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Observation of collider neutrinos without final state muons with the SND@LHC experiment

T0 review · 2 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read The SND@LHC experiment reports the observation of neutrino interactions with no muon in the final state at the LHC, with a significance of 6.4 sigma over an expected background of 0.32 events.

desk verdict First muon-less neutrino observation at the LHC is a solid 6.4-sigma result; the main caveat is an unvalidated spectral cutoff in the neutral-hadron background. read the letter →

arxiv 2411.18787 v1 pith:5DLODSR3 submitted 2024-11-27 hep-ex

SND@LHC Collaboration This is my paper · ORCID
classification hep-ex
keywords neutrinointeractionsLHCSND@LHCelectronneutralcurrentdeepinelasticscatteringforwardphysicsmuon-lessevents
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports the first observation of neutrino interactions at the Large Hadron Collider that leave no muon in the final state. Using 68.6 inverse femtobarns of proton-proton collisions at 13.6 TeV, the SND@LHC experiment finds 9 candidate events against an expected background of 0.32 events, for a 6.4 sigma excess. The selected sample consists mainly of electron-neutrino charged-current and neutral-current interactions. If correct, this establishes that all three neutrino flavours can be studied directly at a hadron collider, not just muon neutrinos, and it is the first step toward flavour classification with this detector.

What carries the argument

The analysis hinges on a discriminating variable called the sum of SciFi hit-density weights, computed for each scintillating-fibre station by counting, for every hit, the number of other hits within 1 cm in the same plane. Neutrino-induced showers produce dense hit clusters and therefore large weight sums, while low-energy neutral hadrons produce sparse patterns. The highest sum among the five SciFi stations is used to define a signal region, with a threshold of 11,000, and a control region below 5,000 that constrains the neutral-hadron background from muon deep-inelastic scattering in the tunnel walls. The selection also requires no hits in the two most downstream muon-system planes, rejecting reconstructible muons and reducing the muon-neutrino background by a factor of 14.

What would settle it

Measure the side-exit muon rate directly: select events that pass the nu0mu selection but keep the final two downstream muon planes active, reconstruct the muon angle for each, and compare the fraction exiting through the detector sides with the simulation used to set the 0.30-event background. A side-exit fraction more than 18.2% larger than simulated would reduce the 6.4 sigma significance below the claimed level.

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Extended reading notes

Core claim

The central claim is that muon-less neutrino interactions, where a neutrino scatters in the detector and produces a shower without a reconstructed outgoing muon, occur in LHC collision data at a rate far above background. The collaboration observes 9 such events in a signal region defined by high SciFi hit density, with an expected signal of 7.2 events and a background of 0.32 events, dominated by muon-neutrino charged-current interactions whose muon escapes through the detector sides. The resulting significance is 6.4 sigma, larger than the 5.5 sigma expected from simulation. Constraining the neutral-current component with the previously measured muon-neutrino charged-current rate yields evidence for electron-neutrino charged-current interactions at 3.7 sigma. This is reported as the first demonstration of flavour classification by the SND@LHC experiment.

Load-bearing premise

The result depends on the estimated background of 0.32 events, and in particular on the 0.30 events expected from muon-neutrino charged-current interactions whose muon escapes through the detector sides; if that rate were substantially underestimated, the 6.4 sigma significance would be correspondingly reduced.

Editorial extensions

If this is right

  • The same data set yields evidence for electron-neutrino charged-current interactions at 3.7 sigma, and the method can be extended to the 2024 dataset for a direct electron-neutrino observation.
  • The event sample opens the door to separating electron neutrinos, tau neutrinos, and neutral-current interactions at a hadron collider, a first step toward flavour-resolved LHC neutrino physics.
  • The observed rate is consistent with the Standard Model expectation of 7.2 signal events, so no new physics is required to explain the excess.
  • The control-region method, validated with test-beam hadron data, provides a template for rejecting neutral-hadron backgrounds in future forward-neutrino detectors.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the 6.4 sigma observation holds with more data, the same hit-density technique could be adapted to measure the neutral-current to charged-current ratio in the forward region, providing a test of electroweak couplings at TeV energies.
  • The background estimate rests on a single muon-tagged control; if future data reveal that side-escaping muons are more frequent than simulated, the significance would shrink, a possibility that can be checked by loosening the DS cut and counting muon-tagged events in the same fiducial volume.
  • With the 2024 data set and the emulsion target, a direct electron-neutrino charged-current detection should be within reach, and the present sample already hints at the flavour composition that the emulsion analysis will confirm.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The paper reports a search for neutrino interactions with no final-state muon (ν0μ events) using 68.6 fb^-1 of p-p collision data recorded by SND@LHC in 2022–2023. The analysis selects shower-like events in the SciFi tracker without reconstructed muons, defines a signal region using a hit-density weight sum, and uses a control region to constrain neutral-hadron backgrounds. Nine candidate events are observed against an expected background of 0.32 events, corresponding to a claimed observation significance of 6.4σ (5.5σ expected). A secondary result extracts evidence for νe charged-current interactions at the 3.7σ level by constraining the NC component with the previously measured νμ CC rate. The paper concludes that this is the first demonstration of flavor classification at a hadron collider neutrino experiment.

Significance. If the result holds, this is a landmark observation: it establishes that SND@LHC can detect neutrino flavors other than νμ in the LHC beam dump, opening the path to νe and ντ physics at colliders. The analysis uses a standard profile-likelihood approach, includes a control region for the dominant neutral-hadron background, and benefits from a previous independent νμ CC measurement for normalizing the largest background component. The paper also provides a test-beam validation of the discriminating variable and is transparent about the assumptions in the background extrapolation. The 6.4σ significance is strong, and the methodology is reproducible in principle.

major comments (2)
  1. [Event selection and data analysis methods, paragraph beginning 'A control region...'] The neutral-hadron background in the signal region is obtained by extrapolating a control region defined by summed SciFi hit-density weights below 5×10^3, using a dedicated simulation of neutrons and neutral kaons from muon DIS in the tunnel walls. The simulation predicts that no neutral hadrons above 100 GeV interact in the target without Veto hits, which yields the 1.5×10^-2 expectation in the signal region. The control region cannot validate this spectral cutoff because any event with a weight sum above 11×10^3 is outside the control band by construction, and the test-beam data in Fig. 1 are at 100 GeV/c, below the energies relevant for the tail. The 100% systematic assigned to this background is applied to the 1.5×10^-2 number and therefore does not cover an order-of-magnitude shape uncertainty. Since the total background is 0.32 events and 9 events are observed, a few tenths of an additional neutral-hadron event would move the significance below 5σ. The authors should provide a data-driven check of the extrapolation, for example by comparing data and simulation in the intermediate region (5–11)×10^3, or by inflating the background uncertainty to reflect the unmeasured spectral tail.
  2. [Results, Table I and surrounding text] The νμ CC background with the muon escaping through the detector sides (0.30 events) is the dominant background, and its estimate relies on the previous SND@LHC νμ CC measurement [4] and on the DS tracking systematic [21]. These cover the overall normalization, but the simulation of the hit-density distribution for these events after the sparse-shower removal and the signal-region threshold is not validated at the high-weight tail. A shape uncertainty in this distribution would directly affect the 0.30 background in the signal region. The authors should state whether any shape systematic on the νμ CC background is included, and if not, why the normalization uncertainty is sufficient for a cut-and-count analysis with a hard threshold.
minor comments (4)
  1. [Event selection, formula for hit weight] The word 'Heavyside' in the definition of the step function should be 'Heaviside'.
  2. [Event selection, control region definition] The control region is described as 'summed SciFi hit-density weights smaller than 5×10^3', but because the preselection already requires the sum to exceed 2×10^3 in at least one station, the control region in practice covers a finite interval; state this explicitly.
  3. [Event selection, signal-region optimization] The paper does not state that no trial factor is applied for the scan over signal-region thresholds; a sentence clarifying that the threshold was chosen from simulation-only expected sensitivity would help the reader interpret the observed significance.
  4. [Results, Table I] Table I lists only the ν1μ simulation in the final rows, while the text gives the neutral-hadron and ντ backgrounds separately; adding these columns or a note would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 6.4 sigma nu0mu observation is an event-count excess over independent background estimates, not a fitted derivation.

full rationale

The central derivation is an event-counting measurement: 9 candidate events are observed in a signal region defined by simulated sensitivity, against a total expected background of 0.32 events. The dominant background, 0.30 nu_mu CC events, is anchored to the SND@LHC Collaboration's previous measurement of nu_mu charged-current interactions (Ref. [4]) plus a DS tracking systematic (Ref. [21]). This is independent support rather than circularity: Ref. [4] is a separate published measurement using a different, muon-tagged event selection and an earlier 36.8 fb^-1 data subset, and the quantity claimed here (neutrino interactions without final-state muons) is not used as an input to construct that background estimate. The neutral-hadron background of 1.5e-2 events is derived from a Pythia6 muon-DIS simulation constrained by the 10-event control region; no parameter is fitted directly to the 9 events in the signal region, and the assigned 100% systematic uncertainty reflects the extrapolation. The paper itself notes the simulation-based assumption that no neutral hadrons above 100 GeV interact in the target without Veto hits; this is a legitimate modeling uncertainty (and the dominant potential weakness of the background estimate) but it is not a circular step, because the signal-region expectation is not defined in terms of the observed signal count. The 3.7 sigma evidence for nu_e CC is obtained by constraining the NC component using the independent nu_mu CC measurement, a standard subtraction, not by redefining the observable. No fitted parameter is renamed as a prediction, and no load-bearing argument reduces to a self-citation that is itself unverified. Significance estimation uses a one-sided profile likelihood with the stated background model; the observed excess is not forced by construction. The appropriate circularity score is 0.

Assumptions & free parameters 3 free parameters · 7 assumptions · 0 invented entities

The analysis rests on established simulation packages (DPMJET-III, Fluka, GENIE, PYTHIA6, Geant4/FairRoot) and prior SND@LHC and FASER measurements. The three listed free parameters are analysis choices and the fitted signal strength, not new physics parameters. No new particles, forces, or conserved quantities are introduced.

free parameters (3)
  • Signal-region threshold on SciFi hit-density weight sum = 11 x 10^3
    Chosen by scanning thresholds from 5 x 10^3 to 20 x 10^3 and maximizing expected significance using simulation only; determines the observed 9 events and 0.32 background.
  • Control-region upper boundary on hit-density weight sum = 5 x 10^3
    Defines the background-rich region used to constrain the neutral-hadron background; the boundary choice affects the sideband extrapolation.
  • Signal strength alpha in profile likelihood = 9 observed events
    Standard parameter of interest for extracting the significance; fitted to data, not a model parameter.
assumptions (7)
  • domain assumption Forward neutrino and hadron fluxes at SND@LHC are modeled by DPMJET-III interfaced with a Fluka model of the LHC.
    Section 'Data and simulation' uses this generator setup for signal and background predictions; the flux model is not independently verified in this paper.
  • domain assumption GENIE simulates neutrino-nucleus interactions using Standard Model deep-inelastic-scattering cross sections.
    Section 'Data and simulation'; the expected signal composition (4.9 nu_e CC, 2.2 NC, 0.1 nu_tau CC) depends on these cross sections.
  • domain assumption PYTHIA6 simulates muon deep-inelastic-scattering interactions in the tunnel material.
    Section 'Data and simulation'; used to set the neutral-hadron background upper limit and to generate neutral hadron samples.
  • domain assumption Geant4/FairRoot detector simulation, validated by hadron test-beam data, reproduces the detector response to showers.
    Section 'Data and simulation' and Figure 1; the hit-density weight distribution for 100 GeV/c hadrons is used to validate the discriminating variable.
  • domain assumption Veto inefficiency during two unaligned timing periods has negligible impact on this analysis.
    Section 'Data and simulation'; relies on downstream detectors rejecting penetrating muons, not directly demonstrated for the degraded periods.
  • ad hoc to paper The neutral-hadron background in the signal region can be extrapolated from the control region using the high-statistics neutral-hadron simulation.
    Section 'Event selection and data analysis methods'; a 100% systematic is assigned to cover the extrapolation, indicating the assumption is not tightly constrained.
  • domain assumption The NC component of the nu0mu signal is constrained using the nu_mu CC measurement of Ref [4], with uncertainties fully correlated.
    Section 'Results'; this constraint is used to extract the 3.7 sigma nu_e CC evidence.

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Cite this review

Pith. "Pith review of Observation of collider neutrinos without final state muons with the SND@LHC experiment." pith.science (2026). https://pith.science/paper/5DLODSR3

@misc{pith2026241118787,
  author       = {Pith},
  title        = {Pith review of: Observation of collider neutrinos without final state muons with the SND@LHC experiment},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5DLODSR3}},
  note         = {Machine review of arXiv:2411.18787}
}
abstract

We report the observation of neutrino interactions without final state muons at the LHC, with a significance of 6.4$ \sigma$. A data set of proton-proton collisions at $\sqrt{s}= 13.6$ TeV collected by SND@LHC in 2022 and 2023 is used, corresponding to an integrated luminosity of 68.6 fb$^{-1}$. Neutrino interactions without a reconstructed muon are selected, resulting in an event sample consisting mainly of neutral-current and electron neutrino charged-current interactions in the detector. After selection cuts, 9 neutrino interaction candidate events are observed with an estimated background of 0.32 events.

Figures

Figures reproduced from arXiv: 2411.18787 by the authors.

Figure 1
Figure 1. FIG. 1. Distribution of the sum of SciFi hit-density weights [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Representative example of a signal-like event. The top panel shows a top-down view of the detector and the bottom [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Distribution of the sum of SciFi hit-density weights [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Forward citations

Cited by 2 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. A SHIFT of Perspective: Observing Neutrinos at CMS and ATLAS

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    A proposed LHC gas target would give CMS/ATLAS O(10^4) muon-neutrino and O(10^3) electron-neutrino interactions, opening forward hadron-production physics to neutrino detection.

  2. Neutrino Experiments at the Large Hadron Collider

    hep-ex 2025-01 accept novelty 2.0 of 10

    A review of LHC collider neutrino physics, covering FASER and SND@LHC results, present measurements, and future plans including the Forward Physics Facility.

Reference graph

Works this paper leans on

21 extracted references · 7 canonical work pages · cited by 2 Pith papers

  1. [4]

    Albanese et al

    R. Albanese et al. (SND@LHC), Observation of collider muon neutrinos with the SND@LHC experiment, Phys. Rev. Lett. 131, 031802 (2023), arXiv:2305.09383 [hep- ex]

    arXiv 2023

  2. [21]

    Albanese et al

    R. Albanese et al. (SND@LHC), Measurement of the muon flux at the SND@LHC experiment, Eur. Phys. J. C 84, 90 (2024), arXiv:2310.05536 [hep-ex]

    arXiv 2024

  3. [1]

    Physics Potential of an Experiment using LHC Neutrinos

    N. Beni et al. , Physics potential of an experiment us- ing LHC neutrinos, J. Phys. G 46, 115008 (2019), arXiv:1903.06564 [hep-ex]

    work page Pith review arXiv 2019

  4. [2]

    Beni et al

    N. Beni et al. , Further studies on the physics potential of an experiment using LHC neutrinos, J. Phys. G 47, 125004 (2020), arXiv:2004.07828 [hep-ex]

    arXiv 2020

  5. [3]

    Kling and L

    F. Kling and L. J. Nevay, Forward neutrino fluxes at the LHC, Phys. Rev. D 104, 113008 (2021), arXiv:2105.08270 [hep-ph]

    arXiv 2021

  6. [5]

    Abreu et al

    H. Abreu et al. (F ASER), First direct observation of col- lider neutrinos with F ASER at the LHC, Phys. Rev. Lett. 131, 031801 (2023), arXiv:2303.14185 [hep-ex]

    arXiv 2023

  7. [6]

    Mammen Abraham et al

    R. Mammen Abraham et al. (F ASER), First Measure- ment of νe and νµ Interaction Cross Sections at the LHC with F ASER’s Emulsion Detector, Phys. Rev. Lett.133, 021802 (2024), arXiv:2403.12520 [hep-ex]

    arXiv 2024

  8. [7]

    Acampora et al

    G. Acampora et al. (SND@LHC), SND@LHC: the scat- tering and neutrino detector at the LHC, JINST 19 (05), P05067, arXiv:2210.02784 [hep-ex]

    arXiv

Show all 21 references
  1. [8]

    Aad et al

    G. Aad et al. (ATLAS), Luminosity determination in pp collisions at √s = 13 TeV using the ATLAS de- tector at the LHC, Eur. Phys. J. C 83, 982 (2023), arXiv:2212.09379 [hep-ex]

    2023 arXiv

  2. [9]

    Preliminary analysis of the luminosity calibration for the ATLAS 13.6 TeV data recorded in 2023, ATL-DAPR- PUB-2024-001 https://cds.cern.ch/record/2900949 (2024)

    2024

  3. [10]

    Roesler, R

    S. Roesler, R. Engel, and J. Ranft, The Monte Carlo event generator DPMJET-III, in Advanced Monte Carlo for radiation physics, particle transport simulation and applications. Proceedings, Conference, MC2000, Lisbon, Portugal, October 23-26 (2000) p. 1033, arXiv:hep- ph/0012252...

    2000

  4. [11]

    Fedynitch, Cascade equations and hadronic inter- actions at very high energies, CERN-THESIS-2015-371 https://cds.cern.ch/record/2231593 (2015)

    A. Fedynitch, Cascade equations and hadronic inter- actions at very high energies, CERN-THESIS-2015-371 https://cds.cern.ch/record/2231593 (2015)

    2015

  5. [12]

    Battistoni et al., Overview of the Fluka code, Annals of Nuclear Energy 82, 10 (2015)

    G. Battistoni et al., Overview of the Fluka code, Annals of Nuclear Energy 82, 10 (2015)

    2015

  6. [13]

    Ahdida et al

    C. Ahdida et al. , New capabilities of the Fluka multi- purpose code, Front. in Phys. 9, 788253 (2022)

    2022

  7. [14]

    Boccone et al

    V. Boccone et al. , Beam-machine interaction at the CERN LHC, Nucl. Data Sheets 120, 215 (2014)

    2014

  8. [15]

    Andreopoulos et al

    C. Andreopoulos et al. , The GENIE neutrino Monte Carlo generator, Nucl. Instrum. Meth. A 614, 87 (2010), arXiv:0905.2517 [hep-ph]

    2010 arXiv

  9. [16]

    Sjostrand, S

    T. Sjostrand, S. Mrenna, and P. Z. Skands, Pythia 6.4 physics and manual, JHEP 05, 026, arXiv:hep- ph/0603175 [hep-ph]

  10. [17]

    Agostinelli et al

    S. Agostinelli et al. (Geant4), Geant4: A Simulation toolkit, Nucl. Instrum. Meth. A 506, 250 (2003)

    2003

  11. [18]

    Al-Turany et al

    M. Al-Turany et al. , F airRoot, Zenodo 10.5281/zen- odo.11210174 (2024)

    2024 doi

  12. [19]

    Moneta et al

    L. Moneta et al. , The RooStats project, PoS ACA T2010, 057 (2010), arXiv:1009.1003 [physics.data- an]

    2010 arXiv

  13. [20]

    C. Vilela (SND@LHC), Recent results from the SND@LHC experiment [Conference presentation], Ren- contres de Moriond QCD and high-energy interactions https://moriond.in2p3.fr/2024/QCD/ (2024)

    2024

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