Electroweak two-loop contribution to the mass splitting within a new heavy SU(2)$_L$ fermion multiplet

Understand

New heavy particles in an SU(2)_L multiplet, sometimes introduced in extensions of the standard model, have highly degenerate tree-level mass M if their couplings to the Higgs bosons are very small or forbidden.

• However, loop corrections may generate the gauge-symmetry-breaking mass splitting within the multiplet, which does not vanish in the large M limit due to the threshold singularity.
• We calculate the electroweak contribution to the mass splitting for a heavy fermion multiplet, to the two-loop order.
• Numerically, two-loop electroweak contributions are typically O(MeV).