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NLO event generation for LHC neutrinos and application to flux measurements at FASER

Peter Krack

TL;DR

The paper tackles the challenge of generating accurate, NLO-consistent predictions for LHC forward neutrinos and integrating these into flux measurements at FASER. It implements a LHAPDF-based neutrino flux grid interfaced to the POWHEG-BOX-RES framework, enabling NLO predictions with realistic parton-shower and hadronisation modelling. Through comparisons with GENIE and systematic studies of NLO/QCD, PS, and soft-QCD variations, it quantifies generator-dependent differences and demonstrates a data-driven approach (NNPDF-style) to extract forward neutrino fluxes from FASER measurements using FK-tables. The results show that forward-neutrino predictions are sensitive to forward hadron production modeling and that even a small dataset can discriminate between event generators, with implications for the physics program of the Forward Physics Facility.

Abstract

The LHC generates an intense beam of high-energy neutrinos in the forward direction, whose scientific potential has been left unexploited for many years. The FASER and SND@LHC experiments, operating since 2023, have recently measured LHC neutrinos for the first time. In this contribution we discuss how to produce accurate predictions, including NLO QCD corrections and modern parton shower algorithms, for present and future LHC neutrino experiments, including those at the proposed Forward Physics Facility (FPF). To this end, the energy and rapidity distribution of the LHC neutrinos is encoded in a LHAPDF grid interfaced to the neutrino DIS event generator in the POWHEG-BOX-RES framework. This Monte Carlo tool enables the modelling of differential distributions that are sensitive to hadronic final states, initial- and final-state radiation, and realistic acceptance and selection cuts. As a first application, we deploy this event generator to compute fast-interpolation grids and carry out a first determination of the LHC forward neutrino fluxes directly from FASER data using the NNPDF fitting methodology.

NLO event generation for LHC neutrinos and application to flux measurements at FASER

TL;DR

The paper tackles the challenge of generating accurate, NLO-consistent predictions for LHC forward neutrinos and integrating these into flux measurements at FASER. It implements a LHAPDF-based neutrino flux grid interfaced to the POWHEG-BOX-RES framework, enabling NLO predictions with realistic parton-shower and hadronisation modelling. Through comparisons with GENIE and systematic studies of NLO/QCD, PS, and soft-QCD variations, it quantifies generator-dependent differences and demonstrates a data-driven approach (NNPDF-style) to extract forward neutrino fluxes from FASER measurements using FK-tables. The results show that forward-neutrino predictions are sensitive to forward hadron production modeling and that even a small dataset can discriminate between event generators, with implications for the physics program of the Forward Physics Facility.

Abstract

The LHC generates an intense beam of high-energy neutrinos in the forward direction, whose scientific potential has been left unexploited for many years. The FASER and SND@LHC experiments, operating since 2023, have recently measured LHC neutrinos for the first time. In this contribution we discuss how to produce accurate predictions, including NLO QCD corrections and modern parton shower algorithms, for present and future LHC neutrino experiments, including those at the proposed Forward Physics Facility (FPF). To this end, the energy and rapidity distribution of the LHC neutrinos is encoded in a LHAPDF grid interfaced to the neutrino DIS event generator in the POWHEG-BOX-RES framework. This Monte Carlo tool enables the modelling of differential distributions that are sensitive to hadronic final states, initial- and final-state radiation, and realistic acceptance and selection cuts. As a first application, we deploy this event generator to compute fast-interpolation grids and carry out a first determination of the LHC forward neutrino fluxes directly from FASER data using the NNPDF fitting methodology.

Paper Structure

This paper contains 12 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. NLO event generation at LHC far-forward experiments
  3. Implementation
  4. Comparison with GENIE
  5. Modelling of high-energy neutrino DIS
  6. Impact of NLO QCD corrections
  7. Impact of the parton shower algorithm
  8. Impact of soft QCD
  9. Extraction of the forward neutrino flux
  10. Methodology
  11. Results
  12. Summary

Figures (5)

  • Figure 1: POWHEG+Pythia8 at LO and NLO compared to GENIE. The distribution for the scattering angle of the final state lepton shows the most drastic disagreement, while the energy of the hadronic state agrees between the two Monte Carlo tools.
  • Figure 2: Differential distribution of the energy of the hadronic system, comparing the NLO and LO result.
  • Figure 3: Differential distribution of the energy of the leading pion for the Pythia8 dipole and the Vincia shower.
  • Figure 4: Differential distribution for the energy of the leading neutron. The default Monash 2013 tune is compared to the LHCf tune.
  • Figure 5: Fit to data from the FASER experiment and the estimate of the neutrino flux from different generator with 68% CL uncertainties. The middle panel shows the pull between the fit and the prediction for the neutrino flux obtained from the four different generators.