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تسجيل مستخدم جديدHints for decaying dark matter from $S_8$ measurements
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Recent weak lensing surveys have revealed that the direct measurement of the parameter combination $S_8equivsigma_8(Omega_m/0.3)^{0.5}$ -- measuring the amplitude of matter fluctuations on 8 $h^{-1}$Mpc scales -- is $sim3sigma$ discrepant with the value reconstructed from cosmic microwave background (CMB) data assuming the $Lambda$CDM model. In this Letter, we show that it is possible to resolve the tension if dark matter (DM) decays with a lifetime of $text{log}_{10}(Gamma^{-1}/ text{Gyr})= 1.75_{-0.95}^{+1.4}$ into one massless and one massive product, and transfers a fraction $varepsilonsimeq 0.7^{+2.7}_{-0.6}%$ of its rest mass energy to the massless component. The velocity-kick received by the massive daughter leads to a suppression of gravitational clustering below its free-streaming length, thereby reducing the $sigma_8$ value as compared to that inferred from the standard $Lambda$CDM model, in a similar fashion to massive neutrino and standard warm DM. Contrarily to the latter scenarios, the time-dependence of the power suppression and the free-streaming scale allows the 2-body decaying DM scenario to accommodate CMB, baryon acoustic oscillation, growth factor and uncalibrated supernova Ia data. We briefly discuss implications for DM model building, galactic small-scale structure problems and the recent Xenon-1T excess. Future experiments measuring the growth factor to high accuracy at $0lesssim zlesssim1$ can further test this scenario.
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The $S_8$ tension is a longstanding discrepancy between the cosmological and local determination of the amplitude of matter fluctuations, parameterized as $S_8equivsigma_8(Omega_m/0.3)^{0.5}$, where $sigma_8$ is the root mean square of matter fluctua
tions on a 8 $h^{-1}$Mpc scale, and $Omega_m$ is the total matter abundance. It was recently shown that dark matter (DM) decaying into a massless (dark radiation) and a massive (warm DM) species, with a lifetime $Gamma^{-1} simeq 55~ (varepsilon/0.007)^{1.4}$ Gyrs -- where $varepsilon$ represent the mass-energy fraction transferred to the massless component -- can resolve the tension. Thanks to a new, fast and accurate approximation scheme for the warm species, we perform a comprehensive study of this 2-body decaying DM scenario, discussing in details its dynamics and its impact on the CMB and linear matter power spectra. We then confront the robustness of the resolution to the $S_8$ tension against a number of changes in the analysis: different $S_8$ priors, marginalization over the lensing information in Planck data, trading Planck high$-ell$ polarization data for those from the SPTpol collaboration, and the inclusion of the recent results from the Xenon1T collaboration. We conclude that the preference for decaying DM, while entirely driven by the local $S_8$ measurements, does not sensibly degrade the fit to any of the cosmological data-sets considered, and that the model could explain the anomalous electron recoil excess reported by the Xenon1T collaboration.
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