[Paper Review] Towards Nuclear Physics of OHe Dark Matter

Maxim Khlopov, A. G. Mayorov+1 more|arXiv (Cornell University)|Nov 15, 2011

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper proposes that dark matter could consist of O-helium (OHe) atoms—neutral bound states of a stable -2 charged particle (O^{--}) and a helium-4 nucleus—whose nuclear interactions with detector nuclei lead to radiative capture and annual modulation of energy release. The model explains the DAMA/LIBRA annual modulation signal via keV-scale binding of OHe to sodium nuclei, offering a mechanism distinct from weakly interacting massive particles (WIMPs).

ABSTRACT

The nonbaryonic dark matter of the Universe can consist of new stable charged particles, bound in heavy "atoms" by ordinary Coulomb interaction. If stable particles $O^{--}$ with charge -2 are in excess over their antiparticles (with charge +2), the primordial helium, formed in Big Bang Nucleosynthesis, captures all $O^{--}$ in neutral "atoms" of O-helium (OHe). Interaction with nuclei plays crucial role in the cosmological evolution of OHe and in the effects of these dark atoms as nuclear interacting dark matter. Slowed down in terrestrial matter OHe atoms cause negligible effects of nuclear recoil in underground detectors, but can experience radiative capture by nuclei. Local concentration of OHe in the matter of detectors is rapidly adjusted to the incoming flux of cosmic OHe and possess annual modulation due to Earth's orbital motion around the Sun. The potential of OHe-nucleus interaction is determined by polarization of OHe by the Coulomb and nuclear force of the approaching nucleus. Stark-like effect by the Coulomb force of nucleus makes this potential attractive at larger distances, while change of polarization by the effect of nuclear force gives rise to a potential barrier, preventing merging of nucleus with helium shell of OHe atom. The existence of the corresponding shallow well beyond the nucleus can provide the conditions, at which nuclei in the matter of DAMA/NaI and DAMA/LIBRA detectors have a few keV binding energy with OHe, corresponding to a level in this well. Annual modulation of the radiative capture rate to this level can reproduce DAMA results. The OHe hypothesis can qualitatively explain the controversy in the results of direct dark matter searches by specifics of OHe nuclear interaction with the matter of underground detectors.

Motivation & Objective

• Explain the DAMA/LIBRA annual modulation signal as arising from O-helium (OHe) dark matter, not weakly interacting massive particles (WIMPs).
• Investigate the nuclear interaction of OHe atoms with detector nuclei, focusing on radiative capture and binding energy formation.
• Assess the feasibility of OHe as a form of strongly interacting dark matter that avoids anomalous isotope overproduction.
• Explore the implications of OHe-nucleus binding for direct detection experiments, especially in NaI and Ge detectors.
• Provide a theoretical framework for OHe as a composite dark matter candidate consistent with Big Bang Nucleosynthesis and cosmological constraints.

Proposed method

• Model OHe as a hydrogen-like bound state of O^{--} (charge -2) and ^4He^{++} (charge +2), using variational methods to compute binding energy corrections due to finite nuclear size.
• Calculate the effective potential for OHe approaching a nucleus, incorporating both Coulomb (Stark-like) attraction and nuclear force-induced polarization barrier.
• Use the potential well formed beyond the nucleus to identify shallow bound states with binding energies in the few keV range, particularly for sodium and germanium.
• Estimate radiative capture rates into these bound states, including annual modulation due to Earth's orbital motion around the Sun.
• Assess detector-specific responses by comparing binding energies in NaI, Ge, and Xe, and evaluate sensitivity differences across cryogenic and scintillating detectors.
• Account for thermal velocity effects in cryogenic detectors (e.g., CoGeNT) to explain apparent compatibility with CDMS constraints despite signal indications.

Experimental results

Research questions

• RQ1 Can the annual modulation observed in the DAMA/NaI and DAMA/LIBRA experiments be explained by radiative capture of OHe dark matter into keV-bound states with sodium nuclei?
• RQ2 What is the role of nuclear polarization in shaping the OHe-nucleus interaction potential, and does it allow for a shallow bound state beyond the nuclear surface?
• RQ3 Why do experiments like CRESST (with germanium) show signals while xenon-based detectors (e.g., XENON) do not, under the OHe model?
• RQ4 How does the OHe-nucleus interaction differ from standard WIMP-nucleus scattering, particularly in terms of energy deposition and detector response?
• RQ5 What are the cosmological and nuclear physics constraints on the existence of OHe as a stable, long-lived dark matter candidate?

Key findings

• The binding energy of OHe to sodium nuclei is estimated at approximately 4 keV, consistent with the energy scale of the DAMA signal.
• A shallow potential well formed by Coulomb and nuclear forces allows for a few keV bound state of OHe with intermediate-mass nuclei like sodium and germanium.
• Annual modulation of the radiative capture rate into this bound state arises from Earth’s orbital motion, matching the observed modulation in DAMA/NaI and DAMA/LIBRA.
• The model predicts the formation of anomalous superheavy isotopes in detectors, with mass increased by ~1.6 TeV (the O^{--} mass), which could serve as a unique experimental signature.
• The OHe model explains the absence of signals in xenon-based detectors and the apparent compatibility of CoGeNT results with CDMS constraints due to enhanced thermal velocity effects in cryogenic detectors.
• The model suggests that a positive WIMP signal in nuclear recoil experiments would be inconsistent with a pure OHe dark matter scenario, implying multicomponent dark matter if both OHe and WIMPs exist.