The Inert Doublet Model and Inelastic Dark Matter

TL;DR

This work examines whether the DAMA annual modulation can be reconciled with the Inert Doublet Model (IDM) in two distinct regimes: elastic scattering of a light WIMP via the Higgs portal and inelastic Dark Matter (iDM) with a tiny neutral-scalar mass splitting protected by a Peccei-Quinn symmetry. The analysis shows that a light, Higgs-portal WIMP can fit DAMA but remains tightly constrained by other direct-detection searches; conversely, iDM in the IDM admits a broad heavy-mass region ($\sim535\,\mathrm{GeV}$ to $\sim50\,\mathrm{TeV}$) that can reproduce DAMA spectra and be consistent with WMAP relic density, with lighter masses possible if a dark-sector asymmetry persists. The results highlight how symmetry protections ($U(1)_{PQ}$) naturally generate the required small splittings and how coannihilation and mass splittings shape the relic abundance and direct-detection signals. Overall, the IDM provides a minimal, testable framework that links DAMA phenomenology to Higgs-portal and Z-exchange DM interactions, potential collider signatures, and dark-sector dynamics such as asymmetries and leptogenesis extensions.

Abstract

The annual modulation observed by DAMA/NaI and DAMA/Libra may be interpreted in terms of elastic or inelastic scattering of dark matter particles. In this paper we confront these two scenarios within the framework of a very simple extension of the Standard Model, the Inert Doublet Model (IDM). In this model the dark matter candidate is a scalar, the lightest component of an extra Higgs doublet. We first revisit the case for the elastic scattering of a light scalar WIMP, M_DM~10 GeV, a scenario which requires that a fraction of events in DAMA are channelled. Second we consider the possibility of inelastic Dark Matter (iDM). This option is technically natural in the IDM, in the sense that the mass splitting between the lightest and next-to-lightest neutral scalars may be protected by a Peccei-Quinn (PQ) symmetry. We show that candidates with a mass M_DM between ~535 GeV and ~50 TeV may reproduce the DAMA data and have a cosmic abundance in agreement with WMAP. This range may be extended to candidates as light as ~50 GeV if we exploit the possibility that the approximate PQ symmetry is effectively conserved and that a primordial asymmetry in the dark sector may survive until freeze-out.

Figures (9)

• Figure 1: Elastic scenario The averaged modulated amplitude $S_{m}$ for the best fit point (see Figure \ref{['fig:mlbda']}). The relevant parameters are given in the table on the right. The $\chi^2_{min}$ is evaluated for 34 d.o.f.
• Figure 2: Elastic scenario (model independent, $f_p=f_n$): Allowed regions in the plane $\sigma^0_{n} - M_{DM}$ consistent with the DAMA annual modulation signal at $90\%$, $99\%$ and $99.9\%$ CL. The solid (brown) curve shows the total unmodulated rate for DAMA. The escape velocity is $v_{esc} = 650\ \rm{km/s}$. The dashed (black) curve is the exclusion contour at $99\%$ CL for CDMS-Ge, while the dotted (green) curve is the exclusion limit at $99\%$ for XENON10.
• Figure 3: Elastic scenario (predictions of IDM) Left panel for $v_{esc}=450$ km/s and right panel for $v_{esc}=650$ km/s. Allowed regions in the plane $|\lambda_{H_0}| - M_{H_0}$ consistent with the DAMA annual modulation signal at $90\%$, $99\%$ and $99.9\%$ CL. The red (solid) lines delimit the WMAP 5 years bounds at $3\ \sigma$ CL Komatsu:2008hk. The solid (brown) curve shows the total unmodulated rate for DAMA. The dashed (black) curve is the exclusion contour at $99\%$ CL for CDMS-Ge, while the dotted (green) curve is the exclusion limit at $99\%$ for XENON10.
• Figure 4: Elastic scenario (predictions of IDM) Allowed regions in the plane $\rho_{\rm{DM}} - M_{H_0}$ consistent with the DAMA annual modulation signal at $90\%$, $99\%$ and $99.9\%$ CL for a fixed coupling $|\lambda_{H_0}| = 0.12$. The escape velocity is fixed to $v_{esc} = 650\ \rm{km/s}$. The bounds are the same as in Figure \ref{['fig:mlbda']}.
• Figure 5: Inelastic scenario (predictions of IDM) On the left, the averaged modulated amplitude $S_{m}$ for the best fit points (for $v_{esc}= 600$ km/s, see also Figure \ref{['fig:mdelta']}). On the right: Values for the global and local minima of the chi-square distribution for the best fit points. The $\chi^2_{min}$ is evaluated for 34 d.o.f. .