Investigation on light dark matter

TL;DR

The paper investigates direct detection of Light Dark Matter (LDM) candidates with sub-GeV masses via inelastic scattering on electrons or nuclei, since elastic scattering yields unobservably small energies. It derives the detectable recoil energy, showing that the energy release is dominated by the mass splitting Δ and the average mass m_bar, with <E_R> ≈ m_bar Δ /(m_H + m_T) and a narrow spread for most targets, enabling targeted energy windows in NaI(Tl) detectors. The authors parameterize the interaction rate with simple velocity-averaged cross sections and show that a modulated component arises from ⟨v^2⟩, allowing DAMA/NaI data to constrain the LDM parameter space. Through a model-dependent analysis of DAMA/NaI time-energy data, they derive 4σ allowed volumes in (m_H, Δ, ξ σ_m) for both electron- and nucleus-driven scenarios, finding viable regions for tens of keV to GeV-scale LDM that could also relate to 511 keV gamma-ray phenomenology in the Galactic center. The work broadens the potential DM candidates consistent with DAMA's annual modulation signal and highlights the role of inelastic channels and detector effects (quenching, channeling) in shaping the observable signatures.

Abstract

Some extensions of the Standard Model provide Dark Matter candidate particles with sub-GeV mass. These Light Dark Matter particles have been considered for example in Warm Dark Matter scenarios (e.g. the keV scale sterile neutrino, axino or gravitino). Moreover MeV scale DM candidates have been proposed in supersymmetric models and as source of the 511 keV line from the Galactic center. In this paper the possibility of direct detection of a Light Dark Matter candidate is investigated considering the inelastic scattering processes on the electron or on the nucleus targets. Some theoretical arguments are developed and related phenomenological aspects are discussed. Allowed volumes and regions for the characteristic phenomenological parameters of the considered scenarios are derived from the DAMA/NaI annual modulation data.

Figures (8)

• Figure 1: Inelastic scattering process considered for the detection of the Light Dark Matter candidate. The 4-momenta are defined in the laboratory frame. The target T can be either an atomic nucleus or an atomic electron.
• Figure 2: Configurations in the plane $\Delta$ vs $m_H$ (shaded areas), corresponding to released energies in NaI(Tl) within the energy interval 1-6 keV electron equivalent. The upper area is due to LDM interaction on Na and I nuclear targets, while the lower area to LDM interaction on electron target. The dashed line ($m_H = \Delta$) marks the case where $\nu_L$ is a massless particle. The configurations characterized by $\Delta \ge 2m_e$ (dark area) are of interest for the positron annihilation line from the galactic center through the decay: $\nu_H \rightarrow \nu_L e^+e^-$. The thresholds of the possible annihilation processes: $\nu_H \bar{\nu}_H \rightarrow e^+ e^-$ (solid vertical line at $m_H$ = 511 keV); $\nu_H \bar{\nu}_L \rightarrow e^+ e^-$ (solid curve); $\nu_L \bar{\nu}_H \rightarrow e^+ e^-$ (solid curve); $\nu_L \bar{\nu}_L \rightarrow e^+ e^-$ (dotted curve) are shown.
• Figure 3: Examples of the energy distributions of the unmodulated (solid) and of the modulated (dotted) parts of the expected differential rate in NaI(Tl) for interactions of LDM with $m_H=100$ MeV on the target nuclei. Left: case of coherent cross section scaling laws; here $\Delta = 4.8$ MeV and $\xi \sigma_0^{coh} \ll \xi \sigma_m^{coh} \simeq 2 \times 10^{-6}$ pb. Right: case of incoherent cross section scaling laws; here $\Delta = 0.95$ MeV and $\xi \sigma_0^{inc} \ll \xi \sigma_m^{inc} \simeq 20 \times 10^{-3}$ pb. The A5 halo model (a NFW halo model with local velocity equal to 220 km/s and density equal to the maximum value, see ref. RNCijmd) has been considered. The quenching factors have been assumed as the case A of ref. RNCijmd. The channeling effect has been included; see text. The vertical dotted lines correspond to the energy threshold of the NaI(Tl) detectors used in DAMA/NaI set-up.
• Figure 4: Case of electron interacting LDM. Projection of the 4$\sigma$ allowed 3-dimensional volume on the plane ($m_H$, $\Delta$) for the same dark halo models and parameters described in ref. RNC;see text. The dashed line ($m_H = \Delta$) marks the case where $\nu_L$ is a massless particle. The decay through the detection channel, $\nu_H \rightarrow \nu_L e^+ e^-$, is energetically not allowed for the selected configurations. The configurations with $m_H \mathrel{\mathop{ \hbox{$>$}} \hbox{$\sim$}} m_e$ (dark area) are interesting for the possible annihilation processes: $\nu_H \bar{\nu}_H \rightarrow e^+ e^-$, $\nu_H \bar{\nu}_L \rightarrow e^+ e^-$, $\nu_L \bar{\nu}_H \rightarrow e^+ e^-$, and $\nu_L \bar{\nu}_L \rightarrow e^+ e^-$ in the galactic center. The three nearly vertical curves are the thresholds of these latter processes as mentioned in the caption of Fig.\ref{['fg:reg']} and in the text.
• Figure 5: Case of electron interacting LDM. Left: examples of some slices of the 4$\sigma$ allowed 3-dimensional volume for various $m_H$ depicted in the ($\xi\sigma^e_m$ vs $\Delta$) plane. Right: slice of the 4$\sigma$ allowed 3-dimensional volume for $m_H = \Delta$, that is for a massless or a very light $\nu_L$ particle, as e.g. either an active neutrino or a nearly massless sterile one or the light axion, etc. The same dark halo models and parameters described in ref. RNC have been used; see text.