Signatures from an extra-dimensional seesaw model

We study the generation of small neutrino masses in an extra-dimensional model, where singlet fermions are allowed to propagate in the extra dimension, while the standard model particles are confined to a brane. Motivated by the fact that extra-dimensional models are nonrenormalizable, we truncate the Kaluza-Klein towers at a maximal Kaluza-Klein number. This truncation, together with the structure of the bulk Majorana mass term, motivated by the Sherk-Schwarz mechanism, implies that the Kaluza-Klein modes of the singlet fermions pair to form Dirac fermions, except for a number of unpaired Majorana fermions at the top of each tower. These heavy Majorana fermions are the only sources of lepton number breaking in the model, and similarly to the type-I seesaw mechanism, they naturally generate small masses for the left-handed neutrinos. The lower Kaluza-Klein modes mix with the light neutrinos, and the mixing effects are not suppressed with respect to the light-neutrino masses. Compared to conventional fermionic seesaw models, such mixing can be more significant. We study the signals of this model at the Large Hadron Collider, and find that the current low-energy bounds on the nonunitarity of the leptonic mixing matrix are strong enough to exclude an observation.

Figure 1

Illustration of the construction of Dirac particles from pairs of modes in the KK tower. Two heavy KK Majorana modes with equal masses, but opposite $CP$ parities, can be grouped together, as shown with double lines, in order to form a Dirac particle. In the case $k=1$ (left column), the heaviest mode $Y^{(N)}$ is left, while for the case $k=3$ (right column), there are three modes left: $Y^{(N-2)}$, $Y^{(N-1)}$, and $Y^{(N)}$.

Figure 2

Feynman diagrams for the potentially interesting LHC signatures with three charged leptons and missing energy in the model under consideration.

Figure 3

The expected number of events for the $2\mu$ and $2e$ signals at the LHC as functions of the inverse radius $R^{-1}$, for an integrated luminosity of $30{\mathrm{fb}}^{-1}$. Note that the masses of the lightest singlet fermions are equal to $R^{-1}/2$. For $R^{-1}<2M_W$, on-shell production of the internal gauge bosons is not possible, and the signal is suppressed. The values of the neutrino oscillation parameters are given in the main text. Left panel: normal neutrino mass hierarchy. Right panel: inverted neutrino mass hierarchy.