# CMB circular and B-mode polarization from new interactions

## TL;DR

This paper explores how new physics interactions violating certain symmetries could generate circular and B-mode polarization in the CMB, which are not predicted by standard models with scalar perturbations.

## Contribution

It introduces a general parametrization of photon-fermion interactions and shows how symmetry violations can produce CMB polarization modes beyond standard predictions.

## Key findings

- CMB circular polarization can be sourced by parity and charge conjugation violation.
- B-mode polarization can be generated through time-reversal symmetry violation.
- Provides a framework to constrain new physics using CMB polarization data.

## Abstract

Standard models describing the radiation transfer of the cosmic microwave background (CMB) through Compton scattering predict that cosmological scalar perturbations at linear order are not able to source V and B polarization modes. In this work, we investigate the possibility that such CMB polarization modes are generated even in the presence of linear scalar perturbations only. We provide a general parametrization of the photon-fermion forward-scattering amplitude and compute mixing terms between different CMB polarization modes. We discuss different general extensions of Standard Model interactions which violate discrete symmetries, while preserving the combination of charge conjugation, parity and time reversal. We show that it is possible to source CMB circular polarization by violating parity and charge conjugation symmetries. Instead, B-mode generation is associated to the violation of symmetry for time-reversal. Our results provide a useful tool to constrain new physics using CMB data.