Layout optimization for the LUXE-NPOD experiment

TL;DR

The paper develops a realistic NPOD sensitivity study for LUXE by embedding ALP-like photon couplings into a full, detailed detector setup. It systematically optimizes the NPOD geometry (dump length, decay volume, detector size) and leverages a silicon–tungsten ECAL with high granularity, NN clustering, and a BDT-based PID to achieve background suppression and precise event reconstruction. The results demonstrate background-free prospects for phase-0 configurations with compact dumps and establish phase-1 reach up to $m_X\sim$330 MeV and $1/\Lambda\sim$ few $\times10^{-6}$ GeV$^{-1}$, highlighting LUXE-NPOD as a unique probe of the $10{-}350$ MeV ALP-like parameter space. With deployment targeted around 2030, the work positions LUXE-NPOD as a key component in the landscape of light, feebly interacting particle searches and a path toward robust ALP constraints in previously unexplored regions.

Abstract

Beam dump experiments represent an effective way to probe new physics in a parameter space, where new particles have feeble couplings to the Standard Model sector and masses below the GeV scale. The LUXE experiment, designed primarily to study strong-field quantum electrodynamics, can be used also as a photon beam dump experiment with a unique reach for new spin-0 particles in the $10-350~\mathrm{MeV}$ mass and $10^{-6}-10^{-3}~\mathrm{GeV}^{-1}$ couplings to photons ranges. This is achieved via the ``New Physics search with Optical Dump'' (NPOD) concept. While prior estimations were obtained with a simplified model of the experimental setup, in this work we present a systematic study of the new physics reach in the full, realistic experimental apparatus, including an existing detector to be used in the LUXE NPOD context. We furthermore investigate updated scenarios of LUXE's experimental plan and confirm that our results are in agreement with the original estimations of a background-free operation.

Figures (14)

• Figure 1: Schematic representation of the LUXE-NPOD concept, showing the NP production. Figure adapted from Bai_2022.
• Figure 2: Sensitivity projections for LUXE-NPOD phase-1 in the plane of the coupling to photon versus ALP mass. The sensitivity is inspected in terms of different dump length $L_D$ (a), decay volume length $L_V$ (b) and minimum di-photon separation at the detector $\Delta_\text{min}$ (c). For (a) and (b) there is no condition on $\Delta_\text{min}$. The exclusion limits indicate where more than three signal events are expected assuming no ALPs are observed, a background-free environment, and a "virtual" ideal detector with $1 ~\mathrm{m}\xspace$ radius for any photon above 0.5 GeV. The gray areas are currently existing bounds from other other experiments PhysRevLett.125.081801PhysRevLett.123.071801PhysRevLett.118.171801PhysRevLett.125.161806Ablikim_2023PhysRevD.110.L031101BERGSMA1985458PhysRevLett.59.755Dolan_2017na62_dumpPhysRevD.108.075019faser_2025.
• Figure 3: Visualization of the full LUXE Geant4 model for the $e$-laser setup (a), and and a enlarged version of the same model, focused on the concrete wall (yellow) with the embedded dump in front of the photon detector (b). The tunnel walls are removed from the model for visualization purpose.
• Figure 4: Photons and neutrons energy (a), $z$-vertex (b) and $z$-vertex with $E_\text{kin} > 0.5 ~\mathrm{GeV}\xspace$ (c) distributions. Long-lived neutral and charged particles energy (d), $z$-vertex (e) and $z$-vertex with $E_\text{kin} > 0.5 ~\mathrm{GeV}\xspace$ (f) distributions. The design used is a core tungsten dump of $R_D=20 ~\mathrm{cm}\xspace$, $L_D=100 ~\mathrm{cm}\xspace$, and lead wrap of $R_D=50 ~\mathrm{cm}\xspace$. The dump is enclosed in concrete, the decay volume is $1 ~\mathrm{m}\xspace$, and the detector has a radius of 1 m. The beginning and end of the dump are depicted with vertical dashed lines in (b), (c), (e) and (f). The simulation is done for 10 BXs.
• Figure 5: Time of arrival at the detector of background photons (a) and background neutrons (b). The simulation considers 10 BXs for a W+Pb dump of length $L_\text{D} = 1~\mathrm{m}\xspace$, decay volume of length $1 ~\mathrm{m}\xspace$, and detector radius of $1 ~\mathrm{m}\xspace$. The time of arrival is measured with respect to the moment of the $e$-laser interaction, when $t=0$. The horizontal dashed line shows the estimated time of arrival of the signal photons, at $t=52.4~\mathrm{ns}\xspace$.