Looking for hints of a reconstructible seesaw model at the Large Hadron Collider

We study the production of heavy neutrinos at the Large Hadron Collider through the dominant $s$-channel production mode as well as the vector boson fusion process. We consider the TeV scale minimal linear seesaw model containing two heavy singlets with the opposite lepton number. This model is fully reconstructible from oscillation data apart from an overall normalization constant which can be constrained from the metastability of the electroweak vacuum and bounds coming from lepton flavor violation searches. The Dirac nature of heavy neutrinos in this model implies suppression of the conventional same-sign-dilepton signal at the Large Hadron Collider. We analyze the collider signatures with the trilepton final state and missing transverse energy as well as vector boson fusion type signals which are characterized by two additional forward tagged jets. Our investigation reveals that due to stringent constraints on light-heavy mixing coming from lepton flavor violation and metastability bounds, the model can be explored only for a light to moderate mass range of heavy neutrinos. We also note that in case of a positive signal, flavor counting of the final trilepton channel can give information about the mass hierarchy of the light neutrinos.

Figure 1

Bound on $y_{\nu }/M_N$ as a function of the Majorana phase $\alpha$, varying the oscillation parameters in the allowed $3\sigma$ range. The red-solid (blue-dashed) curve corresponds to the atmospheric angle (${\theta }_{23}$) residing in the LO (HO) region. (Left plot) The plot is for the NH scenario, where the highest allowed value of $y_{\nu }/M_N$ lies in the LO region. (Right plot) The same plot for the IH scenario.

Figure 2

(Left plot) Leading order $s$-channel diagram for heavy neutrino production at hadron colliders, and (right plot) representative diagram for one of the decay modes of the heavy neutrino. These two figures lead to the trilepton $+{\cancel{E}}_T$ signal considered in the analysis.

Figure 3

Representative parton level diagrams contributing to $N\ell jj$ production through vector boson fusion at hadron colliders. Mirror diagrams are not shown here and also the last diagram is one of the four diagrams with $W^{\pm }$ emitting from each of the quark legs.

Figure 4

The total cross section is shown for the production of the heavy neutrino associated with the light lepton ($pp\to N\ell$, where $\ell =e,\mu$) at the 14 TeV LHC through the leading order $s$-channel process, while dotted lines represent VBF production cross section.

Figure 5

The decay branching ratios of the heavy neutrino ($N$) in different channels as a function of its mass in the case of normal hierarchy, case I, (left) and inverted hierarchy (right). Total decay widths in each case are also demonstrated with the solid line in the same figure.

Figure 6

(Left) The variation of significance $S/\sqrt{S+B}$ for the $s$-channel production signal for benchmark point $M_N=100\mathrm{GeV}$ with the integrated luminosity available for the low luminosity option at 14 TeV LHC. Black-dotted (green-dashed) line parallel to the $x$ axis represents the $5\sigma$ ($3\sigma$) significance. (Right) The lines for the $3\sigma$ (red) and $5\sigma$ (blue) significance in terms of heavy neutrino mass and integrated luminosity. With $300{\mathrm{fb}}^{-1}$ luminosity at LHC14, the heavy neutrino mass in this model can be probed up to $\sim 210(230)\mathrm{GeV}$ with $\sim 5\sigma$ ($3\sigma$) significance. For a very high luminosity of $3000{\mathrm{fb}}^{-1}$ this can reach up to $\sim 270(295)\mathrm{GeV}$.