[Paper Review] CYGNUS: Feasibility of a nuclear recoil observatory with directional sensitivity to dark matter and neutrinos

S. Vahsen, Ciaran A. J. O’Hare+17 more|arXiv (Cornell University)|Aug 28, 2020

Dark Matter and Cosmic Phenomena6 references51 citations

TL;DR

The paper analyzes the feasibility of a large, modular, direction-sensitive gas TPC detector (Cygnus) to detect dark matter via nuclear recoils and to study neutrinos, including below the neutrino floor.

ABSTRACT

Now that conventional weakly interacting massive particle (WIMP) dark matter searches are approaching the neutrino floor, there has been a resurgence of interest in detectors with sensitivity to nuclear recoil directions. A large-scale directional detector is attractive in that it would have sensitivity below the neutrino floor, be capable of unambiguously establishing the galactic origin of a purported dark matter signal, and could serve a dual purpose as a neutrino observatory. We present the first detailed analysis of a 1000 m$^3$-scale detector capable of measuring a directional nuclear recoil signal at low energies. We propose a modular and multi-site observatory consisting of time projection chambers (TPCs) filled with helium and SF$_6$ at atmospheric pressure. Depending on the TPC readout technology, 10-20 helium recoils above 6 keVr or only 3-4 recoils above 20 keVr would suffice to distinguish a 10 GeV WIMP signal from the solar neutrino background. High-resolution charge readout also enables powerful electron background rejection capabilities well below 10 keV. We detail background and site requirements at the 1000 m$^3$-scale, and identify materials that require improved radiopurity. The final experiment, which we name CYGNUS-1000, will be able to observe 10-40 neutrinos from the Sun, depending on the final energy threshold. With the same exposure, the sensitivity to spin independent cross sections will extend into presently unexplored sub-10 GeV parameter space. For spin dependent interactions, already a 10 m$^3$-scale experiment could compete with upcoming generation-two detectors, but CYGNUS-1000 would improve upon this considerably. Larger volumes would bring sensitivity to neutrinos from an even wider range of sources, including galactic supernovae, nuclear reactors, and geological processes.

Motivation & Objective

• Motivate and define the science case for a large directional nuclear recoil observatory capable of probing WIMP parameter space below the neutrino floor.
• Propose a 1000 m^3 scale modular Cygnus detector using helium and SF6 gas and evaluate readout technologies.
• Assess backgrounds, site requirements, and radiopurity needs for a scalable multi-site detector.
• Estimate the potential WIMP discovery reach and neutrino observability for Cygnus-1000 and larger volumes.

Proposed method

• Propose a modular back-to-back TPC architecture with short drift distances to enable directional recoil reconstruction.
• Compare readout technologies and identify high-resolution strip readout TPCs as optimal for performance and cost.
• Model recoil direction and energy distributions for WIMPs and neutrinos using standard SI/SD cross sections and velocity distributions.
• Define neutrino floor for directional detection by combining solar, DSNB, and atmospheric backgrounds and evaluating angular discrimination.
• Compute projected exclusion limits for WIMP-nucleon cross sections across SI and SD channels for various detector volumes and energy thresholds.
• Assess background rejection capabilities and radiopurity requirements for a 1000 m^3 scale Cygnus detector.

Experimental results

Research questions

• RQ1 Can a 1000 m^3 gas TPC with directional readout measure nuclear recoil directions with sufficient angular resolution and head-tail discrimination down to low energies (e.g., 6 keV_nr for He recoils)?
• RQ2 To what extent can directional information circumvent the neutrino floor for SI and SD WIMP searches across a range of WIMP masses and detector volumes?
• RQ3 What are the background, site, and material radiopurity requirements to realize a scalable modular Cygnus detector?
• RQ4 How does the Cygnus reach compare to existing limits, and what neutrino signals (solar, DSNB, atmospheric) can Cygnus observe?

Key findings

• Cygnus-1000 with He:SF6 at 755 Torr and 1000 m^3 can detect WIMP signals down to sub-10 GeV masses and reach below the traditional neutrino floor for certain thresholds.
• Directional capabilities and head-tail recognition enable distinguishing WIMP-induced recoils from solar neutrinos, potentially allowing conclusions about galactic origin.
• An average of 4–5 detected 100 GeV/c^2 WIMP fluorine recoils above 50 keV_r or 10–20 helium recoils above 6 keV_r could rule out isotropy at 90% CL under certain readout assumptions.
• An average of 3–4 helium recoils above 20 keV_r suffices to distinguish a 10 GeV/c^2 WIMP signal from solar neutrinos, given adequate angular resolution.
• The final Cygnus-1000 concept could observe 10–40 solar neutrinos and extend SI cross-section sensitivity into currently unexplored sub-10 GeV/c^2 space; SD sensitivity with smaller volumes is competitive with future detectors.
• A modular, multi-site approach helps scale to larger total masses while managing backgrounds and site constraints.