Investigating electron interacting dark matter

TL;DR

The paper investigates dark matter candidates with a dominant coupling to electrons, proposing direct detection via electronic recoils and exploring potential links to the galactic 511 keV line. It develops the formalism for χ^0–electron scattering, incorporating atomic electron momentum distributions and a 4-fermion contact interaction, and derives the expected modulated counting rate within DAMA/NaI, focusing on the key normalization factor $(\xi \sigma_e^0)/m_{χ^0}$. Applying DAMA/NaI data across halo models yields an allowed range for this normalization and constrains mediator properties, including connections to a light U boson compatible with $g_e-2$ limits. The work highlights the viability of electron-only DM scenarios, provides concrete regions in parameter space, and discusses broader implications for mediator masses and alternative frameworks such as SUSY LSP-electron interactions.

Abstract

Some extensions of the Standard Model provide Dark Matter candidate particles which can have a dominant coupling with the lepton sector of the ordinary matter. Thus, such Dark Matter candidate particles ($χ^{0}$) can be directly detected only through their interaction with electrons in the detectors of a suitable experiment, while they are lost by experiments based on the rejection of the electromagnetic component of the experimental counting rate. These candidates can also offer a possible source of the 511 keV photons observed from the galactic bulge. In this paper this scenario is investigated. Some theoretical arguments are developed and related phenomenological aspects are discussed. Allowed intervals and regions for the characteristic phenomenological parameters of the considered model and of the possible mediator of the interaction are also derived considering the DAMA/NaI data.

Figures (6)

• Figure 1: The $\chi^0$ -- $e^-$ elastic scattering and definition of the momentum variables in the laboratory frame. In the text a contact interaction has been assumed (also see Appendix B) as suitable approximation of the process.
• Figure 2: Few examples of the dependence of the maximum released energy, $E_+$, on the $\chi^0$ mass for electron's momenta of 0.1, 1 and 5 MeV/c, for $v_{\chi^0}$ ranging in the interval $1 \div 2 \times 10^{-3}c$ and for head-on collisions ($\theta=\pi$).
• Figure 3: a) Behaviours of $\rho(p)$ (solid black line) for NaI(Tl) and $I_0$ and $I_m$ for $E_d = 3$ keV in the considered halo model, A5 of ref. HepRNC; see also text. The functions $I_0$ and $I_m$ are in arbitrary units. b) Behaviours of $p^2 p_0 \rho(p) I_m$ for NaI(Tl) at three different values of the released energy: $E_d =$ 3, 6 and 12 keV in the considered halo model, A5 of ref. HepRNC; they show as the main contribution to the counting rate in NaI(Tl) detectors with energy threshold at 2 keV comes from electrons with momenta around few MeV/c.
• Figure 4: An example of the shapes of expected energy distributions in NaI(Tl) due to $\chi^0$ interactions with electrons for the scenario given in the text; the solid line gives the behaviour of the unmodulated part of the expected signal, $S_0$, while the dashed line is the behaviour of the modulated part, $S_m$. In this example the normalization factor is $\frac{\xi \sigma^0_e}{m_{\chi^0}} = 7 \times 10^{-3}$ pb/GeV. The vertical line indicates the energy threshold of the DAMA/NaI experiment.
• Figure 5: The DAMA/NaI region allowed in the ($\xi \sigma_e^0$ vs $m_{\chi^0}$) plane for the same dark halo models and related parameters described in ref. RNC. The region encloses configurations corresponding to likelihood function values distant more than $4\sigma$ from the null hypothesis (absence of modulation). We note that, although the mass region in the plot is up to 2 TeV, $\chi^0$ particles with larger masses are also allowed.