• DOI:10.1103/PhysRevD.97.063519
• Corpus ID: 55722293

Impact of theoretical assumptions in the determination of the neutrino effective number from future CMB measurements

@article{Capparelli2017ImpactOT,
  title={Impact of theoretical assumptions in the determination of the neutrino effective number from future CMB measurements},
  author={L. M. Capparelli and Eleonora Di Valentino and Alessandro Melchiorri and Jens Chluba},
  journal={Physical Review D},
  year={2017},
  volume={97},
  pages={063519}
}

• L. Capparelli, E. D. Valentino, +1 author J. Chluba
• Published 19 December 2017
• Physics
• Physical Review D

One of the major goals of future Cosmic Microwave Background measurements is the accurate determination of the effective number of neutrinos $N_{\rm eff}$. Reaching an experimental sensitivity of $\Delta N_{\rm eff} = 0.013$ could indeed falsify the presence of any non-standard relativistic particles at $95 \%$ c.l.. In this paper, we test how this future constraint can be affected by the removal of two common assumptions: a negligible running of the inflationary spectral index $n_{\rm run}$ and a precise determination of the neutron lifetime $\tau_n$. We first show that the constraints on $N_{\rm eff}$ could be significantly biased by the unaccounted presence of a running of the spectral index. Considering the Stage-IV experiment, a negative running of ${\rm d}n/{\rm d}\ln k= - 0.002$ could mimic a positive variation of $\Delta N_{\rm eff} = 0.03$. Moreover, given the current discrepancies between experimental measurements of the neutron lifetime $\tau_n$, we show that the assumption of a conservative error of $\Delta\tau_n \sim 10$s could bring to a systematic error of $\Delta N_{\rm eff} = 0.02$. Complementary cosmological constraints on the running of the spectral index and a solution to the neutron lifetime discrepancy are therefore needed for an accurate and reliable future CMB bound of $N_{\rm eff}$ at percent level.