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Constraints with CMB lensing on dark matter decays to massive decay products

Anna Bencke*, Nanoom Lee, and Marc Kamionkowski

  • William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA

  • *Contact author: abencke1@jh.edu
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Phys. Rev. D 114, 103503 – Published 3 November, 2026

DOI: https://doi.org/10.1103/75ps-w7b3

Abstract

Motivated by the recent measurements by the Dark Energy Spectroscopic Instrument (DESI) which suggest a late-time matter density approximately 5% lower than that inferred from Planck we investigate models in which dark matter decays to two less massive, and thus warm, states. The decaying dark matter (DDM) models are parametrized by the fraction f of dark matter that decays, the decay rate Γ, and the fraction ϵ of the mass retained by the decay products. To efficiently explore the warm decay product regime, we employ classier-ddm, a modified version of the public Boltzmann solver class that solves the perturbation equations with DDM via an integral-equation method. We consider DESI DR2 baryon acoustic oscillations and Planck 2018 CMB data including lensing, and find that DDM models are not favored over the Λ cold dark matter model. This result arises because CMB lensing tightly constrains the velocity kicks imparted to decay products. For example, for f≃0.5, we find 1σ constraints to the decay-product kick velocities an order of 10−2 to 10−3 times the speed of light for decay redshifts from shortly after recombination until today. Nonetheless, the allowed parameter space includes models with sufficient power suppression at small-scales to potentially address dwarf galaxy anomalies. Our results also suggest that explanations for DESI that involve dark-matter decays to one massive and one massless particle will be constrained by CMB lensing.

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Phys. Rev. D 114, 103503– Published 3 November, 2026
  • Received 14 July 2026
  • Accepted 6 October 2026
Article part of CHORUS
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