A Modest Shift of Centimetres Could Quadruple the LHC’s View of Forward Neutrinos

Deep inside the Large Hadron Collider complex, in a decommissioned transfer tunnel some 480 metres downstream of the ATLAS interaction point, a compact detector has been quietly doing something no machine has done before: catching neutrinos produced directly in collider collisions. The SND@LHC experiment, installed in the TI18 tunnel, has since 2022 demonstrated that a small hybrid detector can survive and operate in one of the most radiation-intense corners of the world’s most powerful accelerator. Now, in a paper published in The European Physical Journal C, the SND@LHC Collaboration has laid out a detailed blueprint for the detector’s upgrade to run throughout the High-Luminosity LHC era, and the numbers it reports are striking. A seemingly trivial change in the detector’s position, lowering it by roughly 40 centimetres and shifting it sideways by about 30, would multiply the total neutrino interaction rate by a factor of five.

The case for the upgrade rests on what the High-Luminosity LHC itself will deliver. When the accelerator completes its current upgrade programme, it will increase the integrated luminosity delivered to experiments by roughly an order of magnitude compared with Run 3, flooding the forward region with an unprecedented flux of neutrinos. These particles, born in the decays of heavy-flavour hadrons such as those containing charm quarks, are produced at extreme angles relative to the beamline, at pseudorapidities between about 7 and 8, and stream hundreds of metres along the tunnel. No general-purpose detector can catch them. SND@LHC, sitting off the collision axis, was conceived precisely to exploit this blind spot, and its first observations of both muon neutrinos and electron neutrinos from LHC collisions confirmed that collider neutrino physics has moved from theory to reality.

The upgraded detector, referred to in the paper as SND@HL-LHC while the collaboration keeps its original name, must contend with a fundamental technological problem: the emulsion-based tracking system that served the experiment during Run 3 cannot cope with the particle rates of the high-luminosity era. Emulsion films integrate everything that passes through them and offer no event-by-event electronic readout. The upgrade therefore replaces them with a fully electronic, silicon-based tracking system capable of real-time operation while preserving the spatial resolution needed to reconstruct neutrino interactions in a high-occupancy environment. The design also adds two capabilities the original detector never had: a precision timing system for rejecting background, and a magnetic spectrometer that measures the momentum and charge of outgoing muons, allowing, for the first time, a clean distinction between neutrino and antineutrino interactions.

The detector itself remains remarkably compact, roughly three metres long with a cross-section of about one square metre, a footprint dictated by the tight confines of the TI18 tunnel. Its heart is the neutrino target region, a stack of 58 tungsten plates, each seven millimetres thick, interleaved with silicon microstrip layers salvaged and repurposed from the CMS Microstrip Tracker. The stack weighs about 1.3 tons and corresponds to 116 radiation lengths and 4 nuclear interaction lengths, dense enough to stop and measure the showers produced when neutrinos strike tungsten nuclei. The silicon modules resolve positions to better than 30 micrometres, and consecutive layers are rotated by 90 degrees to provide precision tracking in both spatial coordinates. To keep the irradiated silicon stable, the target operates at approximately minus 20 degrees Celsius inside an insulated enclosure cooled by a closed-loop dry-air system with full redundancy and safety monitoring.

Downstream of the target sits the second major component: a magnetised hadronic calorimeter built from 34 layers of 50-millimetre-thick iron slabs interleaved with more recycled CMS silicon modules. The iron serves double duty, absorbing hadronic showers so their energy can be measured and providing the medium for a magnetic field of approximately 1.7 tesla, generated by a single-piece copper coil. This field bends the trajectories of muons leaving the target, allowing momentum measurements up to about 1 teraelectronvolt with a resolution of roughly 20 percent. Finite-element simulations confirm a homogeneous field within the active volume, with non-uniformities of only a few percent and negligible stray fields outside. Upstream, three planes of plastic scintillator bars read out by silicon photomultipliers form a veto system with a rejection efficiency at the level of one part in a billion against the few-kilohertz flux of beam muons, while a dedicated timing system, based on either fast scintillator tiles or thin-gap resistive plate chambers, aims at sub-50-picosecond resolution to tie every event to the correct LHC bunch crossing.

The most consequential part of the paper is its comparison of two installation scenarios, both using the identical detector. The Baseline configuration, which is the approved version of the upgrade, places the detector directly on the existing tunnel floor with no civil-engineering work, leaving its centre about 57 centimetres above and 39 centimetres to one side of the collision axis. That position yields an average pseudorapidity coverage of 6.9 to 7.7 but an azimuthal acceptance of only about 66 milliradians, squeezed by the clearance between detector and ceiling. The Extended configuration instead lowers the detector by 43 centimetres and shifts it 29 centimetres horizontally, requiring the removal of 4.5 cubic metres of concrete. The reward is dramatic: the transverse distance to the collision axis shrinks to roughly 14 by 10 centimetres, pseudorapidity coverage extends beyond 7.8, and the azimuthal acceptance grows roughly fourfold to about 250 milliradians, which the authors identify as the dominant contribution to the overall acceptance gain.

Simulations based on the DPMJET, FLUKA, GENIE and POWHEG generators, assuming the full 3000 inverse femtobarns of integrated luminosity expected over roughly a decade of HL-LHC operation, quantify what that geometric gain buys. In the Extended configuration the total neutrino interaction rate rises by approximately a factor of five, and the energy spectra shift toward higher values because the detector intercepts a more energetic component of the forward flux. The consequences ripple through the entire physics programme. For muon neutrino and antineutrino deep-inelastic scattering cross-sections above 350 gigaelectronvolts, a region never previously explored that bridges fixed-target experiments and IceCube, the statistical uncertainty falls from about 3 percent in the Baseline layout to below 1 percent in the Extended one, while the dominant flux systematic can be constrained to about 5 percent thanks to overlapping measurements by the LHCf experiment during Run 3.

The upgrade also promises precision tests of fundamental symmetries. Because electron neutrinos in the detector’s acceptance originate predominantly from charm decays, their measurement probes forward charm production and the gluon distribution of the proton at Bjorken-x values below ten to the minus five, a regime relevant both for future colliders such as the FCC and for predictions of ultra-high-energy atmospheric neutrinos. Per-bin statistical uncertainties on these charm-related measurements improve from about 10 percent in the Baseline to 5 percent in the Extended configuration. Tests of lepton flavour universality sharpen considerably: the ratio of electron-neutrino to tau-neutrino interactions, sensitive to the cross-sections of the two species, would be measured with a statistical uncertainty of about 6 percent in the Baseline and 2 percent in the Extended layout, compared with 30 percent in Run 3. Perhaps most tantalising, the Extended configuration could deliver the first direct observation of tau antineutrinos, which have never been seen experimentally: simulations predict roughly 120 tau antineutrino charged-current interactions in the target, about 9 observable signal events against a background of 2, yielding an observation significance of about 5 sigma, where the Baseline manages only around 3 sigma.

Beyond the Standard Model, the detector doubles as a hunting ground for feebly interacting particles such as dark photons, heavy neutral leptons and axion-like particles produced in the far-forward region, and the paper evaluates sensitivity to light dark matter scattering off protons via a leptophobic baryonic mediator. Here too the Extended geometry wins, since the larger acceptance and closer approach to the collision axis increase the dark matter flux intersecting the detector and extend the reach in mediator coupling into territory not excluded by other experiments. The collaboration also foresees roughly 600 to 1300 neutrino interactions coincident with events recorded by ATLAS, enabled by sub-50-picosecond timing synchronisation, which would directly link a neutrino interaction to its production mechanism. Whatever installation scenario is ultimately realised, the experience gained will feed directly into the SND detector at the proposed SHiP facility, for which the upgraded instrument serves as a full-scale prototype, making this modest tunnel experiment a testing ground for the next generation of collider neutrino physics.

Subject of Research: Upgrade of the SND@LHC forward neutrino detector for the High-Luminosity LHC, comparing installation scenarios and physics reach

Article Title: SND@LHC upgrade for the high-luminosity LHC: physics reach and installation scenarios

Article References: SND@LHC Collaboration, Abbaneo, D., Ahmad, S., Albanese, R., Alexandrov, A., Alicante, F., Aloschi, F., Androsov, K., Anokhina, A., Arellano, L. G., Asawatangtrakuldee, C., Torres, M. A. A., Bangaru, N., Battilana, C., Bay, A., Bertocco, A., Betancourt, C., Bick, D., Biswas, R., … Saá, J. Z. (2026). SND@LHC upgrade for the high-luminosity LHC: physics reach and installation scenarios. The European Physical Journal C, 86(9), Article 1030. https://doi.org/10.1140/epjc/s10052-026-15924-5

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-15924-5

Keywords: SND@LHC, High-Luminosity LHC, neutrinos, CERN, particle physics, forward physics, heavy-flavour decays, dark matter, lepton flavour universality, tau antineutrino, silicon trackers, magnetic spectrometer