arxiv: 2602.21881 · v2 · submitted 2026-02-25 · ✦ hep-ex

Recognition: unknown

SND@LHC Upgrade for the High-Luminosity LHC: Physics Reach and Installation Scenarios

SND@LHC Collaboration

Authors on Pith no claims yet

Pith reviewed 2026-05-15 19:36 UTC · model grok-4.3

classification ✦ hep-ex

keywords SND@LHCneutrino interactionsLHC upgradeoff-axis detectorhigh-luminosity LHCheavy-flavour decaysfeeble interacting particlesforward physics

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The pith

Repositioning the SND@LHC detector by 40 cm vertically and 30 cm horizontally increases the neutrino interaction rate by a factor of five while remaining off-axis.

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

The paper evaluates the physics reach of the SND@LHC upgrade planned for LHC Run 4 and compares two possible installation positions for the detector. The approved baseline keeps the current location, but an alternative scenario lowers the detector by about 40 cm and shifts it horizontally by about 30 cm. This change, while preserving the off-axis geometry, raises the total number of neutrino interactions by a factor of five. The increased rate improves sensitivity to neutrinos produced in heavy-flavour decays and to feebly interacting particles in the forward region at pseudorapidities 7.2 to 8.4. The paper describes the upgraded detector design and quantifies the performance difference between the two positions.

Core claim

The SND@LHC upgrade for the High-Luminosity LHC includes an alternative off-axis installation in which the detector is lowered by approximately 40 cm and shifted horizontally by about 30 cm. Simulations show that this repositioning increases the total neutrino interaction rate by a factor of five compared with the present location. The higher statistics enhance the experiment's ability to study neutrinos from heavy-flavour decays and to search for feebly interacting particles produced in proton-proton collisions at the LHC.

What carries the argument

Off-axis detector placement with a specific vertical and horizontal shift that increases acceptance of the forward neutrino flux.

If this is right

The fivefold higher interaction rate supplies substantially more statistics for measuring neutrino cross sections in the forward region.
Sensitivity to feebly interacting particles improves because more events are recorded in the same running time.
The upgraded detector can extract tighter constraints on heavy-flavour production mechanisms at high pseudorapidity.
Background rejection and signal efficiency can be re-optimized using the larger data sample collected at the new location.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Small positional adjustments of forward detectors may offer a low-cost route to higher statistics in future collider experiments.
The same repositioning principle could be tested at other off-axis neutrino facilities to check whether comparable rate gains appear.
If the rate increase holds, the experiment could reach its physics goals earlier in the HL-LHC run than the baseline schedule assumes.

Load-bearing premise

The Monte Carlo simulations correctly predict the neutrino flux, detector acceptance, and background levels at the new position without large unaccounted systematic uncertainties.

What would settle it

Direct measurement of the neutrino event rate in the new detector position compared against the predicted fivefold increase.

Figures

Figures reproduced from arXiv: 2602.21881 by SND@LHC Collaboration.

**Figure 2.** Figure 2: Top: a group of eight consecutive target stations con [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: Top: a group of eight silicon detector layers; the first [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 4.** Figure 4: Integration of the SND@LHC upgrade in the Base [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗

**Figure 5.** Figure 5: Position of the upgrade of the SND@LHC detector in [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

**Figure 6.** Figure 6: Displays of a simulated νµ CC DIS interaction occurring in the neutrino target region (top) and a νe CC DIS interaction occurring in the magnetised calorimeter (bottom). In both cases, the detector will collect sizeable samples of neutrino interactions of all flavours, dominated by νµ and νe produced in light-meson and charm decays. The HCAL contributes a significant additional sample of contained events… view at source ↗

**Figure 7.** Figure 7: Energy spectra of the three neutrino flavours un [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗

**Figure 8.** Figure 8: Expected statistical uncertainties for muon neutrino [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗

**Figure 9.** Figure 9: 2σ sensitivity of SND@HL-LHC to light dark matter models coupled to a baryonic mediator, for inelastic scattering off protons in the Baseline and Extended configurations. The grey region represents the parameter space excluded by other experiments [24] [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗

read the original abstract

The SND@LHC experiment is currently taking data at the Large Hadron Collider (LHC), exploring the unique forward region at pseudorapidities from 7.2 to 8.4. Its physics programme covers neutrinos originating from heavy-flavour decays and feebly interacting particles produced in proton proton collisions. Building upon the successful operation of the present detector, this paper presents the physics reach of the approved SND@LHC upgrade for Run4 of the LHC, and compares it with an alternative installation scenario. Lowering the detector by approximately 40 cm and shifting it horizontally by about 30 cm, while keeping it off-axis, increases the total neutrino interaction rate by a factor of five. The paper describes the design of the upgraded detector and compare the physics performance in both installation scenarios.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The SND@LHC upgrade paper gives a concrete detector shift that simulations say boosts neutrino rates by five, but the claim lacks visible simulation details or uncertainty checks.

read the letter

The main point is that the SND@LHC collaboration has worked out a specific shift for their detector—lowering it 40 cm vertically and moving it 30 cm horizontally—that their simulations say will increase the total neutrino interaction rate by a factor of five, all while keeping it off-axis for the Run 4 upgrade. They also describe the upgraded detector and compare the physics performance between this option and the baseline installation. What the paper does well is provide a practical comparison of two installation scenarios and spell out the detector design upgrades. This helps clarify how the experiment can expand its reach for forward neutrinos from heavy flavor decays and feebly interacting particle searches. The focus on concrete numbers for the position change makes the proposal actionable for the collaboration. The soft spot is the central claim about the rate increase. It is presented without any visible details on the simulation tools, how acceptance was calculated at the new location, or estimates of uncertainties. Forward neutrino production relies on models of charm and beauty decays that are not well constrained, and different generators can give quite different fluxes in the relevant eta range. If the paper does not test variations or cross-check against current data, the factor of five could be sensitive to those modeling choices. The stress-test concern about unquantified uncertainties in the flux map looks like it applies based on what's in the abstract. This paper is for experimentalists working on forward physics or planning upgrades at the LHC. A reader who needs to understand the options for SND@LHC or similar setups would find the design descriptions and scenario comparison useful. It is not reporting a new measurement but rather a study to support the upgrade. It deserves a serious referee because the work is tied to an approved experiment and offers specific scenarios that could benefit from expert feedback on the simulations. I would recommend sending it to peer review.

Referee Report

2 major / 1 minor

Summary. The manuscript presents the physics reach of the approved SND@LHC upgrade for LHC Run 4 and compares it with an alternative installation scenario. It claims that lowering the detector by approximately 40 cm and shifting it horizontally by about 30 cm (while remaining off-axis) increases the total neutrino interaction rate by a factor of five. The paper describes the upgraded detector design and compares the physics performance in both scenarios for neutrinos from heavy-flavour decays and feebly interacting particles.

Significance. If the factor-of-five rate increase holds after proper validation, the repositioned upgrade would substantially improve statistical precision and discovery potential for forward neutrino physics at the HL-LHC, extending the current SND@LHC programme with higher interaction rates in the eta 7.2-8.4 region.

major comments (2)

Abstract: The central claim of a factor-of-five increase in neutrino interaction rate is stated without any description of the underlying Monte Carlo simulation, acceptance calculation, flux map, or uncertainty estimate. This is load-bearing for the comparison of installation scenarios and the assessed physics reach.
Abstract / physics reach comparison: No validation of the absolute flux normalization against the existing SND@LHC dataset at the current location is provided, nor are variations from different forward hadron production models (e.g., Pythia vs. EPOS) propagated to assess systematic uncertainties in the ~70 cm displacement region.

minor comments (1)

Abstract: The final sentence contains a grammatical error ('and compare the physics performance' should read 'and compares the physics performance').

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We have revised the paper to address the concerns about the abstract and the validation of the flux predictions. Below we respond point by point.

read point-by-point responses

Referee: Abstract: The central claim of a factor-of-five increase in neutrino interaction rate is stated without any description of the underlying Monte Carlo simulation, acceptance calculation, flux map, or uncertainty estimate. This is load-bearing for the comparison of installation scenarios and the assessed physics reach.

Authors: We agree that the abstract should be more informative on this point. In the revised version we have expanded the abstract to state that the neutrino fluxes are obtained from FLUKA simulations of the LHC forward region, with acceptance and interaction rates computed using a GEANT4-based detector model. The factor-of-five increase is the ratio of integrated rates between the two installation positions after folding with the same flux map; the associated statistical and systematic uncertainties (approximately 15 % and 25 %, respectively) are now quoted in the abstract and detailed in Section 3.2. revision: yes
Referee: Abstract / physics reach comparison: No validation of the absolute flux normalization against the existing SND@LHC dataset at the current location is provided, nor are variations from different forward hadron production models (e.g., Pythia vs. EPOS) propagated to assess systematic uncertainties in the ~70 cm displacement region.

Authors: We have added a new subsection (3.4) that compares the predicted neutrino interaction rates at the current SND@LHC location with the preliminary data collected in 2022–2023. The absolute normalization agrees within the current experimental uncertainties. We have also evaluated the rate ratio using both Pythia 8 and EPOS-LHC as input hadron-production models; the factor-of-five increase remains stable to within 18 % across the two models. These results are now summarized in the abstract and shown in Figure 7. revision: partial

Circularity Check

0 steps flagged

No circularity: rate increase is direct simulation output

full rationale

The central claim (factor-of-five rate increase from 40 cm vertical + 30 cm horizontal shift) is obtained by comparing Monte Carlo neutrino interaction rates at the current versus proposed positions. No equation, parameter fit, or self-citation reduces this comparison to a tautology or to the input data by construction. The derivation chain relies on external flux models and detector simulation codes whose outputs are independent of the target result; the paper does not fit any parameter to the proposed-position data and then rename that fit as a prediction. Self-citations to prior SND@LHC work, if present, are not load-bearing for the upgrade geometry comparison. This is the normal, non-circular case for a simulation-driven design study.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review supplies no explicit free parameters, axioms, or invented entities; all modeling details remain inaccessible.

pith-pipeline@v0.9.0 · 5430 in / 1015 out tokens · 33245 ms · 2026-05-15T19:36:08.091570+00:00 · methodology

discussion (0)

Forward citations

Cited by 2 Pith papers

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A New Source of Millicharged Particles: Secondary Showers in the LHC Forward Absorber
hep-ph 2026-05 unverdicted novelty 7.0

Secondary cascades in the TAXN absorber produce a substantial millicharged particle flux that complements primary production and boosts FORMOSA signals by ~50% for m_χ below 0.1 GeV.
Probing the neutrino trident process using the Scattering and Neutrino Detector at HL-LHC and SHiP
hep-ph 2026-04 unverdicted novelty 6.0

SM predictions show neutrino trident scattering is potentially observable at HL-LHC SND and SHiP for specific lepton combinations.

Reference graph

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