LHC signals for singlet neutrinos from a natural warped seesaw mechanism. I

Recently, it was shown in K. Agashe et al. [Phys. Rev. D 94, 013001 (2016)] that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a natural realization of “inverse” seesaw; i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac TeV-mass SM singlet neutrinos. By the $\mathrm{AdS}/\mathrm{CFT}$ correspondence, this scenario is dual to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson (and possibly the top quark), with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2{)}_{\mathrm{L}}\times SU(2{)}_{\mathrm{R}}\times U(1{)}_{\mathrm{X}}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2{)}_{\mathrm{R}}\times U(1{)}_{\mathrm{X}}$ symmetry, thus can be produced from $W_R^{\pm }$ exchange, as in four-dimensional LR symmetric models. However, the direct coupling of light quarks to $W_R^{\pm }$ is negligible, due to $W_R^{\pm }$ also being composite (cf. four-dimensional LR models); nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{\pm }$ bosons. Furthermore, $W_R^{\pm }$ decays dominantly into the singlet and composite partner of charged lepton (cf. SM lepton itself in four-dimensional LR model). This heavy charged lepton, in turn, decays into SM lepton, plus $Z/\mathrm{Higgs}$, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W_R^{\pm }$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the $\mathrm{dilepton}+\text{dijet}+\mathrm{Higgs}$ and $\mathrm{trilepton}+\text{Higgs}$ channels, significant evidence can be seen at the 14 TeV LHC for an integrated luminosity of $300{\mathrm{fb}}^{-1}$ and that even discovery is possible with slightly more luminosity.

Figure 1

RS model with extended bulk gauge symmetry $SU(2{)}_{\mathrm{L}}\times SU(2{)}_{\mathrm{R}}\times U(1{)}_{\mathrm{X}}$ and singlet neutrino. Gauge symmetries together with its breaking pattern are shown on the relevant position along the extra-dimension. The position of the fields shows where the zero-mode profile of the corresponding five-dimensional fields is localized. Fields with (close to) flat zero-mode profile are located in the middle of the bulk.

Figure 2

The figure denotes sine and cosine of mixing $\mathrm{angle}(s_{\star },c_{\star })$ between $W_L^{(1)}$ and $W_R^{(1)}$ as a function of $m_{\star }$, with $g_{\star W}=g_{\star R}=3$. SM parameters are chosen to be standard values $g_W=0.65$ and $v=246\mathrm{GeV}$.

Figure 3

The plot on the left (right) panel shows branching ratios of $W_L$ ($W_R$) as a function of its mass.

Figure 4

The left panel shows Feynman diagram for the signal process of the dilepton channel. The right panel shows Feynman diagram for the signal process of the trilepton channel. Double (single) lines denote composite (elementary) particles. Here composite gauge bosons are in gauge basis $W_L^{(1)}$ and $W_R^{(1)}$ in order to show the mixing induced by Higgs VEV explicitly.

Figure 5

Dilepton channel: Distributions of variables: $M_{\text{All}}$ (top row, left), $M_{\ell \ell }$ (top row, right), $M_{b\overline{b}\ell }$ (mid row, left), $M_{jj\ell }$ (mid row, left), $M_{jj}$ (bottom row, left), $M_{b\overline{b}}$ (bottom row, mid) and MET $({\cancel{E}}_T)$ (bottom row, right) for signal (solid blue) and backgrounds (dotted, $t\overline{t}jj$-purple, $t\overline{t}H/Z$-red, $jj\ell \ell H/Z$-orange, irred-green).

Figure 6

Trilepton channel: Distributions of variables: $M_{W_R}^{\text{recon}}$ (top row, left), $M_{\text{All}}$ (top row, right), $M_{b\overline{b}\ell }$ (mid row, left), $M_{\ell \ell \nu }$ (mid row, right), $M_{\ell \ell \ell }$ (bottom row, left), and $M_{\ell \ell }$ (bottom row, right) for signal (solid blue) and backgrounds (dotted, $t\overline{t}W$-red and irred-green).

Figure 7

Left panel shows the mass spectrum of the first KK fermion as a function of IR BKT $r_{\mathrm{IR}}$. Right panel shows fermion’s coupling to the first KK gauge field, with $(-+)$ boundary condition for KK gauge boson and right chirality of KK fermion. Coupling to the left(right) chirality of the first KK fermions is denoted as $g_{L(R)}$, where $g_0$ is the coupling of right chirality with $r_{\mathrm{IR}}=0$. In both plots, we take $c=-0.27$ and $k\pi r_c=34$.