Dark Radiation and Dark Matter in Large Volume Compactifications

TL;DR

The work demonstrates that in the LARGE Volume Scenario, the decay of the lightest volume modulus naturally produces dark radiation through its ultralight axionic partner, with the Giudice-Masiero term governing the dominant decay to Higgses. It shows a dichotomy: in sequestered LVS, non-thermally produced LSPs can account for the observed DM, while in non-sequestered LVS, the QCD axion serves as the DM candidate. The predicted DR contribution aligns with hints of extra relativistic energy density, requiring a moderate GM coupling parameter (roughly $z\sim 1.5$) and yielding a reheating temperature of a few GeV. The results provide a concrete string-motivated mechanism linking dark radiation and dark matter, with distinct phenomenological implications for each LVS realization and clear pathways for observational tests.

Abstract

We argue that dark radiation is naturally generated from the decay of the overall volume modulus in the LARGE volume scenario. We consider both sequestered and non-sequestered cases, and find that the axionic superpartner of the modulus is produced by the modulus decay and it can account for the dark radiation suggested by observations, while the modulus decay through the Giudice-Masiero term gives the dominant contribution to the total decay rate. In the sequestered case, the lightest supersymmetric particles produced by the modulus decay can naturally account for the observed dark matter density. In the non-sequestered case, on the other hand, the supersymmetric particles are not produced by the modulus decay, since the soft masses are of order the heavy gravitino mass. The QCD axion will then be a plausible dark matter candidate.

Figures (3)

• Figure 1: Contours of the additional effective number of neutrinos, $\Delta N_{\rm eff}$, the modulus reheating temperature, $T_d$ in the plane of the lightest modulus mass $(m_\phi)$ and the coefficient of the GM term $z$. Here the higgsino mass is given by the relation $\mu \simeq \frac{z}{{\log({\cal V})^{1/3}}} (m^{4}_{\phi}/M_{\rm pl})^{1/3} \approx \frac{z}{2.55} (m^{4}_{\phi}/M_{\rm pl})^{1/3}$ in the above two figures.
• Figure 2: Contours of the non-thermally produced Wino dark matter abundance, $\Omega_\chi h^2 = 0.01, 0.03, 0.1, 0.5$ and $1$ (solid (blue) lines) and the Wino mass, $m_{\tilde{W}} = 200, 300, 500, 700,$ and $1000$ GeV (dashed (red) lines), in the plane of the modulus mass $(m_\phi)$ and the coefficient of the GM term ($z$). The relation $m_{\tilde{W}} = 1/(\log({\cal V}) {\cal V}^2)$ is assumed. Here recall that $\mu \sim z m_{\tilde{W}}$.
• Figure 3: The relations between $\Delta N_{\rm eff}$ and the Wino mass $m_{\tilde{W}}$ are shown for the following four cases: (1) $m_{\tilde{W}} = 1/(\log({\cal V}) {\cal V}^2)$, (2) $m_{\tilde{W}} = 0.1/({\cal V}^2)$, (3) $m_{\tilde{W}} = 0.3/({\cal V}^2)$, and (4) $m_{\tilde{W}} = 0.5/({\cal V}^2)$. The right DM abundance, $\Omega_\chi h^2 \simeq 0.11$, is imposed.