Unraveling the Dirac Neutrino with Cosmological and Terrestrial
  Detectors
  We point out a correlation between the effective number of relativistic
degrees of species $\Delta N_\text{eff}$, the cosmologically measured
$m_{\nu,\text{sterile}}^{\text{eff}}$, and the terrestrially measured neutrino
mass sum and effective electron neutrino mass, $\Sigma m_\nu$ and $m_{\nu_e}$,
which arises in the Dirac neutrino hypothesis. If the neutrinos are Dirac
particles, and if the active neutrinos' sterile partners were once thermalized
in the early universe, then this new cosmological relic would simultaneously
contribute to the effective number of relativistic species, $\Delta
N_\text{eff}$, and also lead to a correlation between the
cosmologically-measured effective sterile neutrino mass
$m_{\nu,\text{sterile}}^\text{eff}$ and the terrestrially-measured active
neutrino mass sum $\Sigma m_\nu$. We emphasize that specifically correlated
deviations in $\Delta N_\text{eff}\gtrsim 3$,
$m_{\nu,\text{sterile}}^\text{eff}$ and $\Sigma m_\nu$ above their standard
predictions could be the harbinger revealing the Dirac nature of neutrinos. We
provide several benchmark examples, including Dirac leptogenesis, that predict
a thermal relic population of the sterile partners, and we discuss the relevant
observational prospects with current and near-future experiments. If the
correlation highlighted in this work is observed in future surveys, it could be
interpreted as supporting evidence of Dirac neutrino masses.
