Testing graviton parity and Gaussianity with Planck T -, E -, and B -mode bispectra

Many inflationary theories predict a non-Gaussian spectrum of primordial tensor perturbations, sourced from nonstandard vacuum fluctuations, modified general relativity, or new particles such as gauge fields. Several such models also predict a chiral spectrum in which one polarization state dominates. In this work, we place constraints on the non-Gaussianity and parity properties of primordial gravitational waves utilizing the Planck PR4 temperature and polarization data set. Using recently developed quasioptimal bispectrum estimators, we compute binned parity-even and parity-odd bispectra for all combinations of cosmic microwave background (CMB) T-, E-, and B-modes with 2<500, and perform both blind tests, sensitive to arbitrary three-point functions, and targeted analyses of a well-motivated equilateral gravitational-wave template (sourced by gauge fields) with amplitude f₍₋^ttt. This is the first time B-modes have been included in primordial non-Gaussianity analyses; they are found to strengthen constraints on the parity-even sector by 30% and dominate the parity-odd bounds, without inducing bias. We report no detection of non-Gaussianity (of either parity), with the template amplitude constrained to f₍₋^ttt=900700 (stable with respect to a number of analysis variations), compared to 13001200 in Planck 2018. The methods applied herein can be reapplied to upcoming CMB data sets such as LiteBIRD, with the inclusion of B-modes poised to dramatically improve future bounds on tensor non-Gaussianity.