Latest neutrino results from the FASER experiment and their implications for forward hadron production

TL;DR

This paper reports on neutrino flux measurements from the FASER experiment at the LHC, comparing results with hadronic interaction models to improve understanding of forward particle production relevant to cosmic-ray muon excess.

Contribution

First measurements of electron and muon neutrino fluxes at the LHC's forward region, providing data to test and refine models of hadronic interactions in cosmic-ray physics.

Findings

Flux measurements generally agree with models

Some discrepancies observed in specific energy bins

Data will enable better modeling of forward particle production

Abstract

The muon puzzle -- an excess of muons relative to simulation predictions in ultra-high-energy cosmic-ray air showers -- has been reported by many experiments. This suggests that forward particle production in hadronic interactions is not fully understood. Some of the scenarios proposed to resolve this predict reduced production of forward neutral pions and enhanced production of forward kaons (or other particles). The FASER experiment at the LHC is located 480 m downstream of the ATLAS interaction point and is sensitive to neutrinos and muons, which are the decay products of forward charged pions and kaons. In this study, the latest measurements of electron and muon neutrino fluxes are presented using the data corresponding to 9.5 $\mathrm{fb^{-1}}$ and 65.6 $\mathrm{fb^{-1}}$ of proton-proton collisions with $\sqrt{s}=13.6~\mathrm{TeV}$ by the FASER$\nu$ and the FASER electronic detector, respectively. These fluxes are compared with predictions from recent hadronic interaction models, including EPOS-LHCr, SIBYLL 2.3e, and QGSJET 3. The predictions are generally consistent with the measured fluxes from FASER, although some discrepancies appear in certain energy bins. More precise flux measurements with additional data will follow soon, enabling validation of pion, kaon, and charm meson production with finer energy binning, reduced uncertainties, and multi-differential analyses.