Constraining long-lived dark sector particles with CMB and Lyman-$α$

We use measurements of the intergalactic medium (IGM) temperature from the Lyman-$α$ forest to place new limits on models in which long-lived dark sector (DS) particles, with lifetimes longer than $10^{16}$ s, deposit energy into the IGM through their decays. Such DS decays into Standard Model (SM) states can modify the late-time thermal history of the IGM, making Lyman-$α$ data a sensitive probe of hidden sectors with cosmologically long lifetimes. Our analysis demonstrates that constraints from late-time IGM heating offer a complementary window to those from the Cosmic Microwave Background (CMB), in constraining dark sector parameter space. We further revisit limits on such decaying DS models from Planck’s measurements of the optical depth to reionization and provide updates relevant for DS lifetimes longer than $10^{14}$ s. The model-independent constraints on the DS parameter space we derive in this work can be reinterpreted for a wide range of decaying hidden-sector scenarios, including evaporating primordial black holes and SM-coupled dark photons.

💡 Research Summary

This paper investigates how long‑lived dark‑sector (DS) particles—those with lifetimes longer than 10¹⁶ seconds—affect the thermal and ionization history of the intergalactic medium (IGM) through their decays into Standard Model (SM) states. By exploiting precise measurements of the IGM temperature derived from the Lyman‑α forest, the authors place new, model‑independent constraints on the DS parameter space defined by the fraction of dark matter composed of the DS species (f_DS) and the particle lifetime (τ_DS).

The energy injection rate from DS decay is given by
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