Measurement of the Muon Flux at SND@LHC: Results from the 2023-2025 Proton and Heavy-Ion Periods

Abstract

The SND@LHC experiment investigates neutrinos in the forward pseudorapidity range of $7.2 < \eta < 8.4$. The detector consists of a veto system, a scintillating fiber (SciFi) tracker interleaved with emulsion cloud chambers (ECCs), and a downstream muon system. Muons originating from collisions at ATLAS (IP1) constitute the primary background for CC neutrino interactions and determine the replacement frequency of the emulsion target. A precise characterization of this flux is therefore essential. In this work, we report the muon flux measured in the central $31 \times 31 \text{ cm}^2$ fiducial area of the detector using data from 2023 through 2025. The measured fluxes for proton collisions are: $(1.90 \pm 0.04) \times 10^{-2} \text{ nb/cm}^2$ (2023), $(3.74 \pm 0.06) \times 10^{-2} \text{ nb/cm}^2$ (2024), and $(2.48 \pm 0.04) \times 10^{-2} \text{ nb/cm}^2$ (2025). The measured fluxes for heavy-ion collisions are $(3.13 \pm 0.11) \times 10^4 \text{ nb/cm}^2$, $(5.54 \pm 0.17) \times 10^4 \text{ nb/cm}^2$, and $(3.60 \pm 0.13) \times 10^4 \text{ nb/cm}^2$ in 2023, 2024, and 2025, respectively. Uncertainties are dominated by systematic effects, with the statistical component contributing $\lesssim 1\%$ to the total uncertainty. These results are in agreement with Monte Carlo predictions.