TeV Scale Singlet Dark Matter

TL;DR

This work analyzes TeV-scale neutral scalar singlet dark matter stabilized by a $Z_2$ symmetry, focusing on relic density and indirect detection in theories with a TeV cutoff such as Randall-Sundrum. The relic abundance is set by Higgs-portal annihilation, yielding an order-one coupling $\lambda$ for $M_\Phi \sim \text{TeV}$ in the brane-localized case, while a bulk RS scenario introduces higher-dimension operators that can open additional annihilation channels, potentially altering the relic density. Indirect detection prospects are explored, highlighting monochromatic gamma lines from higher-dimension operators with cross sections scaling as $(M_\Phi/\tilde{\Lambda})^4$, and high-energy positron features arising from annihilations into KK modes, with signals strongly dependent on the DM halo profile and the cutoff scale $\tilde{\Lambda}$. The paper discusses current experimental reach (FERMI, HESS, VERITAS) for gamma lines and the conditions under which high-energy positron signals might be observable, noting that a lighter DM around $\mathcal{O}(100\ \text{GeV})$ could also accommodate PAMELA data via specific operator structures. Overall, the study provides concrete, testable predictions for TeV-scale singlet DM in RS-like theories, connecting relic-density requirements to distinctive gamma-ray and positron signatures.

Abstract

It is well known that stable weak scale particles are viable dark matter candidates since the annihilation cross section is naturally about the right magnitude to leave the correct thermal residual abundance. Many dark matter searches have focused on relatively light dark matter consistent with weak couplings to the Standard Model. However, in a strongly coupled theory, or even if the coupling is just a few times bigger than the Standard Model couplings, dark matter can have TeV-scale mass with the correct thermal relic abundance. Here we consider neutral TeV-mass scalar dark matter, its necessary interactions, and potential signals. We consider signals both with and without higher-dimension operators generated by strong coupling at the TeV scale, as might happen for example in an RS scenario. We find some potential for detection in high energy photons that depends on the dark matter distribution. Detection in positrons at lower energies, such as those PAMELA probes, would be difficult though a higher energy positron signal could in principle be detectable over background. However, a light dark matter particle with higher-dimensional interactions consistent with a TeV cutoff can in principle match PAMELA data.

Figures (8)

• Figure 1: Left panel: annihilation cross section, $\langle \sigma_{\Phi\Phi \rightarrow HH} v \rangle$, for a brane-localized scalar in the non-relativistic regime as a function of $M_{\Phi}$, imposing the WMAP constraint on the DM relic density. The arrows indicate the points where the freeze-out temperature ($\sim M_{\Phi}/25$) crosses the $W^{\pm}$ and $Z^0$ thresholds. Right panel: the corresponding coupling $\lambda$, defined in Eq. (\ref{['4DPhiH']}), as a function of $M_{\Phi}$.
• Figure 2: Annihilation cross section, $\langle \sigma_{\Phi\Phi \rightarrow \psi^{(0)} \psi^{(1)}} v\rangle$, for bulk DM, as a function of $M_{\Phi}$, imposing the WMAP constraint on the DM relic density. The curves marked as "freeze-out" correspond to the annihilation cross section at the time of freeze-out (where the typical velocities were of order $v/c \sim \sqrt{2/25} \sim 0.3$). The lower curves correspond to the annihilation cross section in the ultra non-relativistic regime, as would be relevant for today's conditions. The various curves correspond to different choices of the fermion localization parameter $c_{f}$ that controls their masses and couplings. The arrows indicate the points where the freeze-out temperature ($\sim M_{\Phi}/25$) crosses the $W^{\pm}$ and $Z^0$ thresholds. The curves are terminated (with black dots) when $\lambda_{\psi} = 24\pi^{3}$, which we define as the strong coupling regime (see text). We assume that $\Lambda = 8 k$.
• Figure 3: Left panel: $\bar{J}(\Delta\Omega)$ as a function of $\Delta\Omega$ for three different halo profiles (taken from Ref. Thomas:2005te). Right panel: $\bar{J}(\Delta\Omega) \times \Delta \Omega$ as a function of $\Delta\Omega$.
• Figure 4: Integrated photon flux from $\Phi\Phi \rightarrow HH$ and $H \rightarrow \gamma\gamma$ or $H \rightarrow Z\gamma$, as a function of $M_{\Phi}$. We assume that the branching fractions for these decay modes are $10^{-3}$ and take $\bar{J}(\Delta\Omega)\times \Delta\Omega = 10^{2}$.
• Figure 5: Left panel: positron fraction including the primary positrons/electrons from the annihilation $\Phi\Phi \rightarrow e^{\pm} e^{(1)}$ and the secondary positrons/electrons from the annihilation $\Phi\Phi \rightarrow l^{(1)} l^{(0)}$, followed by a two-body decay $l^{(1)} \rightarrow e^{\pm} X$. We show the spectra for two DM masses, $M_{\Phi} = 500~{\rm GeV}$ and $M_{\Phi} = 2~{\rm TeV}$, assuming that $M_{e^{(1)}} = M_{\Phi}$ and a boost factor such that $B \times \langle \sigma_{e^{(1)}e} v/c \rangle = 1~{\rm pb}$, with $\rho_{0} = 0.3~{\rm GeV/cm^{3}}$. The solid lines represent the individual contributions from primary and secondary production. Right panel: positron fraction for $M_{\Phi} = 1~{\rm TeV}$ and $M_{l^{(1)}} = 0.85\times(2M_{\Phi}) = 1.7~{\rm TeV}$ showing clear peaks at $E^{\rm prim.}_{e^{+}} = (4M^{2}_{\Phi} - M^{2}_{e^{(1)}})/4M_{\Phi}$ and near $M_{\Phi}$ (the endpoint is exactly at $M_{\Phi}$).