Abstract
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 $\sim$ 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 usual LR models. Specifically, we assume that 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 $(B - L)$ gauge boson, without a charged current counterpart. Furthermore, there is no decay for $(B - L)$ 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 $4 - 5 \sigma$ signal can be achieved for both cases described above for the following spectrum with 3000 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.
Highlights
Numerous versions of the seesaw mechanism for explaining the extreme smallness of standard model (SM) neutrino masses have been proposed over the last few decades
The new states beyond SM are the Kaluza-Klein (KK) excitations of the SM particles, as well as singlet neutrino modes, all with mass at the ∼TeV scale. In this and the previous paper, we study the Large Hadron Collider (LHC) signals from production and decay of these singlet neutrinos arising from the decay of other heavier KK particles, in particular, belonging to the extended EW sector alluded to above
In [1], we argued that (i) warped seesaw is a natural implementation of the essence of the original seesaw paradigm and that (ii) composite TeV-mass singlet neutrinos play a crucial role in SM neutrino mass generation
Summary
Numerous versions of the seesaw mechanism for explaining the extreme smallness of standard model (SM) neutrino masses have been proposed over the last few decades. As anticipated earlier, we can have non-negligible production of composite (B − L) (assuming that is lighter) via its mixing with external hypercharge gauge boson which, in turn, couples to light quarks inside proton. In this way, there is no need to involve EWSB for generating this coupling, cf for composite Z0 in the degenerate case. Suppose we make composite WL’s heavier than composite WR’s or vice versa In this case, production of WÆR from light quarks might become negligible, since this coupling requires EWSB mixing, whose effect is damped by the nondegeneracy. Appendixes contain the more technical details of the mixing between elementary hypercharge and composite W3R; ðB − LÞ gauge bosons
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