[Paper Review] Good NEWS for GeV Dark Matter Searches

Stefano Profumo|arXiv (Cornell University)|Jul 27, 2015

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper evaluates the sensitivity of the NEWS spherical proportional counter detector to two classes of low-mass (GeV-scale) dark matter: millicharged dark matter with charges between 10⁻⁹e and 10⁻⁶e, and Dirac fermion dark matter coupled to MeV–GeV-scale scalar or vector mediators. The detector’s sub-keV energy threshold and use of light targets (H, He) enable it to probe previously inaccessible parameter space, particularly for dark matter below 0.3 GeV, surpassing the reach of solid-state experiments like SuperCDMS Si HV.

ABSTRACT

The proposed NEWS apparatus, a spherical detector with a small central electrode sensor operating as a proportional counter, promises to explore new swaths of the direct detection parameter space in the GeV and sub-GeV Dark Matter particle mass range by employing very light nuclear targets, such as H and He, and by taking advantage of a very low (sub-keV) energy threshold. Here we discuss and study two example classes of Dark Matter models that will be tested with NEWS: GeV-scale millicharged Dark Matter, and a GeV-Dirac Fermion Dark Matter model with a light (MeV-GeV) scalar or vector mediator, and indicate the physical regions of parameter space the experiment can probe.

Motivation & Objective

• To assess the scientific potential of the NEWS spherical proportional counter detector for probing low-mass dark matter candidates.
• To evaluate its sensitivity to millicharged dark matter with masses between 0.1 and 10 GeV and charges εe (10⁻⁹ to 10⁻⁶e).
• To study the reach of NEWS for a GeV-Dirac fermion dark matter model with light scalar or vector mediators.
• To identify the parameter space regions—especially in the sub-GeV range—where NEWS offers unique sensitivity compared to existing and planned experiments.
• To quantify the detector’s ability to probe the ratio $ g_n / m_\phi^2 $ in the range $ 2 \times 10^{-6} \lesssim g_n / (m_\phi / \text{GeV})^2 \lesssim 10^{-2} $.

Proposed method

• Modeling the direct detection cross section for millicharged dark matter via electromagnetic interactions, with constraints derived from CMB, neutrino scattering, and galactic shielding effects.
• Using the detector’s low energy threshold (sub-keV) and single electron sensitivity to simulate signal rates for light nuclear targets (H, He, Ne).
• Applying theoretical constraints on mediator lifetime and halo ellipticity to exclude unphysical regions of the parameter space.
• Projecting the detector’s sensitivity using signal-to-noise calculations and comparing with existing experiments like SuperCDMS Si HV and DAMIC100.
• Employing the electric field profile $ E \propto 1/r^2 $ in the spherical geometry to model electron drift and avalanche multiplication near the central sensor.
• Evaluating the impact of dark matter asymmetry and Sommerfeld enhancement on the effective coupling $ g_n / m_\phi^2 $.

Experimental results

Research questions

• RQ1 Can the NEWS detector probe millicharged dark matter with masses below 1 GeV and charges as low as 10⁻⁹e?
• RQ2 What is the sensitivity of NEWS to a GeV-Dirac fermion dark matter candidate coupled to a MeV–GeV-scale scalar or vector mediator?
• RQ3 How does the detector’s performance compare to solid-state experiments like SuperCDMS Si HV in the sub-GeV dark matter mass range?
• RQ4 Which regions of the $ g_n / m_\phi^2 $ parameter space are accessible to NEWS, and what constraints limit them?
• RQ5 To what extent do cosmological constraints from CMB and Big Bang nucleosynthesis affect the viable parameter space for light mediator dark matter models?

Key findings

• NEWS will probe millicharged dark matter with effective charges between $ 10^{-9}e $ and $ 10^{-6}e $, covering a broad range of previously untested parameter space.
• For the light mediator model, NEWS will probe the parameter space $ 2 \times 10^{-6} \lesssim g_n / (m_\phi / \text{GeV})^2 \lesssim 10^{-2} $ for dark matter masses between 0.15 GeV and 6 GeV.
• In the low-mass regime ($ m_X \lesssim 0.3 $ GeV), NEWS will be the only experiment capable of testing the $ g_n / m_\phi^2 $ parameter space for light mediator dark matter.
• The detector’s sub-keV energy threshold and use of light targets (H, He) enable it to access regions of parameter space beyond the reach of ton-scale noble gas and solid-state detectors.
• Constraints from CMB recombination and halo ellipticity exclude parts of the parameter space, but a large region remains viable and accessible to NEWS.
• For $ m_X = 1 $ GeV, the region of $ m_\phi $ and $ g_q $ compatible with direct detection is significantly larger than the excluded regions from BBN and halo ellipticity, indicating strong experimental potential.