DAMA/LIBRA and leptonically interacting Dark Matter

TL;DR

The paper tests whether dark matter that couples only to leptons can explain DAMA/LIBRA's annual modulation. Using a general EFT for DM–lepton interactions, it shows that loop-induced DM–nucleus scattering generically dominates over DM–electron scattering, reimposing the standard DAMA–CDMS/XENON tension; only a forbidden axial-vector case avoids loops, but its spectral predictions and Super-Kamiokande neutrino bounds make it untenable. Finite solar temperature effects on DM capture strengthen neutrino bounds, further disfavoring the leptophilic hypothesis. Collectively, leptophilic DM fails as a viable explanation for DAMA's signal, implying loop-induced nuclear recoils or other mechanisms must be at play.

Abstract

We consider the hypothesis that Dark Matter (DM) has tree-level interactions only with leptons. Such a framework, where DM recoils against electrons bound in atoms, has been proposed as an explanation for the annually modulated scintillation signal in DAMA/LIBRA versus the absence of a signal for nuclear recoils in experiments like CDMS or XENON10. However, even in such a leptophilic DM scenario there are loop induced DM-hadron interactions, where the photons emitted from virtual leptons couple to the charge of a nucleus. Using a general effective field theory approach we show that, if such an interaction is induced at one or two-loop level, then DM-nucleus scattering dominates over DM-electron scattering. This is because the latter is suppressed by the bound state wave function. One obtains a situation similar to standard DM-nucleus scattering analyses with considerable tension between the results of DAMA and CDMS/XENON10. This conclusion does not apply in the case of pseudoscalar or axial vector coupling between DM and leptons, where the loop diagrams vanish. In this case the explanation of the DAMA signal in terms of DM-electron scattering is strongly disfavored by the spectral shape of the signal. Furthermore, if DM can annihilate into neutrinos or tau leptons, the required cross sections are excluded by many orders of magnitude using the Super-Kamiokande bound on neutrinos from DM annihilations in the Sun.

Figures (11)

• Figure 1: Example for generating an effective local DM-electron interaction vertex (right diagram) as used in our analysis by the exchange of a heavy intermediate particle $\phi$ (left diagram).
• Figure 2: DM--nucleus interaction induced by a charged lepton loop and photon exchange at 1-loop (top) and 2-loop (bottom).
• Figure 3: Momentum space wave functions of iodine and sodium. Solid colored curves correspond to shells that contribute to $\rm WES$ in DAMA, while thin light curves are for shells that are not accessible in DAMA.
• Figure 4: Predicted spectrum for the modulated (top) and unmodulated (bottom) event rate in DAMA at the best fit point assuming loop induced WIMP--nucleus scattering resulting from vector-like DM--lepton couplings. Results are shown for using all data points from 2--8 keV (solid) and for omitting the 1st bin (dashed). The parameter values and the $\chi^2$-values are given in the legend.
• Figure 5: DAMA allowed region at 90% and 3$\sigma$ CL in the case of 1-loop induced WIMP--nucleus scattering ($V\otimes V$ coupling) in the plane of the WIMP mass and the WIMP--electron cross section $\sigma^0_{\chi e} = G^2 m_e^2/\pi$. Regions are shown with and without taking into account the channeling effect. Furthermore, we show the bounds at 90% CL from CDMS-II and XENON10. The dashed curves show the 90% CL constraint from the Super-Kamiokande limit on neutrinos from the Sun, by assuming annihilation into $\tau\bar{\tau}$ or $\nu\bar{\nu}$, see sec. \ref{['sec:SK']} for details.