  Extending the Standard Model with right-handed neutrinos (RHNs) is well
motivated by the observation of neutrino oscillations. In the type-I seesaw
model, the RHNs interact with the SM particles via tiny mixings with the active
neutrinos, which makes their discovery in the laboratory, and in particular at
collider experiments in general challenging. In this work we instead consider
an extension of the type-I seesaw model with the addition of a leptoquark (LQ),
and employ a non-minimal production mechanism of the RHN via LQ decay, which is
unsuppressed by neutrino mixing. We focus on relatively light RHN with mass
$\mathcal{O}(10)$ GeV and LQ with mass 1.0 TeV, and explore the discovery
prospect of the RHN at the proposed Large Hadron electron Collider. In the
considered mass range and with the given interaction strength, the RHN is long
lived and, due to it stemming from the LQ decay, it is also heavily boosted,
resulting in collimated decay products. The unique signature under
investigation is thus a displaced fat jet. We use kinematic variables to
separate signal from background, and demonstrate that the ratio variables with
respect to energy/number of displaced and prompt tracks are useful handles in
the identification of displaced decays of the RHN. We also show that employing
a positron beam provides order of magnitude enhancement in the detection
prospect of this signature.
