Using the Large Hadron Collider as a neutrino source is the goal of the SND@LHC experiment. High-energy neutrinos, from a few hundred GeV to a few TeV, have never been studied. Moreover, neutrinos, particularly electron and tau species, in the SND pseudo-rapidity region are portals for charm production, providing, in turn, access to the gluon PDFs in an unexplored x region, with fractions of the proton momentum carried by the gluons below 10^-6.

Data Taking in LHC Run 3

The data taking, which started at the beginning of the LHC Run 3, was immediately successful with the collection of an integrated luminosity of 36.8 fb⁻¹ in 2022. The Collaboration analysed the data to look for muon neutrinos, the most abundant neutrinos in the SND acceptance, by only using the electronic detectors, while the scanning of the emulsion films instrumenting the neutrino target region was ongoing. Muon neutrinos are identified by observing their charged-current interaction: distinctive features are the absence of activity in the upstream veto system as a signature of an incoming neutral particle, the presence of a relatively large energy deposition in the fiducial target and in the HCAL as a sign of the hadronic jet produced in the neutrino scattering, and an outgoing muon track.

Due to the sloping tunnel floor, the coverage of the veto system initially did not include the bottom part of the neutrino target. To cope with this, a cut on the fiducial volume was defined to make the background induced by an unidentified incoming muon negligible. The data sample from 2022 had a total of 8.4 × 10⁹ incoming muon tracks. A rejection power at the level of 10⁻¹² was achieved by combining the veto system and the SciFi stations of the two most upstream walls of the target. The only background left was due to neutral hadrons produced by muon interactions in the upstream rock, which would, in turn, generate hadronic jets with a reconstructed muon track in the final state: this background was estimated to have 0.086 expected events.

Given the eight candidate events found in the data, an observation of collider neutrinos was established with a significance of about 7σ. The results were published in July 2023, together with those obtained by the FASER Collaboration.

2023 Run and Results

The 2023 run was interrupted by an issue in the LHC, which prevented operating it beyond July. Nevertheless, the data taking of SND was successful and achieved its record performance with 99.7% efficiency in the recorded luminosity. An integrated luminosity of 68.6 fb⁻¹ was achieved over the first two years of data taking. This data was used to search for neutrino interactions without a muon in the final state: this sample consists essentially of neutral current neutrino interactions and charged-current electron neutrinos.

Electron neutrinos produce typically an electron in the final state with more than 300 GeV energy on average, thus producing a large density of the energy deposited in the neutrino target. The selection could therefore be tuned to enrich the sample of electron neutrinos by using the energy density. The data accumulated in a test beam carried out at the SPS in 2023 for the calorimeter calibration were used to validate the Monte Carlo simulation. A control region outside of the signal selection cuts was defined which showed a good agreement between data and simulation. As a result of the selection, 9 candidate events were found with an expected background of 0.3 events, dominated by muon neutrinos where the muon is not identified. This has led to the observation of neutrino interactions without a muon in the final state with a significance above 6σ and to the evidence for electron neutrinos with 3.7σ.

Upgrades and Future Plans

In order to cope with the limited coverage of the bottom part of the target by the veto system due to the inclined floor, during the past YETS a trench was dug to lower the veto system and complement it with a third veto station. This led to a doubling of the acceptance to neutrinos, given their higher flux in the bottom part, closer to the beam axis, and to the increase of the efficiency of the veto system, with an inefficiency at the level of 10⁻⁹. This will allow a full study of the charged-current muon and electron neutrino interactions with the electronic detectors data collected between 2022 and 2024.

At the same time, the Collaboration is working on the reconstruction of emulsion data. Given that at the integrated luminosity, the average distance between muon parallel tracks is about 10 microns, the film-to-film alignment requires an extrapolation error at the level of 1 micron, thus requiring sub-micrometric position accuracy. The first challenging task was, therefore, to correct all the local film deformations to achieve the required position resolution of about 0.2 microns. The collaboration has started the event reconstruction on a sample equivalent to about 25 fb⁻¹.

Future Directions

Event displays below show the interactions of two candidate events, a charged-current muon and an electron neutrino.
 

Event displays for two candidate events. Top: a ν_μ interaction (Top) and ν_e-like event (bottom) in the sample of neutrino candidates without a muon in the final state. The coloured circles represent the local density of hits in the SciFi detector, corresponding to the number of hits within 1 cm of each hit. The coloured rectangles represent the amplitude, in arbitrary units, of hits in the upstream hadron calorimeter. Lighter shades correspond to higher values.

In the summer of 2024, the Collaboration submitted a letter of intent, expressing their wish to profit from the HL-LHC. The experiment intends to use silicon strip detector modules inherited from the CMS outer barrel tracker and arrange them in two sections. The upstream section includes tungsten as the neutrino interaction target and denser instrumentation. The downstream section includes magnetized iron as an absorber and has sparser instrumentation. The latter section of the detector acts as a hadronic calorimeter and muon spectrometer. Fast detector planes using either plastic scintillator or resistive-plate chamber technology will be used to trigger the read-out. A detailed design exists for the components of the detector and their integration. A prototype of a silicon strip layer has been assembled with CMS spare components, and a timing detector prototype has been exposed to a hadron test beam in 2024. Significant synergy exists in detector R&D for the neutrino detector of the SHiP experiment, which has a highly complementary physics case.