Parton distribution function uncertainties in theoretical predictions for far-forward tau neutrinos at the Large Hadron Collider

New experiments dealing with neutrinos in the far-forward region at the Large Hadron Collider (LHC) are under design or already in preparation. Two of them, FASER ν and SND@LHC, are expected to be active during Run 3 and have the potential to detect the interactions of ν and ν that come from high-energy collisions in one of the LHC interaction points, extracted along the direction tangent to the beam line. Tau neutrinos and antineutrinos come predominantly from D_s^± production in pp collisions, followed by the leptonic decay of these mesons. Neutrino pseudorapidities in the range of η > 6 . 9 and η > 8 . 9 are relevant to these future experiments. At such pseudorapidities at high energies, QCD theoretical predictions for the flux of ν τ plus ν τ rely on parton distribution functions (PDFs) in a combination of very small and large parton −x values. We evaluate PDF un certainties affecting the flux of ν τ + ν τ produced by D_s^± decay in the far forward region at the LHC. Next-to-leading order (NLO) QCD uncertainties are included in the calculation of D_s^± production and NLO PDF sets are used for consistency. The theoretical uncertainty associated with the 40 PDF sets of the PROSA19 group amounts to ± (20 − 30)