# A New Limit on CMB Circular Polarization from SPIDER

## TL;DR

This paper reports a new upper limit on the circular polarization of the cosmic microwave background (CMB) using data from SPIDER's 2015 balloon flight, significantly improving previous constraints and demonstrating the potential for future, more sensitive measurements.

## Contribution

The study introduces a novel method leveraging the non-zero circular-to-linear polarization coupling of HWP modulators to constrain CMB circular polarization over a wide angular scale range.

## Key findings

- SPIDER sets the first constraints on CMB circular polarization at 95 and 150 GHz.
- The new limits improve previous bounds by several orders of magnitude.
- Constraints on the Stokes V power spectrum range from 141 to 255 μK² at 150 GHz.

## Abstract

We present a new upper limit on CMB circular polarization from the 2015 flight of SPIDER, a balloon-borne telescope designed to search for $B$-mode linear polarization from cosmic inflation. Although the level of circular polarization in the CMB is predicted to be very small, experimental limits provide a valuable test of the underlying models. By exploiting the non-zero circular-to-linear polarization coupling of the HWP polarization modulators, data from SPIDER's 2015 Antarctic flight provide a constraint on Stokes $V$ at 95 and 150 GHz from $33<\ell<307$. No other limits exist over this full range of angular scales, and SPIDER improves upon the previous limit by several orders of magnitude, providing 95% C.L. constraints on $\ell (\ell+1)C_{\ell}^{VV}/(2\pi)$ ranging from 141 $\mu K ^2$ to 255 $\mu K ^2$ at 150 GHz for a thermal CMB spectrum. As linear CMB polarization experiments become increasingly sensitive, the techniques described in this paper can be applied to obtain even stronger constraints on circular polarization.