Dark Radiation in LARGE Volume Models

TL;DR

This paper analyzes reheating in the LARGE Volume Scenario (LVS), where the light volume modulus and its axionic partner inevitably produce dark radiation as it decays, even as decays to Higgses via a Giudice-Masiero term feed the visible sector. The authors compute the leading decay channels of the volume modulus, showing that the dominant invisible channel is Φ→a_b a_b and the principal visible channel is Φ→H_u H_d, with the latter controlled by the Giudice-Masiero parameter Z. They derive the resulting ΔN_eff in terms of the model parameters, finding ΔN_eff ≈ 1.56–1.74 for a minimal Higgs sector and Z=1, and show that introducing additional local axions or Higgs doublets can push ΔN_eff beyond experimental bounds. Consequently, while dark radiation is generic in sequestered LVS, observational constraints strongly limit the allowed axion spectra and Higgs content, posing challenges for an axiverse in these models. Planned precision measurements from Planck and future CMB probes will further tighten these constraints and sharpen the viability of LVS-based reheating scenarios.

Abstract

We consider reheating driven by volume modulus decays in the LARGE Volume Scenario. Such reheating always generates non-zero dark radiation through the decays to the axion partner, while the only competitive visible sector decays are Higgs pairs via the Giudice-Masiero term. In the framework of sequestered models where the cosmological moduli problem is absent, the simplest model with a shift-symmetric Higgs sector generates 1.56 < N_{eff} - N_{eff,SM} < 1.74. For more general cases, the known experimental bounds on N_{eff} strongly constrain the parameters and matter content of the models.

Figures (1)

• Figure 1: Contour plot of $\Delta N_{eff}$ versus $n_H$ ($x$-axis) and $Z$ ($y$-axis) for $n_a=0$ and $T_{reheat}\lesssim 1$ GeV.