Cosmological abundance of primordial black holes in mixed dark matter scenarios incorporating Kaluza-Klein dark matter

TL;DR

This paper addresses the cosmological abundance of primordial black holes in a mixed dark matter scenario with KK dark matter by modeling ultracompact halos formed around PBHs through accretion of KK particles after kinetic decoupling. It computes the KK DM annihilation–driven gamma-ray flux from these UCMHs and compares it to the extragalactic gamma-ray background measured by Fermi-LAT, using bin-wise limits to constrain the PBH fraction $f_{PBH}$. The analysis, aided by DarkSUSY spectra, finds that for KK DM masses in the range $500 \le m_{B^{(1)}} \le 1500$ GeV, the upper limit on the PBH fraction is $f_{PBH} \lesssim 2\times 10^{-5}$ (for $M_{PBH} \gtrsim 10^{-11} M_\odot$), with slightly weaker bounds for lighter PBHs and for p-wave annihilation scenarios. The results provide the first KK DM–specific PBH abundance limits and highlight the role of UCMH density enhancements in shaping the extragalactic gamma-ray background and PBH constraints.

Abstract

The lightest Kaluza-Klein (KK) dark matter particles and primordial black holes (PBHs) emerge as plausible candidates for dark matter. In scenarios where dark matter is a mix of KK particles and PBHs, PBHs can attract surrounding KK dark matter particles post-formation, leading to the creation of ultracompact dark matter halos (UCMHs). The distribution of KK dark matter particles within UCMHs tends to be steeper than in classical dark matter halo structures, such as the Navarro-Frenk-White model. Consequently, the annihilation rate of KK dark matter particles in UCMHs is significant. The high-energy photons resulting from the annihilation of KK particles in UCMHs contribute to the extragalactic gamma-ray background (EGB). Leveraging data from the $\mathtt{Fermi\text{-}LAT}$ experiment, we have derived, for the first time, upper limits on the cosmological abundance of PBHs in the context of KK dark matter annihilation. For a KK dark matter mass range of $500\le m_{\rm B^{(1)}}\le 1500$ GeV, which aligns with the observed present abundance of dark matter, the conservative limits on the fraction of dark matter in PBHs, for massive PBHs with $M_{\rm PBH}\gtrsim 10^{-11}M_{\odot}$, are $f_{\rm PBH} \lesssim 2\times 10^{-5}$.

Figures (2)

• Figure 1: The density profile of KK dark matter around different PBHs [Eq. (\ref{['eq:rho_r']})]. Here we have set the KK dark matter mass $m_{\rm B^{(1)}}=1500$ GeV. The horizontal brown dashed lines show the maximum density $\rho_{\rm max}$ at redshift $z=100$ and 1000 [Eq. (\ref{['eq:rho_max']})].
• Figure 2: Upper limits on the fraction of dark matter in PBHs in the mixed dark matter scenarios consisting of Kaluza-Klein dark matter and PBHs (blue and red solid lines). Constraints derived from several other measurements for comparison (dashed): 1) the ultracompact binary search in advanced LIGO-Virgo (O2)(black) LIGOScientific:2019kanKavanagh:2018ggo; 2) the anisotropy of cosmic microwave background (CMB) including the accreting PBHs in view of the Planck 2018 data (megenta) 2020PhRvR...2b3204S; 3) the gravitational lensing results measured by EROS (green) and HSC (orange) EROS-2:2006ryyCroon:2020ouk; 4) the disruption of neutron stars caused by their capture of PBHs (brown) Capela:2013yf. Data are taken from the zenodo bradley_j_kavanagh_2019_3538999. Note that the grey shaded areas with slashes are those excluded by extragalactic gamma-ray observations, which are applicable to all permitted mass ranges of KK dark matter. Additionally, the constraints for the p-wave annihilation scenario are illustrated (indicated by dot-dashed lines).