Spinless photon dark matter from two universal extra dimensions

TL;DR

The paper investigates the spinless photon $B_H$, the lightest KK-odd state in a two-universal-extra-dimensions model, as a dark matter candidate. It computes annihilation cross sections, establishes the relic abundance constraints, and analyzes detection prospects. The findings show $M_B$ must satisfy $M_B \lesssim 500$ GeV (down to $\sim 200$ GeV for a light Higgs) to match the observed density, with annihilation dominated by boson final states and fermionic channels helicity suppressed; direct detection remains below current experimental reach but could be probed by future experiments, while indirect detection rates resemble neutralinos and neutrino telescope signals are too small for next-gen experiments. The study highlights a neutralino-like phenomenology for 6D KK dark matter, distinguishing it from 5D KK DM and outlining avenues for further exploration of the 6DSM parameter space and alternative DM candidates.

Abstract

We explore the properties of dark matter in theories with two universal extra dimensions, where the lightest Kaluza-Klein state is a spin-0 neutral particle, representing a six-dimensional photon polarized along the extra dimensions. Annihilation of this 'spinless photon' proceeds predominantly through Higgs boson exchange, and is largely independent of other Kaluza-Klein particles. The measured relic abundance sets an upper limit on the spinless photon mass of 500 GeV, which decreases to almost 200 GeV if the Higgs boson is light. The phenomenology of this dark matter candidate is strikingly different from Kaluza-Klein dark matter in theories with one universal extra dimension. Elastic scattering of the spinless photon with quarks is helicity suppressed, making its direct detection challenging, although possible at upcoming experiments. The prospects for indirect detection with gamma rays and antimatter are similar to those of neutralinos. The rates predicted at neutrino telescopes are below the sensitivity of next-generation experiments.

Figures (7)

• Figure 1: The only tree-level contribution to $B_H B_H$ annihilation into $W^+W^-$. The same diagram with the $W$ bosons replaced by $Z$ bosons describes annihilation into $Z$ pairs.
• Figure 2: Tree level diagrams for $B_H B_H$ annihilation into $hh$ (the $u$-channel diagram is not shown).
• Figure 3: Tree-level diagrams for $B_H B_H$ annihilation into $t\bar{t}$.
• Figure 4: Left: The coefficients $a$ and $b$ from the non-relativistic expansion of the total $B_H$ annihilation cross section. The shaded band corresponds to the current range of $a$ measured by WMAP ($0.096 < \Omega_{B_H} h^2 < 0.122$ at 2$\sigma$) and $b$ includes the relativistic correction, $-a/4$. Right: the relative contribution to $a_{\rm total}$ from various final states. Note that the non-relativistic expansion fails near the Higgs $s$-channel resonance, $2 M_B=m_h=500$ GeV.
• Figure 5: The region (shaded) of the $m_h$ vs. $M_B$ plane in which the $B_H$ thermal relic abundance is within the range measured by WMAP ($0.096 < \Omega_{B_H} h^2 < 0.122$).