# Are pure Dirac fermions (electrons, quarks, …) allowed to have effective Majorana mass?

Charged particles such as electrons and quarks are not allowed to have a hard Majorana mass (see here). With 'hard' I mean an explicit mass term in the Lagrangian which would break the corresponding symmetries. However, there are also mechanisms to effectively generate masses, for example, with instantons in the topologically non-trivial QCD vacuum ('t Hooft vertex / determinant). These mass terms do not explicitly arise in the Lagrangian, only occur due to the specific vacuum the Lagrangian 'chooses' and are thus not forbidden by symmetries.

Now my question is: are there any experimental bounds on Majorana masses for charged particles?

The Majorana mass term violates the lepton number or its generalization – the number of "particles minus antiparticles" – by $\Delta L = \pm 2$. It has the form $$m \eta_A \eta_B \epsilon^{AB} + \text{complex conjugate terms}$$ where the first term contains no complex conjugation of any factor, so it creates two equal particles (or annihilates two equal particles; or annihilates a particle and creates an antiparticle, or vice versa).