DarkSHINE: Search for Light Dark Matter at the SHINE Facility in Shanghai

DarkSHINE is an electron fixed target experiment under proposal that aims to probe light dark matter in the MeV-GeV mass range via the invisible decay of dark photons, leveraging the High repetition rate 8 GeV electron beam from the Shanghai High repetition-rate XFEL and Extreme Light Facility. This proceeding presents the core detector design of the experiment, the simulation framework, and the prospects of the physics. The detector system integrates an AC-coupled Low Gain Avalanche Diode silicon tracker, a LYSO crystal electromagnetic calorimeter, and a scintillator-based hadronic calorimeter, all optimized for SHINE high-radiation, high-rate environment. The prototype tests at DESY and CERN have validated key performance metrics, including a spatial resolution of 6.5-8.2 microns for silicon strip sensor, an electromagnetic calorimeter energy resolution of 1.8%. Based on MC simulations and 9E14 EOT, the DarkSHINE experiment is expected to rule out most of the sensitive regions predicted by popular dark photon models.

💡 Research Summary

DarkSHINE is a proposed electron‑fixed‑target experiment that will exploit the high‑repetition‑rate 8 GeV electron beam from the Shanghai High‑Repetition‑Rate XFEL and Extreme Light Facility (SHINE). The experiment is designed to search for light dark matter (LDM) in the MeV–GeV mass range by looking for invisible decays of a dark photon (A′) that mediates interactions between the Standard Model and the dark sector. The authors present a complete description of the detector concept, the simulation framework, and the projected physics reach.

The SHINE linac will generate a dedicated single‑electron bunch (one electron per bunch) at 10 MHz using a low‑power laser and a kicker system that injects the electron into the empty buckets between two FEL bursts. A thin (350 µm, 0.1 X₀) tungsten target is placed inside a 1.5 T dipole magnet. The magnet bends both the incoming beam and the scattered recoil electron, enabling precise momentum measurement and missing‑momentum reconstruction, which is the key observable for invisible A′ decays.

The tracking system is built from AC‑coupled Low‑Gain Avalanche Diode (AC‑LGAD) strip sensors. Each strip has a 50 µm pitch and 50 µm gap; seven layers form the “tagging” tracker upstream of the target and six layers form the “recoil” tracker downstream. Adjacent layers are rotated by 100 mrad to provide two‑dimensional hit information. Laboratory laser tests demonstrated a spatial resolution of 6.5–8.2 µm, and the fast, radiation‑hard response of AC‑LGADs is well suited to the 10 MHz environment. Track reconstruction uses custom clustering and a Kalman‑filter algorithm, achieving high efficiency and low fake rates.

The electromagnetic calorimeter (ECAL) is a homogeneous LYSO crystal array (21 × 21 × 11 crystals, each 2.5 × 2.5 × 4 cm³). Each crystal is read out by a silicon photomultiplier (SiPM) and digitized with 14‑bit, 1 GS/s ADCs employing a dual‑gain scheme. The ECAL energy resolution, obtained from Geant4 simulations and beam tests at DESY and CERN, is 1.8 % / √E(GeV) ⊕ 0.66 %. The high light yield (~30 kph/MeV) and short decay time (~40 ns) allow clean signal extraction at the full 10 MHz rate.

The hadronic calorimeter (HCAL) is a sampling detector composed of plastic scintillator strips (75 cm × 5 cm × 1 cm) interleaved with steel absorbers. The active volume (1.5 m × 1.5 m × 2.5 m) corresponds to >10 interaction lengths, providing excellent veto capability for muons and hadrons, especially low‑energy neutrons that dominate the Standard Model background in the missing‑momentum signal region. A side HCAL surrounding the ECAL further reduces background leakage by a factor of ~3.5. Simulations show neutron veto inefficiency as low as 10⁻⁶ for energies above 1 GeV.

The full simulation and analysis chain, named DSimu‑DAna, is built on Geant4 and ROOT. Signal events (A′ production via bremsstrahlung, t‑channel, and s‑channel processes) are generated with a custom Monte Carlo generator, while Standard Model backgrounds (photon bremsstrahlung, electron‑nucleus scattering, meson decays) are produced with Pythia 8. Detailed detector response models include the AC‑LGAD charge collection, SiPM gain variations, and ADC digitization. Event reconstruction, selection cuts, and background suppression are performed within the DAna module.

Sensitivity studies assume exposures of 3 × 10¹⁴, 9 × 10¹⁴, 1.5 × 10¹⁵, and 10¹⁶ electrons‑on‑target (EOT), corresponding to 1, 3, 5, and 10 years of running. The projected 90 % confidence level exclusion limits on the kinetic‑mixing parameter ε² reach ≈10⁻¹² for dark‑photon masses from a few MeV up to ~100 MeV, surpassing current bounds from NA64e, BaBar, and other fixed‑target experiments by one to two orders of magnitude. The authors also translate these limits into the dimensionless interaction strength y = ε² α_D (m_χ/m_A′)⁴, assuming m_A′ = 3 m_χ and α_D = 0.5. In this framework, DarkSHINE can probe thermal‑relic targets for scalar, Majorana, and pseudo‑Dirac dark matter models, reaching the region where the observed relic abundance could be explained by LDM.

The paper concludes that the combination of a high‑rate, low‑background electron beam, ultra‑precise AC‑LGAD tracking, high‑resolution LYSO calorimetry, and efficient HCAL veto makes DarkSHINE a powerful probe of invisible dark‑photon decays. Ongoing R&D includes integration of a full‑scale prototype, beam tests of the combined system, and development of a machine‑learning‑based trigger to further reduce background rates. With the first beam expected in 2026, DarkSHINE will become China’s first electron‑fixed‑target dark‑matter experiment and will fill a critical gap in the global search for light dark matter.