SHIFT@LHC: Fixed-Target Experiment

• SHIFT@LHC is a proposed fixed-target experiment that repurposes LHC infrastructure to generate high-statistics 113 GeV collisions for novel physics searches.
• It targets long-lived, weakly interacting BSM particles—including dark photons and hidden valley states—using a gaseous target and advanced trigger techniques.
• The design integrates fixed-target methods with existing detectors to deliver precise neutrino cross-section data and complement standard collider operations.

SHIFT@LHC (Shifted Interaction on a Fixed Target at the LHC) designates a proposed experimental program that repurposes existing infrastructure at the LHC by installing a gaseous fixed target in the LHC tunnel, specifically around 160 m from the CMS (and potentially ATLAS) interaction point. This scheme leverages the intense 6.8 TeV proton beam and a relatively modest target and associated technical investments to enable high-statistics fixed-target collisions at a center-of-mass energy of approximately 113 GeV. The resulting particles, including neutrinos and potential long-lived exotic states, propagate through rock and infrastructure and are detected in the central LHC detectors. SHIFT@LHC dramatically expands the accessible region of parameter space for new physics—especially for long-lived, weakly coupled, and low-mass states—and provides a novel, high-energy neutrino beam for detailed cross-section and hadron production studies in a forward kinematic regime relevant for atmospheric neutrino research (Niedziela, 2024, Garcia-Soto et al., 13 Oct 2025).

1. Experimental Concept and Technical Implementation

SHIFT@LHC proposes the installation of a gaseous fixed target, operationally similar to the SMOG/SMOG2 systems at LHCb, but positioned ~160 m upstream or downstream from a general-purpose detector such as CMS. The design locates the gas target in a region of the tunnel with minimal obstructions, enabling secondary particles to travel almost collinearly with the beam axis toward the detector. The key ingredients are:

• Gaseous Target Placement: A localized, low-pressure gas (e.g., hydrogen or noble gases) fills a region near the beam pipe, facilitating proton–nucleus collisions at rest-frame √s ≈ 113 GeV for a 6.8 TeV beam.
• Retention of Standard LHC Operations: SHIFT@LHC is planned to operate parasitically, using a small fraction (∼1%) of the LHC luminosity, with minor impact on ongoing high-luminosity pp collision data-taking.
• Geometric Acceptance and Shielding: The large spatial separation from CMS (∼160 m of intervening LHC infrastructure and rock) naturally attenuates Standard Model backgrounds, especially QCD-induced high-multiplicity events.
• Detector Readout and Triggering: Particles from the shifted vertex arrive with non-standard time-of-flight and angular signatures. Dedicated trigger algorithms or modifications of cosmic/dimuon triggers are envisaged.

This approach is cost-effective (requiring only a modest target installation and no new purpose-built detector) and makes optimal use of existing instruments with little or no interference with standard LHC operation (Niedziela, 2024, Garcia-Soto et al., 13 Oct 2025).

2. Physics Motivation and BSM Discovery Potential

SHIFT@LHC targets feebly-coupled, long-lived, low-mass new physics scenarios that are difficult to probe in traditional collider configurations. The program explicitly benchmarks two classes of BSM models:

• Dark Photon Models (DP/A′): These entail a minimal extension with a new U(1) gauge boson A′ of mass 5–30 GeV and small vector and axial couplings to SM fermions. The DP is forced to decay to a dimuon pair, enabling distinctive resonance and displaced-vertex searches.
• Hidden Valley (HV) Scenarios: Here, a heavier Z′ is produced and decays to dark quarks, which hadronize into dark hadrons. These can then decay to muon pairs after traversing significant distances, creating unusual kinematic distributions in the muonic channel.

Key features enabled by the fixed-target configuration include:

• Access to long lifetimes and extended decay distances (hundreds of meters): Standard pp collision searches are typically only sensitive to decay vertices within a meter-scale fiducial region, whereas SHIFT acceptance encompasses late decays occurring all along the path to the central detector.
• Forward-kinematics acceptance: The geometry is sensitive to particles produced at very high pseudorapidity, a region favored for certain BSM processes and not covered in central LHC analyses.
• Suppression of backgrounds: The rock acts as an effective muon and hadron filter, greatly reducing combinatorial and QCD-induced backgrounds.
• Competitive reach compared to dedicated LLP experiments: The projected sensitivity for dark photons improves on traditional CMS analyses by up to 150×, and for Hidden Valley signals by up to 1000×, all with just 1% of CMS luminosity (Niedziela, 2024).

3. Simulation Methodology and Acceptance Studies

The evaluation of SHIFT@LHC’s reach employs a detailed simulation chain:

• Event Generation: PYTHIA8 is used to model proton–gas collisions at 6.8 TeV beam energy, including hard processes, fragmentation, and BSM decay chains as specified by input scenarios (e.g., dark photons with specified couplings).
• Particle Propagation: Secondary hadrons, muons, and neutrinos are tracked through the LHC tunnel and surrounding rock using GEANT4. Energy thresholds derived from analytic formulae (e.g., E₍crit₎^μ = a·d₍crit₎^μ + b with a ≈ 0.5 GeV/m, b ≈ 1 GeV) determine survival probabilities to CMS for muons produced far upstream.
• Vertex and Angular Constraints: The acceptance is limited by requiring that decay products strike the fiducial detector volume (cylinder with R_D^O ≈ 7.5 m, η_max ≈ 2.4), and that decays occur within a broad range of distances from the target up to hundreds of meters.
• Trigger/Reco Considerations: Anomalous time-of-flight and shallow incident angles demand dedicated reconstruction strategies.

This multi-step approach supports robust reach estimates across dimuon invariant mass windows (e.g., 11–60 GeV), lifetime acceptance, and comparison with backgrounds (Niedziela, 2024, Garcia-Soto et al., 13 Oct 2025).

4. Neutrino Physics Capabilities

SHIFT@LHC provides unprecedented access to forward, high-energy neutrino production and detection:

• Flux and Interaction Rates: Simulations predict O(10^4) muon-neutrino and O(10^3) electron-neutrino charged-current (CC) events in CMS/ATLAS for 1% of Run-4 integrated luminosity, spanning 20 GeV–1 TeV in neutrino energy. This sample is obtained by considering hadron decays in the gas target, their attenuation and propagation, and final neutrino interactions in the complex calorimetric environment.
• Kinematic Coverage: The detectable neutrinos predominantly originate from hadrons decaying at small angles (pseudorapidity 5 < η < 8) with respect to the LHC beam axis, a region not accessed by conventional LHC detectors.
• Relevance to Astroparticle Physics: These conditions emulate the production of atmospheric neutrinos in cosmic-ray interactions, making the measurements complementary to far detector programs (e.g., KM3NeT, DUNE, Hyper-Kamiokande) and invaluable for tuning hadronic interaction and nuclear effect models relevant in the 20 GeV–TeV regime.
• Vertex and Flavor Localizability: The gaseous target and distinct vertex position facilitate flavor-dependent cross-section measurements on a variety of nuclear media in the calorimeters (Garcia-Soto et al., 13 Oct 2025).

5. Complementarity and Comparison with Other Programs

SHIFT@LHC occupies a unique niche among LHC-based and external experimental programs:

Program	Energy Range	Mass Reach	Decay Distance Sensitivity	Detector Type	Luminosity Assumption
LHCb SMOG2	< 100 GeV	~few GeV	Small (vertex near IP)	Dedicated	Lower than CMS
FASER/SND	> 100 GeV	<1 GeV	480 m behind IP	Dedicated, small-scale	pp collisions only
MATHUSLA	~ TeV	< few GeV	60–100 m above IP	Surface detector	Primarily LLP, ≲1 GeV
CMS/ATLAS (pp)	> 100 GeV	high-mass	Decays within ~1 m	General-purpose	Maximum (Run-3/HL-LHC)
SHIFT@LHC	~100 GeV	11–60 GeV (dimuon)	Up to 160 m displacement	General-purpose	1% CMS, cost-effective

SHIFT extends the forward and long-lifetime reach into the dimuon channel for intermediate-mass new bosons, while offering competitive or superior sensitivity for lifetime and long path-length decay searches compared to both dedicated small-scale detectors and the main experiments in standard collider configuration. Its operational mode—using parasitic luminosity—avoids conflicts with core physics operations and leverages existing infrastructure (Niedziela, 2024, Garcia-Soto et al., 13 Oct 2025).

6. Future Directions and Research Impact

The projected performance of SHIFT@LHC suggests several impacts:

• Long-Lived Particle Searches: The extended decay acceptance (>100 m) supports leading sensitivity for BSM models predicting long-lived, feebly-interacting states in the intermediate mass regime.
• Atmospheric and Cosmic Neutrino Model Validation: Forward neutrino production data under controlled, high-luminosity conditions directly inform the systematics of atmospheric neutrino fluxes and cross sections relevant for next-generation oscillation and astrophysics experiments.
• Technological Synergies: Successful deployment would validate scalable methods for integrating low-infrastructure fixed targets with major collider detectors, potentially informing upgrades or similar projects at future colliders.
• Augmented Flavor Physics and Nuclear Effects Studies: The capability to select gas targets of different nuclear composition enables detailed studies of nuclear dependence in hadron production and neutrino interactions in a regime sparsely covered by existing fixed-target experiments.

In summary, SHIFT@LHC synthesizes the cost-efficiency of fixed-target experimentation, the luminosity advantages of modern collider beams, and the deep instrumentation of general-purpose LHC detectors. It dramatically enhances the LHC’s reach in new physics and neutrino frontiers, bridging collider and astroparticle physics and opening new avenues for experimental exploration (Niedziela, 2024, Garcia-Soto et al., 13 Oct 2025).