Neutrino Experiments at the LHC and the HL-LHC

This is a competent and accurate topical review of the LHC neutrino program through mid-2026. It reliably transmits the published Run 3 results from FASER and SND@LHC—I spot-checked the key numbers and they match the cited papers. The description of forward hadron production uncertainties in Sec 2.3 is honest and well laid out. The review gives a clear, useful overview for anyone new to the field.

The novelty is minimal. This is an explicit update of Ref [21] (Ariga et al., 2025), with one of the same co-authors. The new material is a compilation of 2024–2026 preliminary results from conference talks and CERN technical notes, plus a summary of HL-LHC upgrade concepts. The paper does not claim any new result—it is firmly a review.

The main soft spot: several results in Sec 4.4 (FASER's electronic-detector nu_e observation at 5.5 sigma, SND@LHC's updated zero-muon sample, etc.) come from conference talks and CERN notes that have not completed journal peer review. The review labels them as preliminary, which is appropriate, but a reader should check the final publications when they appear. A second concern: Tables 1 and 2 give predicted event yields with no uncertainties, despite Sec 2.3 quoting charm QCD scale uncertainties up to 50%. The authors could have added a footnote or a reference to the full study. This is a minor omission, not a fatal flaw.

The stress-test note correctly observes that the central claim—that Run 3 established collider neutrinos as detectable—rests on counting-experiment significances computed against expected background, not against forward-flux predictions. Those significances are robust. So the review's main conclusion holds up.

Who is this for? A graduate student or a colleague outside the field who wants a single up-to-date summary of where LHC neutrino physics stands. An expert will already know the published results but might find the compilation of recent preliminary numbers handy for reference.

I would accept peer review. This is a well-executed review that does what it sets out to do. The editor should treat it as a review article, not a research paper, and judge it on clarity, completeness, and accuracy rather than novelty.

The central assertion - that Run 3 established collider neutrinos as detectable at the LHC - is supported by counting-experiment observations whose significances are computed against expected background, not against forward-flux predictions. FASER's 153+12-13 nu_mu CC candidates (16 sigma, Ref [17]), SND@LHC's 8 events over 0.086 expected background (~7 sigma, Ref [18]), and the nu_0mu sample (9 events over 0.32 background, 6.4 sigma, Ref [56]) stand even if forward charm production is mis-modeled. The Sec 2.3 forward-hadron modeling limitation is real and correctly identified as the dominant systematic for flux normalization: it feeds the 'consistent with Standard Model' cross-section comparisons in Sec 4.2 and leaves Tables 1-2 without explicit uncertainty bands despite the quoted charm QCD scale uncertainties. But the review flags this explicitly, and the HL-LHC program is itself intended to constrain that flux, so the caveat is contained rather than load-bearing for the historical claim. The only residual risk is that the newest Sec 4.4 results cite CERN technical notes and 2026 conference talks not yet through journal review; the review labels these as preliminary, which is appropriate for a status report. No internal inconsistency or unsupported new claim was found.

I read the manuscript as a topical review whose factual spine is the first-observation and first-measurement results from FASER and SND@LHC. I checked the quoted yields and significances against the cited publications as far as possible; they match. The observations are background-limited counting measurements, so they do not depend on forward-hadron modeling; the flux enters only when converting event counts into cross sections or projecting HL-LHC yields. The review itself, in Sec 2.3, identifies forward charm production as the dominant uncertainty for quantitative interpretations, and the reader's weakest_assumption points at the correct systematic. However, the central historical claim - that collider neutrinos can be detected and studied at a hadron collider - would stand even if the charm flux were off by a factor of two, because the detections are established by background rejection and their role as a precision program is explicitly conditional on future flux constraints. The prospects sections are advocacy by program participants, but the central factual claims are independently published, and the review does not present a new physical result. I therefore find no load-bearing defect in the argument; UNVERDICTED for a non-research review remains appropriate and no verdict adjustment is needed.

The review provides a timely and comprehensive snapshot of a rapidly evolving field. If the reported results hold, the LHC is firmly established as a source of multi-TeV neutrinos, enabling a precision programme at the HL-LHC. The paper collects published, peer-reviewed counting-experiment observations whose significances are computed against expected background, not against forward-flux predictions, so the central claim is robust even if the flux modelling uncertainties (discussed in §2.3) are large. The review correctly identifies the forward hadron production modelling as the dominant systematic for flux normalization and cross-section comparisons, and it references the ongoing efforts to constrain it. As a review article, it does not present new original results, but it updates Ref. [21] and includes the most recent preliminary results from 2026 conferences, with appropriate caveats. It will serve as a useful reference for the collider neutrino community.

This review asserts that collider neutrino physics has moved from concept to reality. During LHC Run 3, the FASER and SND@LHC experiments obtained the first direct observations of neutrino interactions at a hadron collider, at energies up to several TeV. These measurements include the first TeV-scale neutrino–nucleon cross sections and the detection of multiple neutrino flavors. The paper argues that these results establish the LHC as a source of high-energy neutrinos and pave the way for a precision program at the HL-LHC, with upgrades already approved. The central implication is that the far-forward region at the LHC has become a laboratory for neutrino physics, QCD, and beyond-the-Standard-Model searches.

What carries the argument. The key machinery is the forward neutrino flux produced in pp collisions at the LHC: collimated near the beam axis and dominated at high energies by decays of charm and bottom hadrons. Dedicated detectors placed ~480 m downstream of the ATLAS interaction point — FASER on the collision axis and SND@LHC off-axis — intercept this flux using dense tungsten targets with nuclear emulsion for micron-scale vertex reconstruction, backed by electronic spectrometers, calorimeters, and muon identification. The on-axis and off-axis configurations provide complementary acceptance and handle the dominant muon-induced backgrounds.

Core claim. The central discovery reviewed is that neutrino interactions can be cleanly observed and studied in the forward direction at the LHC, despite the challenging hadron-collider environment. The first direct observation of collider neutrinos was reported by FASER (153+12−13 νμ CC candidate events, 16σ) and SND@LHC (8 events, 0.086 expected background, ∼7σ). Subsequently, the first TeV-energy-range νe and νμ charged-current cross sections were measured by FASERν, consistent with Standard Model predictions. SND@LHC further observed neutrino interactions without a final-state muon (6.4σ), opening access to NC and νe channels. These results collectively demonstrate that the LHC forward region provides a viable and rich neutrino source for future precision studies.

The premise it rests on. The load-bearing premise is that the forward neutrino flux can be reliably predicted from hadron production models, especially for charm hadrons at large rapidity, where QCD scale uncertainties reach ~50% and can shift the absolute normalization of all extracted cross sections.

The HL-LHC will yield large neutrino samples (e.g., ~10⁴ νμ CC events in upgraded detectors), enabling differential cross sections, flavor ratios, and antineutrino separation.

Collider neutrino data will constrain forward hadron production models — especially charm production — which directly impact predictions for atmospheric neutrino backgrounds in IceCube and KM3NeT.

The experiments serve as dual-purpose platforms: they also search for light, feebly interacting particles (dark photons, axion-like particles, heavy neutral leptons) with world-leading sensitivity.

ντ interactions, otherwise difficult to study, become accessible via the charm-decay flux, allowing tests of lepton flavor universality at TeV energies.

Cross-section measurements at TeV energies provide benchmarks for high-energy astrophysical neutrino detectors and for future collider facilities (FCC, muon collider).

If forward charm production uncertainties (quoted at up to ~50% for scale variations) are not reduced by dedicated measurements like DsTau/NA65, the absolute flux normalization for νe and ντ will remain the dominant systematic for cross-section extractions in all HL-LHC projections.

The complementarity between on-axis (FASER) and off-axis (SND@LHC) detectors could be exploited to isolate systematic effects from beamline geometry and muon background modeling, strengthening combined flux and cross-section results.

A decision to build the Forward Physics Facility (FPF) would enlarge the program by a factor of 5–20 in event yield and enable detector redundancy; the paper implicitly assumes that the tunnel-based upgrades (FASERν2, SND@HL-LHC) are the baseline for Run 4, but the FPF remains a more ambitious option whose feasibility studies are ongoing.

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