MoEDAL Apparatus for Penetrating Particles (MAPP)

• MAPP is a specialized subdetector that targets feebly interacting particles, including minicharged particles and long-lived states, using a modular scintillator design and stringent coincidence triggers.
• The system integrates an active scintillator array (MAPP-mQP) and a multi-layer hodoscope (MAPP-LLP) to detect rare decays and displaced vertices with high spatial and sub-nanosecond timing resolution.
• Its strategic installation behind substantial rock overburden minimizes background interference, complementing main LHC detectors in exploring new physics signatures.

The MoEDAL Apparatus for Penetrating Particles (MAPP) is a subdetector system integrated with the MoEDAL experiment at the Large Hadron Collider (LHC), purpose-built to explore new domains of physics beyond the Standard Model. It is specifically designed to search for feebly interacting, long-lived, and otherwise elusive particles such as minicharged particles (mCPs), long-lived dark-sector states, and exotic avatars of new physics that are not amenable to triggering or reconstruction in conventional collider detectors. MAPP achieves this through a segmented passive/active detection strategy exploiting low-background environments, modular scintillator design, and advanced shielding to robustly identify rare signals of new physics.

1. Detector Concept, Architecture, and Installation

MAPP’s architecture and methodology are distinct from MoEDAL’s original focus on highly ionizing particles (HIPs). It consists of two primary subsystems:

• MAPP-mQP (minicharged particle detector): A central active volume constructed from an array of plastic scintillator bars (typically 10×10×75 cm³, arranged 10×10 per section, with four collinear sections constituting the active detector). Each bar is instrumented with a dedicated low-noise photomultiplier tube (PMT). The system employs a quadruple-coincidence trigger, requiring signals in all four sections, to suppress backgrounds from ambient noise and radioactivity. A hermetic external veto composed of additional scintillator panels encloses the detector to reject charged Standard Model backgrounds.
• MAPP-LLP (long-lived particle detector): A larger, multi-layered “Russian-doll” hodoscope with three nested scintillator arrays covering a decay volume of order 5 m × 10 m × 3 m, optimized for detecting neutral LLPs decaying visibly within the volume. The system delivers spatial resolution at the centimeter scale and sub-nanosecond timing through SiPM readout.

Installation: MAPP-1 (Phase-1) is deployed in the UA83 gallery at IP8, approximately 98 m from the collision point, shielded by ≳100 m of rock and significant overburden, ensuring minimal exposure to cosmogenic and beam-related backgrounds. The geometry is “pointed” toward the interaction region to maximize acceptance for LHC-produced new particles. A larger MAPP-2 (planned for HL-LHC) will be installed in the UGC1 gallery, offering hundreds of times greater decay volume for LLP exploration (Pinfold, 2022, Mitsou et al., 2023).

2. Physics Motivation and Target Signatures

MAPP substantially extends MoEDAL’s physics reach by targeting weakly coupled, slowly moving, or delayed particle signatures. The principal theoretical targets include:

• Minicharged particles (mCPs): Predicted in scenarios with a dark U(1) gauge group kinetically mixed with the SM hypercharge, mCPs inherit a suppressed electromagnetic coupling $\epsilon = \kappa e' \cos\theta_W$ (Mitsou et al., 2023, Mitsou, 2021, Montigny et al., 2023). Their detection would challenge charge quantization and may illuminate dark sector mass-generation mechanisms or the composition of dark matter.
• Long-lived neutral or charged states: LLPs such as dark Higgs bosons, dark photons, heavy (sterile) neutrinos, or dark pions—which decay meters to hundreds of meters from the IP—often escape prompt or calorimetric detection. Portals realized via mixing terms (e.g., $$L_{\text{portal}} = \epsilon F^{\mu\nu} F'_{\mu\nu}$$) facilitate their production and decay (Mitsou, 2021, Liu et al., 25 Mar 2024, Deppisch et al., 2023).
• Exotic dark sector states: The apparatus is sensitive to strongly interacting massive particle (SIMP) models with quasi-stable dark pions, especially those that are milli-charged via kinetic mixing with a dark photon (Arifeen et al., 2 Sep 2025). The unique ability to probe photon-fusion as well as Drell–Yan production of such objects is enabled by MAPP’s geometry and low threshold.

Key target properties and selection:

Particle Type	Effective Charge	Lifetime	Signal in MAPP
mCP	$10^{-3}$ – $0.1\,e$	Stable or long-lived	Weak ionization, multi-layer hit
Neutral LLP	—	$c\tau$ ~ 1–100 m	Displaced decay vertex
SIMP dark pion	$10^{-3}$ – $0.1\,e$	Stable or meta-stable	Weak ionization, resonance in DY

3. Detection Principles and Technical Features

Detection of feeble ionization: MAPP is tuned for sensitivity to signals corresponding to energy depositions as low as several $10^{-4}\,e$, far below what is accessible in ATLAS, CMS, or LHCb calorimetry (Pinfold, 2022, Mitsou et al., 2023, Montigny et al., 2023). Ionization signals in the scintillators are transduced to photoelectron counts in the PMTs. Resulting signals are subject to stringent coincidence and veto requirements; typically, a signal must be seen in all four main layers but not in veto panels.

LLP detection via displaced decays: For neutral LLPs, visible decays (often to lepton pairs) within the decay volume of MAPP-LLP yield coincident signals in several x–y segmented planes, allowing kinematic reconstruction of the decay vertex. The fiducial volume and precise reconstruction from multi-layered SiPMs enable strong background rejection, even for very small event rates.

Background mitigation: The rock overburden (≳100 m) and the extensive charged-particle veto surrounding the detector drastically suppress both cosmic and beam-related backgrounds. MAPP is designed to be essentially “background-free” under baseline assumptions, setting a limit with as few as three observed events for exclusion at high confidence.

4. Key Theoretical Formulations and Sensitivity

MAPP’s sensitivity calculations are grounded in direct relationships between theoretical cross sections and the actual detector observables. For minicharged particles, the number of expected signal events from Drell–Yan production (dominant at high LHC energies) is: $$N_{\rm hits} = \sigma(\sqrt{s}, m_\chi, \epsilon=1) \cdot \epsilon'^2 \cdot L_{\rm int} \cdot \overline{P}_{\rm acc}(\sqrt{s}, m_\chi)$$ Here, $\sigma$ is the production cross section for unit charge, $L_{\rm int}$ is the integrated luminosity, $\epsilon'$ is the mixing parameter, and $\overline{P}_{\rm acc}$ is the geometric and instrumental acceptance (Montigny et al., 2023, Mitsou et al., 2023).

For SIMP dark pion models, photon fusion cross sections are computed including Wess–Zumino–Witten terms, and the dark pion–SM interaction is controlled by kinetic mixing: $$\mathcal{L}_{\rm mix} = -\frac{\kappa}{2} F^{\mu\nu}_{\rm dark} F_{\mu\nu}^{\rm SM}$$ Resonant Drell–Yan and nonresonant photon-fusion production are both considered, the latter crucial due to its quartic scaling with momentum and $1/F^6$ dependence, where $F$ is the dark pion decay constant (Arifeen et al., 2 Sep 2025).

The excluded region in parameter space is set by requiring

$N_{\rm sig} = N_\chi \cdot A \cdot P \geq 3$

where $A$ is acceptance and $P$ is the probability a true signal exceeds trigger/photoelectron thresholds.

For displaced heavy neutrino searches, sensitivity in mixing strength can reach $V_{\mu N}^2 \approx 10^{-12}$ for visible decays within the MAPP-2 decay zone, with proper decay lengths $O(1 - 100\,{\rm m})$ optimized for the suppression of backgrounds (Deppisch et al., 2023).

5. Discovery Potential, Comparative Reach, and Complementarity

MAPP provides sensitivity to parameter ranges and particle types at the LHC largely inaccessible to the main detectors due to trigger, reconstruction, or background constraints. For instance, MAPP-1 can probe mCPs with mass $0.45\lesssim m_\chi\lesssim 130$ GeV and effective charges as low as $6\times 10^{-4}\,e$ at HL-LHC design luminosities, substantially extending constraints beyond previous limits (Montigny et al., 2023, Mitsou et al., 2023). For scenario-specific LLPs, such as the light pseudoscalar $A$ in Type-I 2HDM, MAPP-2 can reach $\tan\beta$ values up to $10^6$–$10^8$ for $m_A < 10$ GeV, surpassing the parameter space accessible to FASER-2 or FACET for comparable production modes (Liu et al., 25 Mar 2024).

Relative to other dedicated LLP and mCP detectors—including MATHUSLA, milliQan, and FORMOSA—MAPP offers complementary pseudorapidity coverage, unique low-background environments, and an active/passive hybrid approach (especially when used with MoEDAL’s trapping volumes). The quadruple-coincidence and external veto systems provide strong resilience to accidental triggers, while the rock overburden enhances cosmic-ray suppression compared to on-surface deployments (Mitsou, 2021, Pinfold, 2022).

For SIMP dark pion models, sensitivity projections demonstrate the ability to probe uncharted effective charge vs. mass parameter spaces, especially for higher-mass, smaller-coupling regions inaccessible to shallow or less-shielded detectors (Arifeen et al., 2 Sep 2025).

6. Status, Limitations, and Future Prospects

MAPP-1 installation in the UA83 gallery is completed, with data-taking commencing during LHC Run-3. A second, larger MAPP-2 system is planned for the HL-LHC, installed at UGC1 with a significantly larger decay and active detection volume (Pinfold, 2022).

Potential limitations include:

• The absence of a calorimeter (in contrast to MATHUSLA or some forward detectors), which may impact real-time particle identification or detailed energy reconstruction.
• Sensitivity to neutral LLPs is governed by the size of the fiducial decay volume and the geometrical acceptance with respect to the LHC IP.
• For mCPs and SIMP dark pions, accurate modeling of background rates—though assumed to be negligible given shielding and veto requirements—remains under ongoing refinement via detailed GEANT4 simulations (Arifeen et al., 2 Sep 2025).

Prospects for expansion to other new physics scenarios are strong, with ongoing or planned analyses for strongly interacting dark matter candidates, dark sector portals, and right-handed neutrinos. The modular and shielded design allows for rapid deployment and upgrades, and the experiment’s flexibility is intended to match emerging theoretical development across collider and astroparticle physics (Mitsou, 2021, Pinfold, 2022).

7. Theoretical Implications and Impact

MAPP’s results, whether exclusionary or positive, would directly impact several central open questions in particle physics:

• Validation of portal models (e.g., kinetic mixing, Higgs portal): MAPP-probed regions are otherwise inaccessible to prompt signal-based detectors (Mitsou, 2021).
• Direct tests of charge quantization: Discovery of mCPs would alter the foundational understanding of electromagnetic charge and potentially inform string-theoretic model building (Mitsou et al., 2023, Montigny et al., 2023).
• Unique constraints on dark sector structure and dynamics: Especially for composite dark matter models such as SIMPs with collider-visible signatures (Arifeen et al., 2 Sep 2025).
• Contribution to neutrino mass mechanism studies: Sensitivity to right-handed neutrino mixing in the canonical seesaw window ($V^2_{\mu N} \sim 10^{-12}$) would inform both collider and low-energy neutrino mass generation models (Deppisch et al., 2023).
• Complementarity to general-purpose detector programs: Provides largely background-free, triggerless confirmation for any rare, delayed, or slow exotic event observed elsewhere.

By providing a comprehensive, low-background, and technically flexible search apparatus for a wide variety of weak-coupling and long-lived phenomena, MAPP is positioned as a central facility for new physics searches at the LHC and, potentially, at future colliders.