Inert Doublet Model and LEP II Limits

TL;DR

The paper assesses how LEP II data constrain the Inert Doublet Model (IDM), a minimal extension with inert scalars $H^0$, $A^0$, and $H^{ pm}$ stabilized by a $Z_2$ symmetry and with the lightest inert particle as a dark matter candidate. By reinterpreting a DELPHI neutralino pair-production analysis through careful signal generation for $e^+e^- o H^0A^0 o H^0H^0 far{f}$ and matching selection efficiencies to MSSM-based results, the authors derive IDM cross-section limits and map them onto the $(m_{H^0},m_{A^0})$ plane. The results show that LEP II excludes substantial IDM parameter space (e.g., $m_{H^0} aisebox{-0.5ex}{$<$}{}80$ GeV, $m_{A^0} aisebox{-0.5ex}{$<$}{}100$ GeV with $ riangle m>8$ GeV), while regions compatible with the observed dark matter relic density remain viable, particularly for certain $m_h$ values and small coannihilation effects. Overall, the study demonstrates that LEP II constraints, when treated with IDM-specific efficiency considerations, provide nontrivial, robust bounds that complement DM relic-density analyses and guide future collider and astroparticle investigations of the IDM.

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.

Figures (9)

• Figure 1: Representative Feynman diagrams contributing to $\tilde{\chi}_1^0 \tilde{\chi}_2^0$ events at LEP. (a)-(d) show the process factorized into $e^+e^-\rightarrow\tilde{\chi}_1^0\tilde{\chi}_2^0$ production, (a) and (b), and subsequent $\tilde{\chi}_2^0\rightarrow\tilde{\chi}_1^0 f\bar{f}$ decay, (c) and (d). (e)-(h) show the unfactorized process $e^+e^-\rightarrow\tilde{\chi}_1^0\tilde{\chi}_2^0\rightarrow\tilde{\chi}_1^0\tilde{\chi}_1^0f\bar{f}$.
• Figure 2: Feynman diagrams contributing to $H^0 A^0$ events at LEP. (a) and (b) show the process factorized into $e^+e^-\rightarrow H^0 A^0$ production, (a), and subsequent $A^0\rightarrow H^0 f\bar{f}$ decay, (b). (c) shows the unfactorized process $e^+e^-\rightarrow H^0 A^0\rightarrow H^0 H^0 f\bar{f}$.
• Figure 3: Angular distribution of $A^0$ (IDM) versus$\tilde{\chi}_2^0$ (MSSM) produced in $e^+e^-\rightarrow \tilde{\chi}_1^0 \tilde{\chi}_2^0$ and $e^+e^-\rightarrow H^0 A^0$, respectively. The header of each subfigure displays (in units of GeV) the mass $m_1$ of $H^0$ and $\tilde{\chi}_1^0$, and the mass $m_2$ of $A^0$ and $\tilde{\chi}_2^0$. The beam pipe is defined to be along $\cos \theta = \pm 1$ and the center-of-mass energy is $\sqrt s = 206$ GeV. The large difference between the IDM and the MSSM models is due to the scalar versus fermion nature of the outgoing states.
• Figure 4: Angular distribution of final state fermions from $e^+e^-\rightarrow H^0 A^0\rightarrow H^0 H^0 f\bar{f}$ (IDM) versus$e^+e^-\rightarrow \tilde{\chi}_1^0 \tilde{\chi}_2^0$, $\tilde{\chi}_2^0\rightarrow \tilde{\chi}_1^0 f\bar{f}$ (MSSM). The models are the same as in Fig. \ref{['fig:3']}. In (a) the velocity of the mother particle $A^0$/$\tilde{\chi}_2^0$ is large and the energy injected into the fermions during the decay is relatively low, and hence the discrepancy from Fig. \ref{['fig:3']} can survive.
• Figure 5: Fermion opening angle distribution for the same processes and models as in Fig. \ref{['fig:4']}. $\beta$ is the angle between the two outgoing fermions $f$ and $\bar{f}$ as measured in the lab frame. In (a) the IDM and SUSY distributions happen to be overlapping, but one should keep in mind that in the MSSM the decay distribution can depend on the gaugino fraction of $\tilde{\chi}_2^0$. Figs. \ref{['fig:3']}-\ref{['fig:5']} are based on $10^5$ events per model, as generated with MadGraph/MadEventAlwall:2007st.