Large-N_c analysis of two-nucleon neutrinoless double beta decay and charge-independence-breaking contact terms

Analyzing the process nn→ppe^-e^- with the exchange of a light Majorana neutrino, Cirigliano et al. [Phys. Rev. Lett. 120, 202001 (2018)] showed that in chiral effective field theory (ChEFT) a lepton-number-violating contact term is required at leading order in the ChEFT power counting. To estimate the impact of this new contribution, Cirigliano et al. related its size to a combination of isotensor charge-independence-breaking (CIB) nucleon-nucleon interactions by making an assumption about the relative sizes and signs of the isotensor CIB terms. We show that these assumptions can be justified in the limit of the number of color charges N_c becoming large, lending support to the numerical estimates of Cirigliano et al.