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We present an $8.1\sigma$ detection of the non-Gaussian 4-Point CorrelationFunction (4PCF) using a sample of $N_{\rm g} \approx 8\times 10^5$ galaxiesfrom the BOSS CMASS dataset. Our measurement uses the $\mathcal{O}(N_{\rmg}^2)$ NPCF estimator of Philcox et al. (2021), including a new modification tosubtract the disconnected 4PCF contribution (arising from the product of two2PCFs) at the estimator level. This approach is unlike previous work andensures that our signal is a robust detection of gravitationally-inducednon-Gaussianity. The estimator is validated with a suite of lognormalsimulations, and the analytic form of the disconnected contribution isdiscussed. Due to the high dimensionality of the 4PCF, data compression isrequired; we use a signal-to-noise-based scheme calibrated from theoreticalcovariance matrices to restrict to $\sim$ $100$ basis vectors. The compressionhas minimal impact on the detection significance and facilitates traditional$\chi^2$-like analyses using a suite of mock catalogs. The significance isstable with respect to different treatments of noise in the sample covariance(arising from the limited number of mocks), but decreases to $4.7\sigma$ when aminimum galaxy separation of $14 h^{-1}\mathrm{Mpc}$ is enforced on the 4PCFtetrahedra (such that the statistic can be modelled more easily). Thedetectability of the 4PCF in the quasi-linear regime implies that it willbecome a useful tool in constraining cosmological and galaxy formationparameters from upcoming spectroscopic surveys.

A First Detection of the Connected 4-Point Correlation Function of Galaxies Using the BOSS CMASS Sample