Abstract
The reach of sub-GeV dark-matter detectors is at present severely affected by low-energy events from various origins. We present the theoretical methods to compute the single- and few-electron events that arise from secondary radiation emitted by high-energy particles as they pass through detector materials and perform a detailed simulation to quantify them at (Skipper) CCD-based experiments, focusing on the SENSEI data collected at Fermilab near the MINOS cavern. The simulations account for the generation of secondaries from Cherenkov and luminescent recombination radiation; photo-absorption in the bulk, backside layer, pitch adapter, and epoxy; the photon reflection and refraction at interfaces; thin-film interference; the roughness of the interfaces; the dynamics of charges produced in the highly doped CCD-backside-layers; and the partial charge collection on the CCD backside. We consider several systematic uncertainties, notably those stemming from the backside modeling, which we estimate with a “fiducial” and an “extreme” charge-diffusion model, with the former model being preferred due to better agreement with partial-charge collection data. We find that Cherenkov photons constitute about 30% of the observed single-electron events for both diffusion models; radiative recombination contributes negligibly to the event rate for the fiducial model, although it can dominate over Cherenkov for the extreme model. We also estimate the fraction of 2-electron events that arise from 1-electron event coincidences in the same pixel, finding that the entire 2-electron rate can be explained by coincidences of radiative events and spurious charge. Accounting for both radiative and non-radiative backgrounds, we project the sensitivity of future Skipper-CCD-based experiments to different dark-matter models. For light-mediator models with dark-matter masses of 1, 5, and 10 MeV, we find that future experiments with 10-kg-year exposures and successful background mitigation could have a sensitivity that is larger by 9, 3, and 2 orders of magnitude, respectively, when compared to an experiment without background improvements.
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Acknowledgments
The authors wish to thank Jerry Vavra for useful discussions on transmission and absorption measurements of the Epotek 301-2 epoxy resin, and Steve Holland for many discussions about the detailed properties of CCDs. We also thank the SENSEI Collaboration for many useful discussions, in particular Mariano Cababie, Javier Tiffenberg, and Sho Uemura. The work of P.D. is supported by the US Department of Energy under grant DE-SC0010008. R.E. acknowledges support from DoE Grant DE-SC0009854, Simons Investigator in Physics Award 623940, the US-Israel Binational Science Foundation Grant No. 2016153, and the Heising-Simons Foundation Award No. 79921 (for SENSEI). DEU is supported by Perimeter Institute for Theoretical Physics and by the Simons Foundation. Research at Perimeter Institute is supported in part by the Government of Canada through the Department of Innovation, Science and Economic Development Canada and by the Province of Ontario through the Ministry of Colleges and Universities. The work of M.S. is supported by Department of Energy Grants DE-SC0009919 and DE-SC0022104.
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Du, P., Egaña-Ugrinovic, D., Essig, R. et al. Low-energy radiative backgrounds in CCD-based dark-matter detectors. J. High Energ. Phys. 2024, 164 (2024). https://doi.org/10.1007/JHEP01(2024)164
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DOI: https://doi.org/10.1007/JHEP01(2024)164