TeV-scale seesaw mechanism with quintuplet fermions

We propose a new seesaw model based on a fermionic hypercharge-zero weak quintuplet in conjunction with an additional scalar quadruplet which attains an induced vacuum expectation value. The model provides both tree-level seesaw $\sim v^6/M^5$ and loop-suppressed radiative $\sim (1/16{\pi }^2)·v^2/M$ contributions to active neutrino masses. The empirical masses $m_{\nu }\sim {10}^{-1}\mathrm{eV}$ can be achieved with $M\sim \mathrm{TeV}$ new states, accessible at the LHC. For $5{\mathrm{fb}}^{-1}$ of accumulated integrated luminosity at the LHC, there could be $\sim 500$ doubly-charged ${\Sigma }^{++}$ or $\overline{{\Sigma }^{++}}$ fermions with mass $M_{\Sigma }=400\mathrm{GeV}$, leading to interesting multilepton signatures. The neutral component of the fermion quintuplet, previously identified as a minimal dark matter candidate, becomes unstable in the proposed seesaw setup. The stability can be restored by introducing a $Z_2$ symmetry, in which case neutrinos get mass only from radiative contributions.

Figure 1

Tree-level diagram corresponding to dimension-nine operator in Eq. (15). The fermion line flow indicates a Majorana nature of the seesaw mediator.

Figure 2

One-loop diagrams generating the light neutrino masses.

Figure 3

For the portion of the parameter space left from the equality-division line, where the tree-level contribution to neutrino masses dominates, we plot $m_{\mathrm{tree}}/m_{\mathrm{loop}}$. Right from the equality-division line, the one-loop contribution to neutrino masses dominates, and we plot $m_{\mathrm{loop}}/m_{\mathrm{tree}}$.

Figure 4

The cross sections for production of quintuplet 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 quintuplet mass $M_{\Sigma }$.

Figure 5

Same as Fig. 4, but for designed $\sqrt{s}=14\mathrm{TeV}$ at the LHC.

Figure 6

Number of ${\Sigma }^{++}$ and $\overline{{\Sigma }^{++}}$ particles produced for three characteristic LHC collider setups, in dependence on the heavy lepton mass $M_{\Sigma }$.

Figure 7

Partial decay widths of ${\Sigma }^0$ quintuplet lepton for $|V_{l\Sigma }|=3.5\times {10}^{-7}$, in dependence on heavy quintuplet mass $M_{\Sigma }$.

Figure 8

Partial decay widths of ${\Sigma }^{+}$ quintuplet lepton for $|V_{l\Sigma }|=3.5\times {10}^{-7}$, in dependence on heavy quintuplet mass $M_{\Sigma }$.

Figure 9

Partial decay widths of ${\Sigma }^{++}$ quintuplet lepton for $|V_{l\Sigma }|=3.5\times {10}^{-7}$, in dependence on heavy quintuplet mass $M_{\Sigma }$.