[Paper Review] First Dark Matter Limits from the COUPP 4kg Bubble Chamber at a Deep Underground Site

Drew Fustin|arXiv (Cornell University)|Jan 14, 2024

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper presents the first dark matter search results from the COUPP 4 kg bubble chamber operated at SNOLAB’s 6000 m.w.e. deep underground site, using 4.0 kg of CF₃I as a target. It sets a new world-leading limit on the spin-dependent WIMP-proton scattering cross section and demonstrates sensitivity to spin-independent WIMP-nucleon interactions, with 20 candidate events attributed to internal neutron background.

ABSTRACT

The COUPP 4 kg bubble chamber employs 4.0 kg of CF$_3$I as a WIMP scattering target for use as a dark matter direct detection search. This thesis reports the first experimental results from operating this bubble chamber at the deep underground site (6000 m.w.e.) of SNOLAB, near Sudbury, Ontario. Twenty dark matter candidate events were observed during an effective exposure of 553.0 kg-days, when operating the bubble chamber at three different bubble nucleation thresholds. These data are consistent with a neutron background internal to the detector. Characterization of this neutron background has led to the recommendation to replace two detector components to maximize dark matter signal sensitivity in a future run with this bubble chamber. A measurement of the gamma-ray flux has confirmed that this detector should not be sensitive to a gamma-induced background for more than three orders of magnitude below current sensitivity. The dark matter search data presented here set a new world-leading limit on the spin-dependent WIMP-proton scattering cross section and demonstrate significant sensitivity to spin-independent WIMP-nucleon scattering.

Motivation & Objective

• To conduct the first direct dark matter search using a 4 kg bubble chamber at a deep underground site.
• To measure the spin-dependent WIMP-proton scattering cross section with improved sensitivity.
• To characterize internal neutron backgrounds affecting signal interpretation.
• To assess the detector's sensitivity to gamma-ray-induced backgrounds.
• To optimize detector components for enhanced dark matter signal sensitivity in future runs.

Proposed method

• Operation of a 4.0 kg CF₃I-based bubble chamber at SNOLAB’s 6000 m.w.e. depth to reduce cosmic ray backgrounds.
• Use of three distinct bubble nucleation thresholds to vary detection sensitivity and identify event characteristics.
• Analysis of 553.0 kg-days of effective exposure data to search for WIMP-induced nuclear recoils.
• Characterization of neutron background sources via event rate and energy spectrum analysis.
• Gamma-ray flux measurement to evaluate potential gamma-induced background contamination.
• Component replacement recommendation based on neutron background analysis to improve future sensitivity.

Experimental results

Research questions

• RQ1 What is the sensitivity of the COUPP 4 kg bubble chamber to spin-independent and spin-dependent WIMP-nucleon scattering at a deep underground site?
• RQ2 What is the origin and magnitude of the observed 20 candidate events in the data?
• RQ3 How does the internal neutron background affect the interpretation of dark matter search results?
• RQ4 To what extent is the detector sensitive to gamma-ray-induced backgrounds?
• RQ5 Which detector components should be replaced to maximize future dark matter signal sensitivity?

Key findings

• The COUPP 4 kg bubble chamber set a new world-leading limit on the spin-dependent WIMP-proton scattering cross section.
• The 20 observed dark matter candidate events are consistent with an internal neutron background source.
• Neutron background characterization led to recommendations to replace two detector components to improve future sensitivity.
• The detector's gamma-ray flux measurement confirms insensitivity to gamma-induced backgrounds for more than three orders of magnitude below current sensitivity.
• The experiment demonstrated significant sensitivity to spin-independent WIMP-nucleon scattering, validating the detector's potential for future dark matter searches.
• The 553.0 kg-days of effective exposure provided a robust dataset for background and signal analysis at the deep underground SNOLAB site.