Minicharged Particle Sensitivity of the MAPP Outrigger Detector

TL;DR

The study quantifies the background-free sensitivity of the MAPP Outrigger Detector (OD) to minicharged particles (mCPs) at the HL-LHC by modeling production via Drell–Yan and meson decays and simulating detector response. The signal yield is computed as $N_{ m sig} = N_{ m \chi} \times A \times P$, with $N_{ m \chi}$ estimated from $2 \sigma_{q\bar{q} \rightarrow \chi \bar{\chi}} L^{\mathrm{int}}_{\mathrm{LHCb}}$ for Drell–Yan and meson-decay BRs scaled by $\epsilon^2$ and phase-space factors; the detection probability is $P = (1 - e^{-N_{\rm PE}})^{n}$, with $N_{\rm PE}$ tied to $\epsilon^2$, scintillation photon yield $N_{\gamma}$, and PMT quantum efficiency. Under a background-free CL$_s$ framework with a 3-event criterion, the MAPP OD projects sensitivity up to $\sim$200 GeV mCP mass at the HL-LHC for $L^{int} = 300$ fb$^{-1}$, and provides complementary reach to MAPP-1. The work also examines a minicharged strongly interacting dark matter (mC-SIDM) scenario, mapping limits to this parameter space and highlighting regions inaccessible to direct-detection experiments where accelerator searches are advantageous. Overall, the MAPP OD advances the dark-sector exploration at the LHC by strengthening sensitivity to high-mass, intermediate-charge mCPs and probing new mC-SIDM territory.

Abstract

We present a detailed study of the projected background-free sensitivity of the MAPP Outrigger Detector (OD) to minicharged particles (mCPs) at the High-Luminosity Large Hadron Collider (HL-LHC). As the first upgrade to the MAPP Experiment, the MAPP OD is a standalone detector designed to offer enhanced sensitivity to high-mass mCPs with intermediate effective charges. The MAPP OD is planned for installation in a duct adjacent to the MAPP-1 detector, located between the LHC's UA83 gallery and the beamline. Considering mCP production via the Drell-Yan mechanism and various meson decays, the results show that, at the 95% confidence level, the MAPP OD can extend the experiment's upper mass reach to mCP masses of approximately 200 GeV at the HL-LHC.

Figures (3)

• Figure 1: Upper: An overview of the UA83 gallery, indicating the location of Duct 4 relative to IP8, the beamline, and the MAPP-1 detector. Lower: A schematic diagram illustrating the design and placement of the MAPP Outrigger Detector in Duct 4. The bottom-left panel shows a basic scintillator plank used in the MAPP Outrigger Detector; MAPP-1 is displayed in the bottom-right of the diagram.
• Figure 2: The projected $95\%$ CL exclusion limits for the MAPP Outrigger Detector (blue) for minicharged particles vs. those for the MAPP-1 detector (red) Kalliokoski2024. The dashed lines correspond to $L^{\mathrm{int}}_{\mathrm{LHCb}} = 30$ fb$^{-1}$ and the dashed-dotted lines correspond to $L^{\mathrm{int}}_{\mathrm{LHCb}} = 300$ fb$^{-1}$. The shaded areas represent the regions of parameter space excluded by previous searches at the $95\%$ CL (except the BEBC bounds, which correspond to the $90\%$ CL) Prinz1998Davidson2000Magill2019Acciarri2020Ball2020Plestid2020Marocco2021Barak2024alcott2025search; milliQan* denotes the milliQan demonstrator Ball2020. The grey dashed and black dotted lines represent the most stringent indirect $2\sigma$ upper limits derived from the Planck full-mission results Planck2018cosmo on the effective number of different neutrino species ($N_{\mathrm{eff}}$) Adshead_2022 and the region associated with a potential resolution of the EDGES anomaly ($f_{\chi} = 0.4\%$) Kovetz2018, respectively.
• Figure 3: The projected $95\%$ CL exclusion limits for the MAPP Outrigger Detector (blue) for mC-SIDM vs. those for the MAPP-1 detector (red) Kalliokoski2024. The dashed lines correspond to $L^{\mathrm{int}}_{\mathrm{LHCb}} = 30$ fb$^{-1}$ and the dashed-dotted lines correspond to $L^{\mathrm{int}}_{\mathrm{LHCb}} = 300$ fb$^{-1}$. Additional exclusion limits shown on the plot in grey correspond to projections of constraints set by several strongly interacting dark matter searches assuming a small minicharged fraction of dark matter of $f_{\chi} = 0.4$%: terrestrial direct-detection experiments Mahdawi2018Emken2019, the X-ray quantum calorimetry (XQC) experiment Erickcek2007, and a high-altitude balloon-based experiment (RRS) Rich1987. The solid grey line denotes the critical reference cross-section ($\bar{\sigma}_{e\mathrm{,ref,crit}}$); milliQan* denotes the milliQan demonstrator Ball2020.