Correlation between Dark Matter and Dark Radiation in String Compactifications

TL;DR

The paper investigates whether a link exists between dark matter (DM) and dark radiation (DR) in string compactifications, focusing on modulus decays during reheating. By computing the reheating temperature $T_{ m rh}$ and the DR abundance quantified via $N_{ m eff}$ within perturbatively stabilised models, the authors derive a correlation between $\Delta N_{ m eff}$ and the DM mass $m_{ m DM}$ that is constrained by Planck and Fermi data; they show that DR production from light bulk axions is generic and imposes strong bounds on the DM production mechanism. The analysis, especially in sequestered LVS models, finds that the allowed parameter space largely prefers non-thermal Higgsino-like DM with $T_{ m rh}\gtrsim 1$ GeV, while thermal DM is only viable if $\Delta N_{ m eff}$ is pushed toward the SM value. These results connect the high-scale SUSY-breaking structure to cosmological observables and yield testable lower bounds on $m_{ m DM}$ and reheating, with implications for collider and astrophysical searches.

Abstract

Reheating in string compactifications is generically driven by the decay of the lightest modulus which produces Standard Model particles, dark matter and light hidden sector degrees of freedom that behave as dark radiation. This common origin allows us to find an interesting correlation between dark matter and dark radiation. By combining present upper bounds on the effective number of neutrino species N_eff with lower bounds on the reheating temperature as a function of the dark matter mass m_DM from Fermi data, we obtain strong constraints on the (N_eff,m_DM)-plane. Most of the allowed region in this plane corresponds to non-thermal scenarios with Higgsino-like dark matter. Thermal dark matter can be allowed only if N_eff tends to its Standard Model value. We show that the above situation is realised in models with perturbative moduli stabilisation where the production of dark radiation is unavoidable since bulk closed string axions remain light and do not get eaten up by anomalous U(1)s.

Figures (3)

• Figure 1: Lower bound on $T_{\rm rh}$ (solid line) based on Fermi data for $b \overline{b}$ final state fermi. The result for $WW$ final state is similar. We have taken $T_{\rm f}\simeq \frac{m_{\rm DM}}{20}$ (dashed line). The shaded region is ruled out due to DM overproduction both in the thermal case (for $m_{\rm DM}\lesssim 40$ GeV and above the dashed line) and in the branching scenario (below the solid and dashed lines).
• Figure 2: Constraints on the $(\Delta N_{\rm eff},m_{\rm DM})$-plane for $c_{\rm hid}=1$, $g_*=68.5$ and $m_\phi= 5\cdot 10^6$ GeV: the solid line is based on Fermi data whereas the dashed line represents the freeze-out temperature. The shaded region is ruled out due to DM overproduction both in the thermal case (for $m_{\rm DM}\lesssim 40$ GeV and below the dashed line) and in the non-thermal branching scenario (above the solid and dashed lines).
• Figure 3: Lower bound on the DM mass as a function of $\Delta N_{\rm eff}$ for different values of the modulus mass: $m_\phi=4\cdot 10^6$ GeV (solid line), $m_\phi= 5\cdot 10^6$ GeV (dashed line) and $m_\phi= 6\cdot 10^6$ GeV (dotted line). The shaded region is ruled out based on Fermi data fermi. Here we have set $g_*=68.5$ and $c_{\rm hid}=1$.