The Waning of the WIMP? A Review of Models, Searches, and Constraints (1703.07364v1)

Abstract: Weakly Interacting Massive Particles (WIMPs) are among the best-motivated dark matter candidates. In light of no conclusive detection signal yet despite an extensive search program that combines, often in a complementary way, direct, indirect, and collider probes, we find it timely to give a broad overview of the WIMP paradigm. In particular, we review here the theoretical foundations of the WIMP paradigm, discuss status and prospects of various detection strategies, and explore future experimental challenges and opportunities.

• The paper reviews Weakly Interacting Massive Particle (WIMP) dark matter theory, experimental searches, and current constraints from experimental data.
• Experimental searches for WIMPs, including direct, indirect, and collider methods, have placed increasingly stringent constraints on the particle's mass and interaction cross-section.
• While WIMP models remain theoretically compelling, current experimental constraints are stringent, necessitating refined theoretical models and continued experimental sensitivity improvements to explore remaining parameter space.

The Waning of the WIMP? A Review of Models, Searches, and Constraints

This paper undertakes a comprehensive evaluation of Weakly Interacting Massive Particles (WIMPs) as dark matter candidates, examining the theoretical underpinnings, detection methods, and current constraints on these models. The analysis addresses various WIMP models, exploring how different assumptions and parameters impact their viability in light of recent experimental data.

Theoretical Framework and Motivation

WIMPs are compelling dark matter candidates because they naturally arise in several extensions of the Standard Model and can account for the observed dark matter density through thermal freeze-out. This involves a new particle species $\chi$, in equilibrium with the early universe's thermal bath, which freezes out when its annihilation rate drops below the expansion rate. The mass and interaction cross-section of WIMPs are such that they result in the correct relic abundance when $$\langle \sigma v \rangle \sim 10^{-26} \ \text{cm}^3\text{s}^{-1}$$. This "WIMP miracle" connects the weak interaction scale with cosmological constraints, motivating searches for these particles in various contexts.

Detection Strategies

The extensive search for WIMPs includes three main approaches: direct detection, indirect detection, and collider searches:

1. Direct Detection: Experiments like LUX and XENON1T aim to measure nuclear recoils induced by WIMP interactions with target materials. The main challenge is differentiating potential WIMP signals from background events, requiring low-threshold detectors and strategies to reduce or distinguish background interactions. The cross-sectional limits for spin-independent and spin-dependent WIMP-nucleon interactions have become very stringent, particularly with exposures and materials that optimize sensitivity to expected WIMP signatures.
2. Indirect Detection: This approach looks for byproducts of WIMP annihilation or decay—typically gamma rays, neutrinos, and cosmic rays—using instruments such as Fermi-LAT and AMS. Regions of interest include the Galactic center, dwarf spheroidal galaxies, and galaxy clusters where enhanced signal rates are expected. The constraints obtained from null results in these regions provide complementary limits to direct detection.
3. Collider Searches: The LHC can potentially produce WIMPs directly in high-energy collisions, manifesting as missing transverse energy amidst an event with visible Standard Model particles (mono-X searches). Constraints from these searches are considered alongside those from direct detection and indirect observation to delineate the parameter space of viable WIMP models.

Model Review and Constraints

The paper classifies WIMP models into various categories based on interaction mediators and DM types, each with different theoretical motivations and implications:

• Standard Model (SM) Portals: Higgs and $Z$ boson mediated interactions represent the minimal extensions where the dark sector communicates with SM fields. However, current direct detection limits heavily constrain such scenarios.
• BSM s-channel Portals: Extensions with additional scalar or vector mediators allow more freedom in model parameters. These models are subject to strong direct detection limits due to the predictable coupling with nucleons.
• Portals Evading Direct Detection: Scenarios like the pseudoscalar mediator model might evade direct limits due to suppressed interaction terms. However, such models remain constrained by indirect signals and collider searches, necessitating significant model tuning.
• Secluded Sector Portals: These involve mediators that primarily interact with dark matter, only indirectly affecting SM fields through mixing or loop-induced processes, such as kinetic mixing models.
• t-channel Portals: In these configurations, the new physics modifies the topology of dark matter interaction diagrams, which can still produce observable effects but with different signatures and constraints.

Future Directions and Implications

The paper speculates on future developments in the search for WIMPs, highlighting the growing capability of next-generation direct detection experiments and the continuing role of collider physics. The interaction landscape of WIMP models suggests potential areas of focus, such as elucidating the nature of DM interactions and addressing the parameter spaces still allowed by current limits.

The evolution of theoretical models in response to experimental results underscores the dynamism of this field. Researchers are encouraged to refine models to incorporate broader frameworks, such as those involving multiple particle sectors or non-standard cosmologies, while remaining receptive to unexpected findings that may arise as sensitivity increases.

In conclusion, while WIMP models remain compelling in the quest to identify dark matter, ongoing developments necessitate a refined understanding of their theoretical bases and experimental implications. The nuanced interplay between different detection strategies continues to shape the search landscape and guides the theoretical adjustments necessary to accommodate new findings.