CMB-S4 forecasts for constraints on f_NL through μ -distortion anisotropy

Diffusion damping of the cosmic microwave background (CMB) power spectrum results from imperfect photon-baryon coupling in the pre-recombination plasma. Energy release at redshifts 5×104<z<2×106 can create μ-type spectral distortions of the CMB. These μ distortions trace the underlying photon density fluctuations, probing the primordial power spectrum in short-wavelength modes kS over the range 50 Mpc-1k104 Mpc-1. Small-scale power modulated by long-wavelength modes kL from squeezed-limit non-Gaussianities introduces cross correlations between CMB temperature anisotropies and μ distortions. Under single-field inflation models, μ×T correlations measured from an observer in an inertial frame should vanish up to a factor of (kL/kS)21. Thus, any measurable correlation rules out single-field inflation models. We forecast how well the next-generation ground-based CMB experiment CMB-S4 will be able to constrain primordial squeezed-limit non-Gaussianity, parametrized by fNL, using measurements of CμT as well as CμE from CMB E modes. Using current experimental specifications and foreground modeling, we expect σ(fNL)1000. This is roughly 4 times better than the current limit on fNL using μ×T and μ×E correlations from Planck and is comparable to what is achievable with LiteBIRD, demonstrating the power of the CMB-S4 experiment. This measurement is at an effective scale of k≈740 Mpc-1 and is thus highly complementary to measurements at larger scales from primary CMB and large-scale structure.