Scalar-induced Neutrinoless Double Beta Decay in $SU(5)$

We discuss the role of heavy scalar fields in mediating neutrinoless double beta decay $(0νββ)$ within the $SU(5)$ Grand Unified Theory framework, extended suitably to include neutrino mass. In such a minimal realistic $SU(5)$ setup for fermion masses, the scalar contributions to $0νββ$ are extremely suppressed as a consequence of the proton decay bound. We circumvent this problem by imposing a discrete Z₃ symmetry. However, the scalar contributions to $0νββ$ remain suppressed in this SU(5) × Z₃ model due to the neutrino mass constraint. We find that the $0νββ$ contribution can be enhanced by extending the scalar sector with an additional 15-dimensional scalar representation with suitable Z₃ charge. Such an extension not only yields realistic fermion mass spectra but also leads to experimentally testable predictions in upcoming ton-scale $0νββ$ searches, which can be used as a sensitive probe of the new scalars across a broad range, from LHC-accessible scales up to ~ 10¹⁰\,GeV.