Constraints on the Optical Depth to Reionization from Balloon-Borne CMB Measurements by Josquin Errard et al. on Thursday 24 November

We assess the uncertainty with which a balloon-borne experiment, nominally

called Tau Surveyor ($\tau S$), can measure the optical depth to reionization

$\sigma(\tau)$ with given realistic constraints of instrument noise and

foreground emissions. Using a $\tau S$ fiducial design with six frequency bands

between 150 and 380 GHz with white and uniform map noise of 7 $\mu$K arcmin,

achievable with a single mid-latitude flight, and including Planck's 30 and 44

GHz data we assess the error $\sigma(\tau)$ obtained with three foreground

models and as a function of sky fraction $f_{\rm sky}$ between 40% and 54%. We

carry out the analysis using both parametric and blind foreground separation

techniques. We compare $\sigma(\tau)$ values to those obtained with low

frequency and high frequency versions of the experiment called $\tau S$-lf and

$\tau S$-hf that have only four and up to eight frequency bands with narrower

and wider frequency coverage, respectively. We find that with $\tau S$ the

lowest constraint is $\sigma(\tau)=0.0034$, obtained for one of the foreground

models with $f_{\rm sky}$=54%. $\sigma(\tau)$ is larger, in some cases by more

than a factor of 2, for smaller sky fractions, with $\tau S$-lf, or as a

function of foreground model. The $\tau S$-hf configuration does not lead to

significantly tighter constraints. Exclusion of the 30 and 44 GHz data, which

give information about synchrotron emission, leads to significant $\tau$

mis-estimates. Decreasing noise by an ambitious factor of 10 while keeping

$f_{\rm sky}$=40% gives $\sigma(\tau) =0.0031$. The combination of

$\sigma(\tau) =0.0034$, BAO data from DESI, and future CMB B-mode lensing data

from CMB-S3/S4 experiments could give $\sigma(\sum m_{\nu}) = 17$ meV.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2206.03389v2