chosen small mixing between heavy and light leptons, to $\sim 10$ golden decays ${\Sigma }^{+++}(\overline{{\Sigma }^{+++}})\to W^{\pm }{W}^{\pm }{l}^{\pm }$ with a specific decay signature.

Figure 1

Dim 9 tree-level seesaw diagram produced by exotic fields in Eqs. (2, 3).

Figure 2

Dim 5 operator arising at the loop level: the two-loop seesaw diagram given by the ${\lambda }_1$ and ${\lambda }_2$ couplings in Eq. (13), and the one-loop seesaw diagram given by the ${\lambda }_3$ coupling in Eq. (13).

Figure 3

The cross sections for production of 5-plet lepton pairs on LHC proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ via neutral $\gamma$, $Z$ (a) and charged $W^{\pm }$ currents (b), in dependence on the heavy 5-plet mass $M$.

Figure 4

Same as Fig. 3, but for designed $\sqrt{s}=14\mathrm{TeV}$ at the LHC.

Figure 5

Same as Fig. 3, but for $\sqrt{s}=1.96\mathrm{TeV}$ at the Tevatron.

Figure 6

Partial decay widths of ${\Sigma }^0$ 5-plet lepton for $|V_1^{l\Sigma }|=|V_2^{l\Sigma }|={10}^{-6}\sqrt{\frac{20}{M_{\Sigma }(\mathrm{GeV})}}$, in dependence on heavy 5-plet mass $M_{\Sigma }$.

Figure 7

Partial decay widths of ${\Sigma }^0$ 5-plet lepton for $|V_1^{l\Sigma }|={10}^{-6}\sqrt{\frac{0.5}{M_{\Sigma }(\mathrm{GeV})}}$ different from $|V_2^{l\Sigma }|={10}^{-6}\sqrt{\frac{800}{M_{\Sigma }(\mathrm{GeV})}}$, in dependence on heavy 5-plet mass $M_{\Sigma }$.

Figure 8

Feynman diagram contributing to the triply-charged golden decay.

Figure 9

Selected partial decay widths of ${\Sigma }^{+++}$ 5-plet lepton for $|V_2^{l\Sigma }|={10}^{-6}\sqrt{\frac{20}{M_{\Sigma }(\mathrm{GeV})}}$ in dependence of heavy 5-plet mass $M_{\Sigma }$.

Figure 10

Selected partial decay widths of ${\Sigma }^{+++}$ 5-plet lepton for $|V_2^{l\Sigma }|={10}^{-6}\sqrt{\frac{800}{M_{\Sigma }(\mathrm{GeV})}}$ in dependence of heavy 5-plet mass $M_{\Sigma }$.