Constraints on a seesaw model leading to quasidegenerate neutrinos and signatures at the LHC

We consider a variant of TeV-scale seesaw models in which three additional heavy right-handed neutrinos are added to the standard model to generate the quasidegenerate light neutrinos. This model is theoretically interesting since it can be fully rebuilt from the experimental data of neutrino oscillations except for an unknown factor in the Dirac-Yukawa coupling. We study the constraints on this coupling coming from metastability of electroweak vacuum. An even stronger bound comes from the lepton flavor violating decays on this model, especially in a heavy neutrino mass scenario which is within the collider’s reach. Bestowed with these constrained parameters, we explore the production and discovery potential coming from these heavy neutrinos at the 14 TeV run of the Large Hadron Collider. Signatures with trilepton final state together with backgrounds are considered in a realistic simulation.

Figure 1

Parametric plot of $(\mathrm{Tr}[Y_{\nu }^{†}{Y}_{\nu }]{)}^{1/2}$ with $\omega$ and common light neutrino mass scale $m_0$, and heavy neutrino mass fixed at 100 GeV. The numbers in the plot indicate the corresponding values for the different sets of parameters $\omega$ and $m_0$.

Figure 2

Left panel: Contours of allowed lepton flavor violating regions with $\mathrm{Br}(\mu \to e\gamma )=5.7\times {10}^{-13}$ in the parameter plane of Majorana phases ${\alpha }_1$ and ${\alpha }_2$ with different values of Dirac $CP$ phase $\delta$. Considering all the neutrino oscillation parameters and mass differences in the global best-fit values, the area within each contour is consistent with the experimental LFV upper bound from the decay rate of $\mu \to e\gamma$. Right panel: Variation of these LFV equality contours for different choices of the heavy neutrino mass $M_R$ and parameter $\omega$ considering one example ($\delta =\pi /2$) contour from the left panel. As expected, decreasing the $M_R$ or increasing the $\omega$ would make the contour narrower, retaining a smaller window for choices of these unknown parameters.

Figure 3

Allowed region of the Yukawa norm $\mathrm{Tr}[Y_{\nu }^{†}{Y}_{\nu }]$ as a function of the heavy neutrino mass $M_R$ by imposing combined constraints coming from metastability of the electroweak vacuum as well as lepton flavor violating decay ($\mu \to e\gamma$). Choice of Higgs mass fixed at $m_h=126\mathrm{GeV}$. The horizontal slanting lines represent the upper bound on $\mathrm{Tr}[Y_{\nu }^{†}{Y}_{\nu }]$ consistent with the metastability bound, as in Eq. (18). Three lines are due to three different set of values for top mass and strong coupling [87, 88]. The shaded area below the curved line is allowed from the lepton flavor violating constraint as used in Eq. (23). This is after putting global best-fit values of oscillation parameters together with the particular values of unknown phases within their full range as tabulated in Table 1. The yellow line corresponds to $\omega =11.9$ and gives us the best choice for study within the bound of LFV. Hence, the region marked “Disallowed” is ruled out from LFV for such choice of $\omega$.

Figure 4

Left panel: Total cross section is plotted for leading order $s$-channel heavy neutrino production (solid line) associated with charged leptons at the 14 TeV LHC. Basic preselection cuts $p_{T\ell }\ge 20\mathrm{GeV}$ and $|{\eta }_{\ell }|\le 2.5$ are applied and the choice of parameters is compatible with the neutrino oscillation data constrained with vacuum metastability and LFV. The dotted line shows the corresponding VBF production cross section, where basic VBF cuts were used in addition to the preselection cuts. Right panel: Demonstration of the decay branching ratios of the heavy neutrino in different channels as a function of mass. Total decay width is also shown with a red solid line.

Figure 5

Contours of constant $3\sigma$ and $5\sigma$ significance at the 14 TeV LHC in terms of heavy neutrino mass $M_R$ and integrated luminosity. With $300{\mathrm{fb}}^{-1}$ data, the trilepton signal can probe up to $M_R=160(140)\mathrm{GeV}$ with $3\sigma$ $(5\sigma )$ significance, whereas with $3000{\mathrm{fb}}^{-1}$ luminosity LHC can reach up to 230 (190) GeV. Inset shows variation of significance for the $s$-channel trilepton production signal and backgrounds with heavy neutrino mass $M_R=100\mathrm{GeV}$.