Taurus is a balloon-borne telescope that will map CMB polarization on the largest angular scales to obtain a more precise measurement of the optical depth to reionization. Also known as tau, the optical depth to reionization represents the probability that a photon from the CMB will scatter as it travels towards our instruments.  Cosmic reionization is the epoch when the first stars formed, and the value of tau is affected by specific details of the reionization process.  Additionally, more precise measurements of tau can also enable CMB measurements of the sum of neutrino masses by breaking an important parameter degeneracy.

Taurus will be deployed on a mid-latitude super-pressure balloon, where it can observe approximately 70% of the sky.  The instrument combines low temperature (100 mK) detectors with compact, cold optics and polarization modulators to provide a sensitive and robust view of the microwave sky in many frequency bands.  Observing at high frequencies inaccessible from the group enables the removal of polarized Galactic dust signals, which would otherwise obscure the cosmological information.  The Taurus mission builds upon heritage from the SPIDER payload, and the science is highly complementary to the ground-based CMB-S4 experiment.