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

A natural seesaw mechanism for obtaining the observed size of SM neutrino masses can arise in a warped extra-dimensional/composite Higgs framework. In a previous paper, we initiated the study of signals at the LHC for the associated $\sim \mathrm{TeV}$ mass SM singlet neutrinos, within a canonical model of $SU(2{)}_L\times SU(2{)}_R\times U(1{)}_{B-L}$ (LR) symmetry in the composite sector, as motivated by consistency with the EW precision tests. Here, we investigate LHC signals in a different region of parameter space for the same model, where production of singlet neutrinos can occur from particles beyond those in the usual LR models. Specifically, we assume that the composite ($B-L$) gauge boson is lighter than all the others in the EW sector. We show that the composite ($B-L$) gauge boson can acquire a significant coupling to light quarks simply via mixing with elementary hypercharge gauge boson. Thus, the singlet neutrino can be pair-produced via decays of the($B-L$) gauge boson, without a charged current counterpart. Furthermore, there is no decay for the ($B-L$) gauge boson directly into dibosons, unlike for the usual case of $W_R^{\pm }$ and $Z^{\prime }$. Independently of the above extension of the EW sector, we analyze production of singlet neutrinos in decays of composite partners of $SU(2{)}_L$ doublet leptons, which are absent in the usual LR models. In turn, these doublet leptons can be produced in composite $W_L$ decays. We show that the $4–5\sigma$ signal can be achieved for both cases described above for the following spectrum with $3000{\mathrm{fb}}^{-1}$ luminosity: 2–2.5 TeV composite gauge bosons, 1 TeV composite doublet lepton (for the second case) and 500–750 GeV singlet neutrino.

Figure 1

The left panel shows Feynman diagram for the signal process of ${\rho }_{B-L}$-channel. The right panel shows Feynman diagram for the signal process of composite $SU(2{)}_L$ doublet-channel. Double (single) lines denote composite (SM) particles.

Figure 2

${\rho }_{B-L}$-Channel: $p_{T{\ell }_1}$ (top row, left), $M_{jj}$ (top row, right), $M_{\ell \ell \ell }$ (middle row, left), $M_{\text{All}}$ (middle row, right), $M_{\ell jj}$ (bottom row, left), $M_{\ell \ell \nu }$ (bottom row, right) for signal (solid blue) and backgrounds (solid, $jj\ell \ell W$-red, $t\overline{t}\ell \ell$-orange, $t\overline{t}W$-green, $\ell \ell WW$-brown)

Figure 3

Composite $SU(2{)}_L$ doublet-channel: Distributions of variables: $M_{bb}$ (top row, left), $M_{{\ell }_{\nu }\nu }$ (top row, right), $M_{j{j}_1}$ (second row, left), $M_{\ell \ell }$ (second row, right), $M_{N_1}$ (third row, left), $M_{{\ell }^h}$ (third row, right), $M_{{\rho }_{W_L}}$ (bottom row, left), $P_{T{\ell }_1}$ (bottom row, middle) and $M_{\ell \ell \ell }$ (bottom row, right) for signal (solid blue) and backgrounds (solid, $t\overline{t}jj\ell \ell$-red, $t\overline{t}t\overline{t}$-orange, $t\overline{t}\ell \ell W$-green)