A Renormalizable Model for the Galactic Center Gamma Ray Excess from Dark Matter Annihilation (1404.3716v2)

Abstract: Evidence for an excess of gamma rays with O(GeV) energy coming from the center of our galaxy has been steadily accumulating over the past several years. Recent studies of the excess in data from the Fermi telescope have cast doubt on an explanation for the excess arising from unknown astrophysical sources. A potential source of the excess is the annihilation of dark matter into standard model final states, giving rise to gamma ray production. The spectrum of the excess is well fit by 30 GeV dark matter annihilating into a pair of b quarks with a cross section of the same order of magnitude as expected for a thermal relic. Simple models that can lead to this annihilation channel for dark matter are in strong tension with null results from direct detection experiments. We construct a renormalizable model where dark matter-standard model interactions are mediated by a pseudoscalar that mixes with the CP-odd component of a pair of Higgs doublets, allowing for the gamma ray excess to be explained while suppressing the direct detection signal. We consider implications for this scenario from Higgs decays, rare B meson decays and monojet searches and also comment on some difficulties that any dark matter model explaining the gamma ray excess via direct annihilation into quarks will encounter.

• The paper introduces a renormalizable model that explains the gamma ray excess from the Galactic Center via dark matter annihilation into bottom quarks with a thermal relic cross section.
• The model employs a pseudoscalar mediator mixed with a CP-odd two-Higgs doublet to suppress direct detection signals while aligning with Higgs decay observations.
• The framework rigorously addresses collider, B physics, and direct detection constraints, offering clear predictions for future experiments to test dark matter properties.

Overview of a Renormalizable Model for the Galactic Center Gamma Ray Excess from Dark Matter Annihilation

The paper presents a theoretical framework addressing the observed gamma ray excess in the Galactic Center detected by the Fermi Gamma-Ray Space Telescope. This phenomenon has led researchers to consider the possibility of such excess originating from dark matter (DM) annihilation, particularly into standard model (SM) particles resulting in gamma-ray production.

The authors investigate models that describe DM annihilating into a pair of bottom quarks ($b\bar{b}$) with a mass around 30 GeV. This scenario aligns the annihilation cross section with the expected value for a thermal relic: $$\langle \sigma v_{\rm rel}\rangle \approx 3\times10^{-26}~{\rm cm^3}/{\rm s}$$. A significant challenge in constructing such a model is reconciling the excess annihilation events with non-detection in direct detection experiments, where the strong limits on spin-independent nucleon scattering apply especially rigidly in this mass range.

Model Construction

The authors propose a renormalizable model that introduces a mechanism to suppress the signal in direct detection experiments. The model includes a pseudoscalar mediator that interfuses with the CP-odd component of a two-Higgs doublet model (2HDM). This mixing allows the model to evade the stringent bounds placed by direct detection experiments by suppressing the velocity-independent spin-independent cross section. The major aspects of the model are:

• Dark Sector Interaction: The dark matter candidate, a Dirac fermion, interacts with a pseudoscalar mediator, producing gamma rays as a result of annihilation into SM particles like $b\bar{b}$.
• Higgs Portal and Extended Higgs Sector: Utilizing a 2HDM framework, the model enables a CP-odd Higgs that mixes with the DM mediator through a Higgs portal. This construction circumvents the problems of excessive spin-independent interaction rates by leveraging off-diagonal CP properties of the dark sector interaction Hamiltonian.

Theoretical and Experimental Considerations

Key experimental constraints examined in the paper pertain to:

• Direct Detection Limits: The spin-independent cross section remains drastically suppressed below current experimental detection capabilities. The one-loop diagrams generate scalar-scalar interactions that still evade current state-of-the-art direct detection parameters, while offering an avenue for potential future detection improvements.
• Higgs Decay Modes: Exotic decays of the 125 GeV Higgs such as $h \rightarrow aa$ where $a$ represents the pseudoscalar mediator, play a critical role. These contribute to final states like $4b$ or $2b2\mu$, directly influencing branching ratios and necessitating recourse to heavy Higgs search guidelines for experimental verifications.
• B Physics Observables: Contributions to processes such as $B_s \rightarrow \mu^+\mu^-$ serve as pivotal tests, given their precise agreement with Standard Model predictions. These processes further narrow the parameter space for lighter mediator masses.
• Collider Probes: Collider-based analyses, focusing on monojet plus missing energy signatures with a $b$-tagged jet, provide avenues for constraining this model, although monojet searches remain less sensitive due to suppressed couplings in the model's large $\tan\beta$ regime.

Future Implications

The theoretical structure offers a comprehensive framework for addressing the Galactic Center gamma ray excess. Future experimental efforts, from more refined dark matter direct detection capabilities to enhanced collider experiments probing Higgs physics, hold potential to further scrutinize or support such model parameters. The emphasis on pseudoscalar mediating interactions displays a considered approach in model-building to meet both theoretical underpinnings and experimental reality without overstepping observational evidence.

In conclusion, the paper presents a cohesive theoretical model that not only constrains itself to existing empirical standards but also sets a groundwork for future explorations in particle and astroparticle physics to unravel the mysteries of dark matter. It serves as a blueprint indicating pathways for reconciling observed cosmic phenomena with extensions of the established physics frameworks.