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 $7.2 < η< 8.4$. 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.

Figures (6)

• Figure 1: Schematic side-view (YZ plane) of the SND@LHC detector layout (not to scale). The setup includes the third veto plane (installed in 2024) and the two mini-drift tube (mDT) planes (added in 2025).
• Figure 2: DS Hough transform track density distribution at $z = 430$ cm in data. Higher density is observed at the upper part for proton collisions and in the lower part for heavy-ion collisions. The box illustrates the fiducial region of uniform efficiency for both SciFi and DS tracking. The diagonal lines of lower occupancy are artifacts of the 2D binning.
• Figure 3: Track angle distributions and source separation. Panel (a) is in total counts, while panels (b, c, e, f) are normalized to unit area. Panel (d) shows a 2D histogram of track angle versus muon energy for heavy-ion simulations in arbitrary units. The labels IP1, B1Only, and B2noB1 denote events coincident with an IP1 collision, a non-colliding beam 1, or a non-colliding beam 2 bunch slot, respectively. Contributions from non-colliding beams are negligible and are excluded from the final muon flux results.
• Figure 4: The LHC elements LEHR.11R1 and MQ.11R1. Pions and kaons, generated through ion interactions at LEHR.11R1, decay into muons. These muons are subsequently directed toward the detector by the quadrupole magnet MQ.11R1 and following dipoles.
• Figure 5: Muon production distribution along the beamline in heavy-ion simulations. The coordinate system is centered at IP1 with the $z$-axis pointing along the beam collision axis, away from SND@LHC. Concentration of muon production within the boxed area, approximately 415 m from IP1, aligns with the position of the LHC half-cell 11.