Constraining the phase shift of relativistic species in DESI BAOs

In the early Universe, neutrinos decouple quickly from the primordial plasma and propagate without further interactions. The impact of free-streaming neutrinos is to create a temporal shift in the gravitational potential that impacts the acoustic waves known as baryon acoustic oscillations (BAOs), resulting in a non-linear spatial shift in the Fourier-space BAO signal. In this work, we make use of and extend upon an existing methodology to measure the phase shift amplitude β_φ and apply it to the Dark Energy Spectroscopic Instrument (DESI) Data Release 1 (DR1) BAOs with an anisotropic BAO fitting pipeline. We validate the fitting methodology by testing the pipeline with two publicly available fitting codes applied to highly precise cubic box simulations and realistic simulations representative of the DESI DR1 data. We find further study towards the methods used in fitting the BAO signal will be necessary to ensure accurate constraints on β_φ in future DESI data releases. Using DESI DR1, we present individual measurements of the anisotropic BAO distortion parameters and the β_φ for the different tracers, and additionally a combined fit to β_φ resulting in β_φ = 2.7 ± 1.7. After including a prior on the distortion parameters from constraints using Planck we find (Formula presented) suggesting β_φ > 0 at 4.3σ significance. This result may hint at a phase shift that is not purely sourced from the standard model expectation for N_eff or could be a upwards statistical fluctuation in the measured β_φ; this result relaxes in models with additional freedom beyond Lambda-cold dark matter.