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

Abstract

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, FASERν and SND@LHC, are expected to be active during Run 3 and have the potential to detect the interactions of ν and $$ \overline{\nu} $$ 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 $$ {D}_s^{\pm } $$ 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 ντ plus $$ \overline{\nu} $$ 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 ντ + $$ \overline{\nu} $$ ν ¯ τ produced by $$ {D}_s^{\pm } $$ D s ± decay in the far forward region at the LHC. Next-to-leading order (NLO) QCD uncertainties are included in the calculation of $$ {D}_s^{\pm } $$ 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)% for the (ντ + $$ \overline{\nu} $$ ν ¯ τ) number of charged-current (CC) events. Scale uncertainties are much larger, resulting in a range of CC event predictions from ~70% lower to ~90% higher than the central prediction. A comparison of the predictions with those obtained using as input the central PDFs from the 3-flavour NLO PDF sets of the CT14, ABMP16 and NNPDF3.1 collaborations show that far-forward neutrino energy distributions vary by as much as a factor of ~2 – 4 relative to the PROSA19 predictions at TeV neutrino energies. The Forward Physics Facility in the high luminosity LHC era will provide data capable of constraining NLO QCD evaluations with these PDF sets.

Feebly-interacting particles: FIPs 2020 workshop report

“…allow to validate the predictions of collinear factorization and BFKL-based approaches; to constrain gluon PDFs at very low x ∼ 10 −7 ; to probe gluon saturation effects and non-linear dynamics; or to test models of intrinsic charm [1129,1130,1131,1132,1133,1134]. First experimental constraints on forward charm production would also constrain prompt atmospheric neutrino fluxes and hence provide valuable input for neutrino telescopes in their search for extragalactic neutrinos.…”

Tau neutrinos in the next decade: from GeV to EeV

“…Predictions for the number of ν τ + ντ CC events for pp collisions at √ s = 14 TeV evaluated with NLO QCD with PROSA PDF sets [723], renormalization and factorization scales set to (2m c ) 2 + p 2 T and k T Gaussian smearing with k T = 0.7 GeV. Reproduced from [720]. CC BY 4.0.…”

“…Tau neutrinos, again, come almost exclusively from the direct and chain decays of D ± s . Neutrino fluxes from a number of leading order MC simulations [394] and from NLO QCD [410,720] show that thousands to tens of thousands of tau neutrino CC events can be accumulated in detectors at a FPF with L = 3 ab −1 .…”

“…Transverse momentum smearing corrections to the collinear parton model can impact predictions at very forward rapidities where even a small k T can move a neutrino trajectory out of the detectable rapidity range [410,720]. An example of the flux of ν τ + ντ from a NLO QCD evaluation with several parton distribution functions and two values for Gaussian smearing k T is shown in figure 55 [720]. A table showing corresponding predicted numbers of events for FASERν and SND@LHC, and for a detector of 60 tons with η > 6.9, is also shown.…”

“…The green band shows the scale uncertainties for the m T,2 scale dependent result for PROSA PDFs. Reproduced from [720]. CC BY 4.0.…”