Novel bispectrum estimator of the kinematic Sunyaev-Zel’dovich effect using projected fields

With the advent of current and future high-resolution cosmic microwave background (CMB) experiments, the kinematic Sunyaev-Zel'dovich (kSZ) effect has become a unique observational probe of the distribution of baryons and velocities in the Universe. In this work, we propose a novel binned bispectrum of the form temperature-temperature-density to extract the late-time kSZ effect from cleaned CMB maps. Unlike ``kSZ tomography'' methods, this estimator can use any tracer of the large-scale structure density field projected along the line-of-sight and does not require individual redshifts. With our method, we forecast signal-to-noise ratios (SNR) of $\ensuremath{\sim}100--200$ for the upcoming Simons Observatory (SO) and CMB-S4 correlated with a galaxy sample from WISE that is restricted to the linear regime. We also extend galaxy modes into the nonlinear regime and explore this harmonic space to show that the SNR peaks for squeezed triangles that have a short (linear) density mode and long temperature modes in harmonic space. The existing ${\mathrm{kSZ}}^{2}$-density projected-fields estimator compresses the rich information contained in this bispectrum across various scales and triangle shapes. Moreover, we find that the lensing correction to this estimator's signal is relatively small for high-SNR squeezed triangle configurations. We study the dependence of this kSZ signal on $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ parameters for SO and CMB-S4 and forecast initial constraints on the sum of neutrino masses while restricting to the linear galaxy bias regime. Our work illustrates the potential of the projected-fields kSZ bispectrum as a novel probe of baryonic abundance and beyond-$\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ cosmology with upcoming precision measurements.