Stellar cooling limits on KK gravitons and dark dimensions

TL;DR

This paper refines bounds on KK gravitons in dark dimensions by analyzing stellar cooling, introducing new production channels, and providing updated constraints from supernovae, especially SN 1987A.

Contribution

It introduces novel production mechanisms for KK gravitons in stellar environments and updates the bounds on the KK mass scale considering these channels.

Findings

SN 1987A provides the strongest limits on KK gravitons.

Pion-induced processes can dominate KK graviton production in supernovae.

Bounds on the KK mass scale depend on the number of extra dimensions, with stronger limits for more dimensions.

Abstract

We revisit cooling bounds on light Kaluza-Klein (KK) gravitons, as arise in the dark dimension scenario, considering red giants, neutron stars, and supernovae. In addition to bremsstrahlung, we account for two novel production channels: resonant mixing with the in-medium photon and a pion-induced process in supernovae. The strongest limits arise from SN 1987A, with the emissivity from the pion process exceeding that from bremsstrahlung by a factor of a few. Given present uncertainties, we obtain a bound on the KK mass scale of $m_{\rm KK}\gtrsim 0.6\,{\rm eV}$ $(\gtrsim 500\,{\rm eV})$ for 2 (3) extra dimensions. Improved understanding of the properties of pions in supernovae could strengthen these limits to roughly ${\rm eV}$ $({\rm keV})$. For 1 extra dimension, the bounds are weaker than those from laboratory searches. We also show that constraints from KK graviton decays to Standard Model particles are less stringent than the cooling bounds if there is KK number violation at the level typically assumed in the dark dimension scenario, although these bounds could be strengthened by future observations.