Accidental dark matter: Case in the scale invariant local B - L model

We explore the idea of accidental dark matter (aDM) stability in the scale invariant local U(1)(B-L) model, which is a theory for neutrino and at the same time radiatively breaks scale invariance via quantum mechanical dynamics in the U(1)(B-L) sector. A real singlet scalar can be accidental DM with an accidental Z(2), by virtue of both extended symmetries. A U(1)(B-L) charged complex scalar can also be a viable accidental DM due to an accidental (or remanent) Z(3). They can reproduce correct relic density via the annihilations through the conventional Higgs portal or dark Higgs portal. The dark Higgs portal scenario is in tension with the LHC bound on Z(B-L), and only heavy DM of a few TeVs can have correct relic density. In particular, DM may trigger classical scale invariance spontaneous breaking. The situation is relaxed significantly in the Z(3) case due to the effective semiannihilation mode, and then light DM can be accommodated easily. In addition, the Z(3) model can accommodate the GeV scale gamma-ray excess from the Galactic center via semiannihilation into pseudo-Goldstone boson. The best fit is achieved at a DM about 52 GeV, with an annihilation cross section consistent with the thermal relic density. The invisible Higgs branching ratio is negligible because the Higgs portal quartic coupling is very small lambda(h phi) less than or similar to 10(-3).