First Ly- α 1D bispectrum measurement in eBOSS

We present the first robust measurement of the one-dimensional Lyman alpha (Ly-α) forest bispectrum using the complete extended Baryon Oscillation Spectroscopic Survey (eBOSS) quasar sample, corresponding to the sixteenth data release (DR16) of the Sloan Digital Sky Survey (SDSS). The measurement employs an FFT estimator over 12 redshift bins, ranging from z = 2.2 to z = 4.4, and extends to scales of 0.017 (km/s)-1. The used sample consists of 122,066 quasar spectra; however, only the first six redshift bins contain sufficient data to extract a physical bispectrum. To validate and correct the bispectrum measurement, we use synthetic datasets generated from lognormal and 2LPT mocks. Additionally, we detect clear evidence of correlations between SiIII absorption lines and the Ly-α forest within the bispectrum signal, which we model using an extension of the approach applied to the analogous one-dimensional power spectrum signal. In this context, the pipeline developed for this study addresses the impact of instrumental and methodological systematics and is ready for application to larger spectroscopic datasets, such as those from the Dark Energy Spectroscopic Instrument (DESI). Finally, we compare the signal to a simple perturbation theory model, where a χ 2 analysis shows a reasonable fit for specific bispectrum configurations with z ≤ 3.2; suggesting that higher-order one-dimensional statistics in the Ly-α forest can complement cosmological inference based on the power spectrum in future analyses.