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 $7.2<\hat{I}\cdot <8.4$ forward pseudorapidity range. The detector consists of a veto system, a scintillating fiber tracker interleaved with emulsion cloud chambers, 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), $(3.76\pm 0.09)\times 10^{-2}{\text{ nb/cm}}^2$ (2024), and $(2.48\pm 0.05)\times 10^{-2}{\text{ nb/cm}}^2$ (2025). A 2024 reference proton run at $\sqrt{s}=5.36\text{ TeV}$ yielded $(4.21\pm 0.14)\times 10^{-2}{\text{ nb/cm}}^2$, providing a direct baseline for the heavy-ion energy regime. The measured fluxes for heavy-ion collisions are $(3.11\pm 0.12)\times 10^4{\text{ nb/cm}}^2$, $(5.53\pm 0.22)\times 10^4{\text{ nb/cm}}^2$, and $(3.24\pm 0.13)\times 10^4{\text{ nb/cm}}^2$ in 2023, 2024, and 2025, respectively. Uncertainties are dominated by systematic effects, with the statistical component contributing $\lesssim 0.1\%$ to the total uncertainty. These results are in agreement with Monte Carlo predictions.

12 pages, 9 figures, 3 tables, to appear in The European Physical Journal C (EPJC)