Constraining Primordial Black Holes via p-wave annihilation in light of CMB Spectral Distortion and 21-cm global signal

Abstract

Primordial black holes (PBHs) can form spike density halos through the accretion of weakly interacting massive particles (WIMPs). In these halos, the enhanced density significantly boosts the annihilation rate of WIMPs. For Majorana dark matter annihilation into light fermions, the s-wave part of the annihilation cross section is helicity-suppressed, making the p-wave contribution dominant. We study the velocity-dependent p-wave annihilation case, whose resulting energy injection can modify the thermal and ionization history of the Universe, leaving observable imprints on the cosmic microwave background (CMB) spectrum and the global 21-cm signal. From the predicted energy injection into the plasma, we derive stringent upper limits on the fraction of dark matter in form of PBHs for p-wave annihilation models, based on the observational constraints of the CMB spectral distortions ($y$-type), and from the measurement of the 21-cm absorption signal at cosmic dawn. Our results highlight that accounting for the p-wave nature of annihilation is crucial for deriving robust constraints on the PBH abundance.

Figures (6)

• Figure 1: The density profile of WIMP annihilation as a function of radial distance. The black solid line corresponds to the density profile before WIMP annihilation. In the solid and dashed Coloured lines denote the effect of p-wave and s-wave WIMP annihilation, respectively, at redshift $z=1000$ for $\rm M_{PBH}=10^3\, M \odot$. The gray solid line indicates $r=2R_s$; for $r < 2 R_{S}$, the WIMPs are absorbed by PBH.
• Figure 1: Evolution of CMB spectral distortions, $|dy/dz|$, as a function of redshift z for $\rm M_{PBH}=10^3\, M \odot$. In both figures, the solid and dashed Coloured lines denote the CMB spectral distortion for p-wave and s-wave WIMP annihilation, respectively.
• Figure 2: Upper limits on the fraction of DM in the form of PBHs, $f_{\rm PBH}= \Omega_{\rm PBH}/\Omega_{\chi}$ as a function of PBH mass, $M_{\rm PBH}$. The bounds have been obtained using FIRAS and PIXIE limits on $y$-type of spectral distortion. In this plot, Solid, dashed, and dotted lines represent the upper bound for p-wave annihilation with WIMP mass $m_\chi=1\,\rm GeV,\,10\,\rm GeV,$ and $100\, \rm GeV$, respectively. Our upper bound compared with other observational constraints, from extragalactic gamma-rays observation of PBH evaporation(blue region), femto-lensing (brown region) Barnacka:2012bm, microlensing HSC/Subaru data (sky-blue region) Niikura:2017zjd, microlensing EROS/MACHO(green region) Niikura:2017zjd, X-ray emitted by gas accretion CMB (cyan region), and WMAP3(gray region) Ricotti:2007au.
• Figure 2: Upper limits on the fraction of DM in the form of PBHs, $f_{\rm PBH}= \Omega_{\rm PBH}/\Omega_{\chi}$ as a function of PBH mass, $M_{\rm PBH}$. In both plots, Solid, dashed, and dotted lines represent the upper bound for the s-wave case with thermally averaged WIMP annihilation cross section $\langle \sigma v \rangle_s\approx 3\times10^{-26}$$\rm cm^3 s^{-1}$ for WIMP mass $m_\chi=1\,\rm GeV,\,10\,\rm GeV,$ and $100\, \rm GeV$, respectively. In the left panel, the bounds have been obtained using FIRAS and PIXIE limits on $y$-type of spectral distortion, while in the right panel, the bounds are obtained by requiring ($\delta T_b=0$) at $z=17$ due to WIMP annihilation.
• Figure 3: Upper limits on the fraction of DM in the form of PBHs, $f_{\rm PBH}$ as a function of PBH mass, $M_{\rm PBH}$. We obtained bounds such that the WIMP annihilation will erase the amplitude of the 21 cm brightness temperature ($\delta T_b=0$) at $z=17$. Here, Solid, dashed, and dotted lines represent the upper bound for p-wave annihilation with DM mass $m_\chi=1\,\rm GeV,\,10\,\rm GeV,$ and $100\, \rm GeV$, respectively.