Inert Doublet Model and LEP II Limits (0810.3924v2)

Abstract: The inert doublet model is a minimal extension of the standard model introducing an additional SU(2) doublet with new scalar particles that could be produced at accelerators. While there exists no LEP II analysis dedicated for these inert scalars, the absence of a signal within searches for supersymmetric neutralinos can be used to constrain the inert doublet model. This translation however requires some care because of the different properties of the inert scalars and the neutralinos. We investigate what restrictions an existing DELPHI collaboration study of neutralino pair production can put on the inert scalars and discuss the result in connection with dark matter. We find that although an important part of the inert doublet model parameter space can be excluded by the LEP II data, the lightest inert particle still constitutes a valid dark matter candidate.

• The paper leverages DELPHI neutralino data to impose exclusion limits on inert scalar masses in the Inert Doublet Model.
• It analyzes the IDM parameter space with emphasis on vacuum stability and perturbativity constraints along specific mass differences.
• The study highlights future collider opportunities to explore remaining IDM regions, informing dark matter and new physics research.

Inert Doublet Model and LEP II Constraints: An Expert Review

This paper by Lundström, Gustafsson, and Edsjö provides a detailed analysis of the Inert Doublet Model (IDM), a minimal extension of the Standard Model (SM) that introduces an additional SU(2) doublet with scalar particles. The authors focus on the phenomenological implications of the IDM in connection with data from the LEP II accelerator, specifically regarding the limits that can be set on its parameter space given the absence of dedicated analyses for these inert scalars in the LEP II experimental results.

Summary of Key Findings

The IDM introduces three new scalar fields: two neutral (H^0 and A^0) and one charged (H^{\pm}). These scalars are stabilized by a Z_2 parity, which prohibits their direct coupling to SM fermions, rendering them "inert." This parity ensures the stability of the lightest inert particle, making H^0 or A^0 viable dark matter candidates.

A notable aspect discussed is the neutrality of LEP II data regarding inert scalars, given no specialized analyses were performed. However, the authors skillfully leverage existing DELPHI collaboration data on neutralino production to extract constraints on the IDM. By comparing the kinematics and production characteristics of neutralinos and inert scalars, they outline an approach to derive meaningful exclusion limits on the inert scalar masses using the existing LEP II neutralino search results.

Detailed Analysis and Impact

In examining IDM scenarios, the authors assess theoretical constraints such as vacuum stability (ensured by specific quartic coupling conditions) and perturbativity (requiring couplings to remain within perturbative limits). They also evaluate experimental constraints, including collider data (LEP I and II) and cosmological observations (WMAP) pertinent to the relic dark matter density.

The methodology revolves around simulating potential IDM signal events and evaluating them against the cuts and efficiencies used in the DELPHI neutralino analysis. A central finding is that for H^0 and A^0 scalars with significant mass differences ($\Delta m \gtrsim 8$ GeV), and where $m_{H^0} \lesssim 80$ GeV and $m_{A^0} \lesssim 100$ GeV, the IDM models are excluded by LEP II data.

Future Implications

On the theoretical frontier, the IDM serves as a prototype for other scalar-driven extensions of the SM, offering a simpler alternative to supersymmetric models for addressing issues like electroweak naturalness and dark matter. The constraints derived set a precedent for evaluating other non-SUSY models with similar characteristics.

Practically, the work hints at promising avenues for future experimental searches. The limited reach of LEP II at higher masses and $\Delta m \lesssim 8$ GeV underscores a significant parameter space for exploration with new or upgraded facilities, such as the LHC or future colliders.

Conclusion

The paper’s careful extraction of LEP II constraints showcases the utility of existing data in probing new physics models and reinforces the IDM as a viable framework within particle cosmology. While the parameter space for IDM models is partially constrained, the persistence of allowed regions suggests further innovation and application within both theoretical and experimental realms, advocating for adaptive methodologies for future collider experiments. The research establishes a foundational understanding, fostering continued inquiry into the complex interplay between particle physics extensions and observational realities.