Gamma-rays from ultracompact minihalos: potential constraints on the primordial curvature perturbation

TL;DR

This paper explores how gamma-ray observations of ultracompact minihalos (UCMHs) can constrain the primordial curvature perturbation spectrum, providing tighter limits than previous methods under specific dark matter assumptions.

Contribution

It introduces a method to use Fermi satellite data to set new bounds on UCMH abundance and primordial power spectrum, assuming WIMP dark matter and intact UCMHs.

Findings

Detection threshold for UCMHs with Fermi is about 10^{-7} halo fraction.

Non-detection constrains the primordial power spectrum to P_R < 10^{-6.5} to 10^{-6}.

Constraints are tighter than those from primordial black hole formation.

Abstract

Ultracompact minihalos (UCMHs) are dense dark matter structures which can form from large density perturbations shortly after matter-radiation equality. If dark matter is in the form of Weakly Interacting Massive Particles (WIMPs), then UCMHs may be detected via their gamma-ray emission. We investigate how the {\em{Fermi}} satellite could constrain the abundance of UCMHs and place limits on the power spectrum of the primordial curvature perturbation. Detection by {\em Fermi} would put a lower limit on the UCMH halo fraction. The smallest detectable halo fraction, $f_{\rm UCMH} \gtrsim 10^{-7}$, is for $M_{\rm UCMH} \sim 10^{3} M_{\odot}$. If gamma-ray emission from UCMHs is not detected, an upper limit can be placed on the halo fraction. The bound is tightest, $f_{\rm UCMH} \lesssim 10^{-5}$, for $M_{\rm UCMH} \sim 10^{5} M_{\odot}$. The resulting upper limit on the power spectrum of the primordial curvature perturbation in the event of non-detection is in the range $\mathcal{P_R} \lesssim 10^{-6.5}- 10^{-6}$ on scales $k \sim 10^{1}-10^{6} \, {\rm Mpc}^{-1}$. This is substantially tighter than the existing constraints from primordial black hole formation on these scales, however it assumes that dark matter is in the form of WIMPs and UCMHs are not disrupted during the formation of the Milky Way halo.