[Paper Review] Fuelling the search for light dark matter-electron scattering

Louis Hamaide, Christopher McCabe|arXiv (Cornell University)|Oct 8, 2021

Dark Matter and Cosmic Phenomena4 references4 citations

TL;DR

This paper proposes using the DarkSphere Spherical Proportional Counter (SPC) detector, capable of single-electron sensitivity and low background rates, to search for light dark matter via electron scattering. By simulating DM-electron scattering in five gaseous targets—helium, neon, xenon, methane, and isobutane—using quantum chemistry tools, the study shows DarkSphere could improve current exclusion limits for dark matter masses above 4 MeV by up to five orders of magnitude, with neon as the optimal single gas and gas mixtures enhancing sensitivity.

ABSTRACT

Dark matter (DM) detectors employing a Spherical Proportional Counter (SPC) have demonstrated a single-electron detection threshold and are projected to have small background rates. We explore the sensitivity to DM-electron scattering with SPC detectors in the context of DarkSphere, a proposal for a 300 cm diameter fully-electroformed SPC. SPCs can run with different gases, so we investigate the sensitivity for five targets: helium, neon, xenon, methane, and isobutane. We use tools from quantum chemistry to model the atomic and molecular systems, and calculate the expected DM induced event rates. We find that DarkSphere has the potential to improve current exclusion limits on DM masses above 4 MeV by up to five orders of magnitude. Neon is the best all-round gas target but using gas mixtures, where methane and isobutane constitute 10% of the gas, can improve the sensitivity, especially when combined with helium. Our study highlights the currently untapped potential of SPCs to search for DM-electron scattering in the MeV-to-GeV DM mass range.

Motivation & Objective

• To evaluate the sensitivity of Spherical Proportional Counter (SPC) detectors to dark matter-electron scattering in the MeV-to-GeV mass range.
• To identify optimal gaseous targets for SPC-based dark matter detection by comparing atomic and molecular response functions.
• To assess the potential of gas mixtures—particularly those with 10% methane or isobutane—relative to pure gases in enhancing detection sensitivity.
• To quantify the expected event rates for dark matter scattering on electrons using quantum chemistry-based modeling of electron orbitals and transition matrix elements.

Proposed method

• Employing quantum chemistry methods to compute electron density distributions and transition matrix elements for atomic and molecular systems in various gases.
• Modeling dark matter-electron scattering rates using the calculated electronic structure data and the non-relativistic effective field theory framework.
• Simulating detector response for SPCs with a 300 cm diameter, fully electroformed structure, assuming single-electron energy resolution and low intrinsic background rates.
• Evaluating sensitivity across five target gases: helium, neon, xenon, methane, and isobutane, including mixtures with 10% methane or isobutane by volume.
• Projecting exclusion limits on dark matter-nucleon and dark matter-electron coupling strengths using projected event rate spectra and statistical sensitivity analysis.

Experimental results

Research questions

• RQ1 What is the sensitivity of the DarkSphere SPC detector to dark matter-electron scattering across different gaseous targets?
• RQ2 How does the choice of gas—pure or mixed—affect the projected detection sensitivity for MeV-scale dark matter?
• RQ3 What is the expected event rate for dark matter scattering on electrons in neon, helium, xenon, methane, and isobutane under SPC operating conditions?
• RQ4 Can gas mixtures with 10% methane or isobutane enhance sensitivity beyond that of pure gases in the SPC configuration?
• RQ5 To what extent can the SPC’s single-electron threshold and low background rates improve exclusion limits compared to existing experiments?

Key findings

• DarkSphere has the potential to improve current exclusion limits for dark matter masses above 4 MeV by up to five orders of magnitude.
• Neon emerges as the best single-gas target due to favorable electron density and scattering cross-section characteristics.
• Incorporating 10% methane or isobutane into a gas mixture with helium enhances sensitivity, particularly in the lower MeV dark matter mass range.
• The use of quantum chemistry tools enables accurate modeling of electron response functions, crucial for predicting scattering rates in complex molecular systems.
• The SPC’s single-electron detection threshold and low background rates are critical for achieving high sensitivity in the MeV-to-GeV dark matter mass window.
• Gas mixtures offer a viable path to extend sensitivity beyond pure gas targets, highlighting an untapped opportunity in SPC-based dark matter searches.