Primordial Lepton Asymmetries in the Precision Cosmology Era: Current Status and Future Sensitivities from BBN and the CMB. (arXiv:2208.03201v1 [hep-ph])
<a href="http://arxiv.org/find/hep-ph/1/au:+Escudero_M/0/1/0/all/0/1">Miguel Escudero</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Ibarra_A/0/1/0/all/0/1">Alejandro Ibarra</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Maura_V/0/1/0/all/0/1">Victor Maura</a>
Using a new sample of extremely metal poor systems, the EMPRESS survey has
recently reported a primordial helium abundance that is $3sigma$ smaller than
the prediction from the Standard BBN scenario. This measurement could be
interpreted as a hint for a primordial lepton asymmetry in the electron
neutrino flavor. Motivated by the EMPRESS results, we present a comprehensive
analysis of the lepton asymmetry using measurements of the abundances of
primordial elements, along with CMB data from Planck. Assuming that there is no
dark radiation in our Universe, we find an electron neutrino chemical potential
$xi_{nu_e} = 0.037 pm 0.013$, which deviates from zero by $2.8sigma$. If no
assumption is made on the abundance of dark radiation in the Universe, the
chemical potential is $xi_{nu_e} = 0.037 pm 0.020$, which deviates from zero
by $1.9sigma$. We also find that this result is rather insensitive to the
choice of nuclear reaction rates. If the true helium abundance corresponds to
the EMPRESS central value, future CMB observations from the Simons Observatory
and CMB-S4 will increase the significance for a non-zero lepton asymmetry to
$4sigma$ and $5sigma$ respectively, assuming no dark radiation, or to
$3sigma$ when no assumption is made on the abundance of dark radiation.
Using a new sample of extremely metal poor systems, the EMPRESS survey has
recently reported a primordial helium abundance that is $3sigma$ smaller than
the prediction from the Standard BBN scenario. This measurement could be
interpreted as a hint for a primordial lepton asymmetry in the electron
neutrino flavor. Motivated by the EMPRESS results, we present a comprehensive
analysis of the lepton asymmetry using measurements of the abundances of
primordial elements, along with CMB data from Planck. Assuming that there is no
dark radiation in our Universe, we find an electron neutrino chemical potential
$xi_{nu_e} = 0.037 pm 0.013$, which deviates from zero by $2.8sigma$. If no
assumption is made on the abundance of dark radiation in the Universe, the
chemical potential is $xi_{nu_e} = 0.037 pm 0.020$, which deviates from zero
by $1.9sigma$. We also find that this result is rather insensitive to the
choice of nuclear reaction rates. If the true helium abundance corresponds to
the EMPRESS central value, future CMB observations from the Simons Observatory
and CMB-S4 will increase the significance for a non-zero lepton asymmetry to
$4sigma$ and $5sigma$ respectively, assuming no dark radiation, or to
$3sigma$ when no assumption is made on the abundance of dark radiation.
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