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We present new results on the calculation of the dark matter relic abundancewithin the Higgs induced right-handed (RH) neutrino mixing model, solving theassociated density matrix equation. For a benchmark value of the dark mattermass $M_{\rm DM} = 220\,{\rm TeV}$, we show the evolution of the abundance andhow this depends on reheat temperature, dark matter lifetime and source RHneutrino mass $M_{\rm S}$, with the assumption $M_{\rm S} < M_{\rm DM}$. Wecompare the results with those obtained within the Landau-Zener approximation,showing that the latter largely overestimates the final abundance giving someanalytical insight. However, we also notice that since in the density matrixformalism the production is non-resonant, this allows source RH neutrino massesbelow the W boson mass, making dark matter more stable at large mass values.This opens an allowed region for initial vanishing source RH neutrinoabundance. For example, for $M_{\rm S} \gtrsim 1\,{\rm GeV}$, we find $M_{\rmDM}\gtrsim 20\,{\rm PeV}$. Otherwise, for $M_{\rm S} > M_W\sim 100\,{\rm GeV}$,one has to assume a thermalisation of the source RH neutrinos prior to thefreeze-in of the dark matter abundance. This results into a large allowed rangefor $M_{\rm DM}$, depending on $M_{\rm S}$. For example, imposing $M_{\rm S}\gtrsim 300\,{\rm GeV}$, allowing also successful leptogenesis, we find $0.5\lesssim M_{\rm DM}/{\rm TeV} \lesssim 500$. We also discuss in detailleptogenesis with two quasi-degenerate RH neutrinos, showing a case whenobserved dark matter abundance and matter-antimatter asymmetry aresimultaneously reproduced. Finally, we comment on how an initial thermal sourceRH neutrino abundance can be justified and on how our results suggest that alsothe interesting case where $M_{\rm DM} < M_{\rm S}$, embeddable in usual highscale two RH neutrino seesaw models, might be viable.

Density matrix calculation of the dark matter abundance in the Higgs induced right-handed neutrino mixing model