[Paper Review] Directional Detection of Light Dark Matter in Superconductors

Yonit Hochberg, Eric David Kramer+2 more|arXiv (Cornell University)|Sep 9, 2021

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes directional detection of keV-scale light dark matter in superconductors by exploiting the anisotropic angular distribution of quasiparticle (QP) excitations produced during dark matter scattering. It demonstrates that initial QP momentum directionality is preserved during down-conversion into secondary QPs and phonons, enabling directional sensitivity even in isotropic superconducting targets, with directionality strongest at low energy deposits (E < 3Δ).

ABSTRACT

Superconducting detectors have been proposed as outstanding targets for the direct detection of light dark matter scattering at masses as low as a keV. We study the prospects for directional detection of dark matter in isotropic superconducting targets from the angular distribution of excitations produced in the material. We find that dark matter scattering produces initial excitations with an anisotropic distribution, and further show that this directional information can be preserved as the initial excitations relax. Our results demonstrate that directional detection is possible for a wide range of dark matter masses, and pave the way for light dark matter discovery with bulk superconducting targets.

Motivation & Objective

• To investigate whether directional detection of light dark matter is feasible in isotropic superconducting targets.
• To determine if the angular distribution of initial quasiparticle excitations from dark matter scattering can be preserved through down-conversion processes.
• To evaluate the potential for directional sensitivity in superconducting detectors across a wide range of dark matter masses and interaction models.
• To assess the viability of using quasiparticle angular asymmetry as a signature to distinguish dark matter signals from backgrounds.
• To provide a theoretical foundation for next-generation superconducting dark matter experiments with directional sensitivity.

Proposed method

• Models dark matter scattering in superconductors using Bogoliubov quasiparticles as the fundamental degrees of freedom, replacing single-electron scattering for low-energy deposits.
• Computes the initial quasiparticle momentum distribution resulting from dark matter scattering, showing angular correlation with the incoming dark matter direction.
• Develops a new numerical code to simulate the down-conversion of initial quasiparticles into secondary quasiparticles and phonons, tracking angular and energy distributions.
• Introduces a two-bin asymmetry measure $\mathcal{A}_2$ to quantify directionality, defined as $\left|2 \times \frac{n_{\text{on}}}{n_{\text{on}} + n_{\text{off}}} - 1\right|$, where 'on-axis' has $|\cos\theta| > 1/2$.
• Analyzes the energy partition between quasiparticles and phonons during relaxation, showing that sub-gap phonons preserve directionality while above-gap phonons disrupt it.
• Evaluates the directionality across different dark matter masses and mediator types (light vs. heavy), comparing on-axis and off-axis event rates.

Experimental results

Research questions

• RQ1 Can the angular distribution of quasiparticle excitations from light dark matter scattering in superconductors preserve directional information?
• RQ2 To what extent is the initial directionality of quasiparticles maintained during the down-conversion process into secondary quasiparticles and phonons?
• RQ3 What is the dependence of directional sensitivity on the deposited energy and dark matter mass?
• RQ4 How does the nature of the mediator (light or heavy) affect the angular distribution of final-state excitations?
• RQ5 What is the maximum achievable directional asymmetry $\mathcal{A}_2$ for realistic superconducting targets?

Key findings

• Initial quasiparticle excitations from dark matter scattering in superconductors exhibit a directional distribution correlated with the incoming dark matter momentum.
• Directionality is preserved during down-conversion for quasiparticle deposits below $3\Delta$, where only sub-gap phonons are emitted, preventing secondary QP production.
• For deposits above $3\Delta$, emission of above-gap phonons leads to additional QP production with weak angular correlation, reducing directionality.
• The two-bin asymmetry $\mathcal{A}_2$ reaches a maximum of approximately 0.8 for low-energy deposits ($E < 3\Delta$), indicating strong directionality.
• The fraction of energy remaining in the quasiparticle system asymptotes to $\sim 0.60$ at high energies, consistent with prior studies in aluminum superconductors.
• Heavy mediators preserve directionality better than light mediators, with the on-axis/off-axis ratio remaining below 1 for a wider range of energies, enabling better signal-background discrimination.