Observation of Collider Muon Neutrinos with the SND@LHC Experiment

We report the direct observation of muon neutrino interactions with the SND@LHC detector at the Large Hadron Collider. A dataset of proton-proton collisions at $\sqrt{s}=13.6\mathrm{TeV}$ collected by SND@LHC in 2022 is used, corresponding to an integrated luminosity of $36.8{\mathrm{fb}}^{-1}$. The search is based on information from the active electronic components of the SND@LHC detector, which covers the pseudorapidity region of $7.2<\eta <8.4$, inaccessible to the other experiments at the collider. Muon neutrino candidates are identified through their charged-current interaction topology, with a track propagating through the entire length of the muon detector. After selection cuts, 8 ${\nu }_{\mu }$ interaction candidate events remain with an estimated background of 0.086 events, yielding a significance of about 7 standard deviations for the observed ${\nu }_{\mu }$ signal.

Figure 1

Schematic layout of the SND@LHC detector front view (left) and side view (right). The side view includes an illustration of a ${\nu }_{\mu }$ charged-current interaction in the target with a hadronic shower sampled in the emulsion detector, target trackers, and hadronic calorimeter, and a muon track visible in the muon system.

Figure 2

Display of a ${\nu }_{\mu }$ CC candidate event. Hits in the SciFi, and hadronic calorimeter and muon system are shown as blue markers and black bars, respectively, and the line represents the reconstructed muon track.

Figure 3

Distribution of SciFi hits for candidate events, along with the expectation from the neutrino signal. The dashed line shows the background-only hypothesis scaled up to a deviation from the nominal expectation at a level of 5 standard deviations. The vertical bars represent 68.3% confidence intervals of the Poisson means.

Figure 4

Energy spectrum of neutral hadrons produced by muon interactions in the rock and concrete entering the SND@LHC acceptance. The shaded area shows the spectrum after rejecting events with hits in the veto detector.