MAPP Outrigger Detector (OD) Overview

• MAPP Outrigger Detector (OD) is a scintillator-based extension of the MAPP program that enlarges the geometric acceptance for milli-charged particles at LHC IP8.
• It employs tilted scintillator planes in a four-layer coincidence setup to target particles with higher effective charges while mitigating background noise.
• The OD design has evolved from a Phase-1 configuration to a standalone hodoscope, extending sensitivity to mCP masses up to around 200 GeV at the HL-LHC.

Searching arXiv for papers on the MAPP Outrigger Detector and closely related MAPP documentation. The MAPP Outrigger Detector (OD) is a scintillator-based extension of the MoEDAL Apparatus for Penetrating Particles (MAPP) at LHC Interaction Point 8, introduced to enlarge the geometric acceptance of the MAPP milli-charged-particle program while operating at a higher charge threshold than the main MAPP-mQP bar detector. In the MoEDAL–MAPP facility description, the OD is a Phase-1 acceptance extender for milli-charged particles (mCPs), especially in a smaller-angle forward band and at higher masses; in a later dedicated sensitivity study, it is treated as a standalone detector adjacent to MAPP-1 and is projected, under a background-free assumption, to extend MAPP’s upper mass reach to mCP masses of approximately 200 GeV at the HL-LHC (Pinfold, 2022, Kalliokoski et al., 6 Aug 2025).

1. Definition and position within the MoEDAL–MAPP program

Within the MoEDAL–MAPP program, the OD is explicitly presented as a component of MAPP-1 rather than as part of the much larger MAPP-2 displaced-vertex detector. The 2022 facility paper describes MAPP-1 as the Run-3 installation in the UA83 tunnel near IP8 and states that the OD is a Phase-1 extension to the baseline MAPP-mQP scintillator detector. Its stated purpose is to increase geometric acceptance for mCPs while complementing the main bar detector’s lower charge threshold and different angular coverage (Pinfold, 2022).

The OD’s role is therefore distinct from that of MAPP-2. MAPP-2 is described as an HL-LHC detector in the UGC1 gallery, designed to make MoEDAL–MAPP competitive for neutral LLP decays through a large fiducial volume and dedicated veto structure, whereas the OD is described as an acceptance extender for feebly ionizing charged particles in the UA83 environment. In that programmatic sense, the OD belongs to the Run-3 mCP infrastructure rather than to the HL-LHC displaced-vertex volume.

This distinction matters because the term “OD” is not used uniformly across all MAPP phenomenology. The sterile-neutrino study of MAPP-2 models the detector as two polyhedra in UGC1 and explicitly notes that it does not distinguish an OD versus a “main” MAPP module, which indicates that “OD” is not a generic label for all MAPP sub-detectors (Deppisch et al., 2023).

2. Physics motivation and complementarity with MAPP-mQP

The OD was motivated by the need to enlarge acceptance for mCP trajectories that would miss the main MAPP-mQP bar array. In the Phase-1 description, the main MAPP-mQP detector targets effective charges down to about $10^{-3}e$, whereas the OD is intended to operate at a higher threshold, about $10^{-2}e$, in exchange for larger-area coverage in a different angular region. The central complementarity is therefore geometric rather than threshold-driven: the OD sacrifices per-track light yield for acceptance, particularly for particles emitted at smaller angles with respect to the beam and for higher-mass mCPs (Pinfold, 2022).

That trade is made explicit in the geometry. The OD is placed so as to intercept forward-emitted mCPs that could bypass the main bar detector, and its tilted scintillator planes are used to increase effective path length through scintillator for particles pointing back to IP8. The instrument is thus optimized for lightly ionizing, through-going charged states rather than for displaced decays of neutral LLPs.

Later sensitivity work sharpens the same physics target. The 2025 dedicated OD study states that the first upgrade to the MAPP Experiment is designed specifically to improve sensitivity to high-mass mCPs with intermediate effective charges. In that treatment, Drell–Yan production and various meson decays are considered, and the high-mass regime is the principal motivation for the standalone OD concept (Kalliokoski et al., 6 Aug 2025).

A common misconception is that the OD is an LLP decay detector analogous to MAPP-2. The published MAPP descriptions do not support that reading: the OD is presented primarily as an mCP detector, while large-volume neutral LLP sensitivity is assigned to MAPP-2 (Pinfold, 2022).

3. Published detector layouts and hardware realizations

Published descriptions of the OD give two concrete implementations. The first appears in the MoEDAL–MAPP facility paper as a Phase-1 extension in the UA83–beam-tunnel passage; the second appears in the dedicated OD sensitivity study as a standalone detector in Duct 4 adjacent to MAPP-1. Both are scintillator–PMT systems tilted at $45^\circ$ toward IP8, but they differ substantially in segmentation and channel count (Pinfold, 2022, Kalliokoski et al., 6 Aug 2025).

Aspect	Phase-1 facility description	Dedicated OD sensitivity study
Location	Circular passage joining UA83 gallery and beam tunnel	Duct 4 in UA83, between gallery and beamline
Angular placement	Roughly $2^\circ$–$6^\circ$	Approximately 120 m from IP8
Active structure	4 planks	80 scintillator slabs
Unit dimensions	$$800\,\mathrm{cm}\times 60\,\mathrm{cm}\times 5\,\mathrm{cm}$$ per plank	$$60\,\mathrm{cm}\times 30\,\mathrm{cm}\times 5\,\mathrm{cm}$$ per slab
Segmentation	16 plates per plank, each $50\,\mathrm{cm}\times 60\,\mathrm{cm}$	Ten installation subunits, eight slabs each
Photosensors	2-inch 12-stage PMT per plate	First three layers: 2-inch Hamamatsu R2154-02; fourth layer: 3.5-inch HZC Photonics XP82B2FNB
Tilt	$45^\circ$	$45^\circ$

In the Phase-1 design, each of the four planks comprises 16 scintillator plates of size $10^{-2}e$0, each read out by a 2-inch 12-stage PMT. The plates are tilted at $10^{-2}e$1, giving about 28 cm of scintillator per traversing mCP, and each plank has area $10^{-2}e$2. The detector is placed in a forward band spanning roughly $10^{-2}e$3–$10^{-2}e$4 relative to the beam axis (Pinfold, 2022).

In the later standalone design, the OD is built from 80 BC-408 scintillator slabs configured as a layered “bricklayer” hodoscope. The hardware is divided into ten installation subunits, each containing eight slabs in four layers mounted on a rail at $10^{-2}e$5 relative to the incoming direction from IP8. The first three layers use 2-inch Hamamatsu R2154-02 PMTs, and the fourth uses 3.5-inch HZC Photonics XP82B2FNB PMTs. Readout and calibration are integrated into the MAPP-1 electronics rack (Kalliokoski et al., 6 Aug 2025).

These two descriptions are not identical. This suggests an evolution from an initial plank-based acceptance extender toward a more explicitly standalone four-layer hodoscope, while preserving the core design logic of scintillation, IP8 pointing, and coincidence-based background suppression.

4. Detection principle, coincidence logic, and background suppression

The OD exploits the $10^{-2}e$6 scaling of ionization loss, expressed in the MAPP literature as $10^{-2}e$7 or, for an mCP with charge $10^{-2}e$8, approximately $10^{-2}e$9. Because the expected scintillation yield is suppressed by $45^\circ$0, the detector relies on coincidence logic across multiple layers rather than on a single large pulse in one channel (Pinfold, 2022, Kalliokoski et al., 6 Aug 2025).

In the dedicated OD sensitivity model, the expected number of detected photoelectrons per layer is written as

$45^\circ$1

with $45^\circ$2. The four-layer detection probability is then

$45^\circ$3

The same study uses Geant4 photon transport with surface reflectivity 98%, light yield $45^\circ$4, bulk attenuation length 2.6 m, a $45^\circ$5 light guide with refractive index 1.44, Tyvek wrapping, and the UNIFIED and LUT Davis optical models. For 1 GeV muons, the simulated benchmark is $45^\circ$6 optical photons reaching the PMT per slab (Kalliokoski et al., 6 Aug 2025).

Background suppression is a central element of the OD concept. In the Phase-1 description, the OD uses four-fold coincidence across layered plates to suppress dark counts and isolated noise, while the main MAPP-mQP detector adds a hermetic veto layer for rejection of entering particles and radiological backgrounds. In the standalone OD study, the four-layer bricklayer hodoscope provides strict coincidence and self-veto capabilities for non-IP-aligned tracks, and an additional layer of iron shielding is placed between the OD and the duct opening facing the beamline to reduce beam-induced backgrounds (Pinfold, 2022, Kalliokoski et al., 6 Aug 2025).

The dedicated OD study does not specify a detailed timing window $45^\circ$7 or an explicit hardware trigger threshold. Its sensitivity estimates are instead based on the photoelectron-probability model above. That omission is material: the paper motivates a background-free sensitivity projection through geometry, shielding, low-noise PMTs, and coincidence logic, but does not present a dedicated quantitative background study for cosmics, radiogenic sources, or beam-induced backgrounds (Kalliokoski et al., 6 Aug 2025).

5. Sensitivity methodology and projected reach

The OD’s later sensitivity projections are formulated for the HL-LHC at $45^\circ$8 with $45^\circ$9, with $2^\circ$0 also shown for comparison. The study considers mCP production via leading-order Drell–Yan pair production and a set of meson decays, with Drell–Yan simulated in MadGraph5_aMC@NLO v2.7.3 using an implementation of the Holdom kinetic-mixing scenario, and meson production normalized using Pythia 8.240 and PDG inputs (Kalliokoski et al., 6 Aug 2025).

The geometric acceptance $2^\circ$1 is computed by Monte Carlo. An mCP is accepted only if it traverses all four collinear OD layers. The expected signal yield is written as

$2^\circ$2

where $2^\circ$3 is the produced mCP yield, $2^\circ$4 is the geometric acceptance, and $2^\circ$5 is the four-layer detection probability. Limits are set with the $2^\circ$6 method under a background-free assumption, and the 95% CL exclusion is defined by

$2^\circ$7

The published result is that, at $2^\circ$8, the OD can extend the experiment’s upper mass reach to mCP masses of approximately 200 GeV for intermediate effective charges (Kalliokoski et al., 6 Aug 2025).

Earlier MoEDAL–MAPP facility projections already indicated the same qualitative role. In comparative sensitivity plots, the main MAPP-mQP bars provide reach down to $2^\circ$9 over a broad mass range, while the OD curves lie at higher $6^\circ$0 but extend coverage in mass due to larger acceptance at small angles. That is the detector’s defining complementarity: lower geometric acceptance but lower threshold for the bars, higher threshold but larger forward acceptance for the OD (Pinfold, 2022).

The phenomenological scope has broadened beyond vector-portal fermionic mCPs. A later study of SIMP dark pions uses “MAPP-1/OD” terminology and projects sensitivity for Drell–Yan and photon-fusion production under a background-free assumption, again relying on a four-layer coincidence model and IP8-pointing geometry. This suggests that the OD-style detection strategy has become a generic template for MoEDAL–MAPP studies of weakly ionizing charged states, even when the detector label is not used identically across papers (Arifeen et al., 2 Sep 2025).

6. Nomenclature, scope limits, and relation to other “OD” usages

The MAPP literature does not use the label “OD” uniformly. The 2022 facility paper defines a distinct MAPP Outrigger Detector as a Run-3 extension to MAPP-mQP in UA83. The 2025 dedicated sensitivity study preserves that identity but describes a more detailed standalone detector in Duct 4. By contrast, the sterile-neutrino MAPP-2 analysis explicitly states that OD-specific information is not specified, and the earlier R-parity-violating neutralino study states that an OD is not defined as a distinct hardware concept in that paper (Deppisch et al., 2023, Dreiner et al., 2020).

This inconsistency is most likely to generate two misunderstandings. The first is to equate the OD with MAPP-2; the cited detector papers do not do so. The second is to assume that every MAPP phenomenology paper uses the same hardware definition. The dark-pion study’s “MAPP-1/OD” language and its use of a $6^\circ$1 four-section scintillation geometry are not identical to the 80-slab standalone OD description. This suggests that “OD” can function either as a specific hardware proposal or as a shorthand for an OD-like four-layer scintillator acceptance model, depending on the paper (Arifeen et al., 2 Sep 2025).

A separate terminological issue is that “OD” is not unique to MoEDAL–MAPP within arXiv detector literature. In HAWC, the same acronym denotes a sparse water-Cherenkov outrigger array used to improve core localization for partially contained multi-TeV air showers, which is an unrelated detector concept with different technology, geometry, and physics goals (Sandoval, 2015, Joshi et al., 2017).

Taken together, the published record supports a narrow and technically specific definition. The MAPP Outrigger Detector is a scintillator–PMT, coincidence-based, forward-acceptance extension to the MAPP milli-charged-particle program in the UA83 environment. Its purpose is not to replace the main MAPP-mQP bars or the MAPP-2 LLP detector, but to occupy an intermediate niche: higher threshold than the bars, far greater usefulness for small-angle and higher-mass weakly ionizing charged particles, and a detector architecture designed around layered coincidence, passive shielding, and IP8-pointing geometry (Pinfold, 2022, Kalliokoski et al., 6 Aug 2025).