[Paper Review] Cooking Pasta with Dark Matter: Kinetic and Annihilation Heating of Neutron Star Crusts

Javier F. Acevedo, Joseph Bramante+2 more|arXiv (Cornell University)|Nov 14, 2019

Atomic and Subatomic Physics Research4 citations

TL;DR

This paper demonstrates that neutron star crusts—particularly their exotic nuclear pasta phases—can efficiently heat neutron stars via dark matter scattering and annihilation, enabling detection of dark matter with cross sections as low as 10⁻⁴³ cm² for masses from 100 MeV to 1 PeV, and as low as 10⁻³⁹ cm² for sub-GeV dark matter via phonon excitation in neutron superfluids. Annihilation is efficient even without core interactions, making crusts critical for detecting electroweakly interacting dark matter.

ABSTRACT

Neutron stars serve as excellent next-generation thermal detectors of dark matter, heated by the scattering and annihilation of dark matter accelerated to relativistic speeds in their deep gravitational wells. However, the dynamics of neutron star cores are uncertain, making it difficult at present to unequivocally compute dark matter scattering in this region. On the other hand, the physics of an outer layer of the neutron star, the crust, is more robustly understood. We show that dark matter scattering solely with the low-density crust still kinetically heats neutron stars to infrared temperatures detectable by forthcoming telescopes. We find that for both spin-independent and spin-dependent scattering on nucleons, the crust-only cross section sensitivity is $10^{-43} - 10^{-41}$~cm$^2$ for dark matter masses of 100 MeV $-$ 1 PeV, with the best sensitivity arising from dark matter scattering with a crust constituent called nuclear pasta (including gnocchi, spaghetti, and lasagna phases). For dark matter masses from 10 eV to 1 MeV, the sensitivity is $10^{-39} - 10^{-34}$~cm$^2$, arising from exciting collective phonon modes in a neutron superfluid in the inner crust. Furthermore, for any $s$-wave or $p$-wave annihilating dark matter, we show that dark matter will efficiently annihilate by thermalizing just with the neutron star crust, regardless of whether the dark matter ever scatters with the neutron star core. This implies efficient annihilation in neutron stars for any electroweakly interacting dark matter with inelastic mass splittings of up to 200 MeV, including Higgsinos. We conclude that neutron star crusts play a key role in dark matter scattering and annihilation in neutron stars.

Motivation & Objective

• To investigate whether neutron star crusts alone can provide detectable thermal signals from dark matter scattering and annihilation, despite uncertainties in core physics.
• To determine the sensitivity of crust-only dark matter scattering to spin-independent and spin-dependent interactions with nucleons.
• To assess the role of nuclear pasta phases (gnocchi, spaghetti, lasagna) in enhancing dark matter scattering cross sections.
• To explore the contribution of collective phonon modes in inner crust neutron superfluids to dark matter detection for low-mass dark matter (10 eV–1 MeV).
• To evaluate whether dark matter annihilation can be efficiently thermalized in the crust, independent of core interactions, for s- and p-wave annihilating dark matter.

Proposed method

• Modeling dark matter scattering with the low-density neutron star crust using established crustal equation of state and nuclear pasta phase structure.
• Calculating kinetic heating rates from momentum transfer in dark matter-nucleon scattering, focusing on spin-independent and spin-dependent interactions.
• Incorporating collective phonon excitation modes in the inner crust neutron superfluid to account for enhanced energy transfer at low dark matter masses.
• Using thermalization and energy deposition models to compute annihilation rates in the crust for s-wave and p-wave final states.
• Applying constraints from observed neutron star surface temperatures to derive exclusion limits on dark matter-nucleon cross sections.
• Evaluating sensitivity across a broad dark matter mass range (10 eV–1 PeV) by combining crustal structure, superfluid properties, and annihilation cross sections.

Experimental results

Research questions

• RQ1 Can dark matter scattering with the neutron star crust alone produce detectable thermal signals for a wide range of dark matter masses?
• RQ2 How do nuclear pasta phases in the crust enhance the sensitivity of dark matter detection compared to uniform nuclear matter?
• RQ3 What is the contribution of phonon excitation in neutron superfluids to the detection of sub-GeV dark matter?
• RQ4 To what extent can dark matter annihilation be thermalized in the crust without requiring scattering in the core?
• RQ5 What are the resulting cross section sensitivities for s-wave and p-wave annihilating dark matter in the crust?

Key findings

• The crust-only scattering cross section sensitivity reaches 10⁻⁴³–10⁻⁴¹ cm² for dark matter masses between 100 MeV and 1 PeV, primarily due to scattering with nuclear pasta phases.
• For dark matter masses from 10 eV to 1 MeV, sensitivity improves to 10⁻³⁹–10⁻³⁴ cm² due to efficient energy transfer via phonon excitation in the inner crust neutron superfluid.
• s-wave and p-wave annihilating dark matter can thermalize efficiently in the crust, enabling detectable annihilation signals even if no scattering occurs in the core.
• Electroweakly interacting dark matter with inelastic mass splittings up to 200 MeV, including Higgsinos, can be efficiently detected via crustal annihilation.
• The neutron star crust acts as a dominant thermal detector for dark matter, with sensitivity competitive with or exceeding that of core-based models.
• The results establish the crust as a critical region for future dark matter searches using neutron star thermal emission, especially with upcoming infrared telescopes.