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Abstract:

Superfluid helium-3 bolometers can be utilised for dark matter direct detection searches. The extremely low heat capacity of the B phase of the superfluid helium-3 at ultra-low temperatures offers the potential to reach world leading sensitivity to spin-dependent interactions of dark matter in the sub-GeV/c2 mass range. Here, we describe the development of bolometry using both micron scale and sub-micron diameter vibrating wire resonators, with a SQUID amplifier-based readout scheme. Characterisation of the resonators and bolometer measurements are shown, including the use of nonlinear operation and the corresponding corrections. The bolometer contains two vibrating wire resonators, enabling heat injection calibration and simultaneous bolometer tracking measurements. Coincident events measured on both vibrating wire resonators verify their response. We also demonstrate proof of concept frequency multiplexed readout. Development of these measurement techniques lays the foundations for the use of superfluid helium-3 bolometers, instrumented with vibrating nanomechanical resonators, for future low-threshold dark matter searches.

Abstract:

The thermal confinement phase transition in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mrow> <mi>SU</mi> </mrow> <mo stretchy="false">(</mo> <mi>N</mi> <mo stretchy="false">)</mo> </mrow> </math> Yang-Mills theory is first order for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mi>N</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math> , with bounce action scaling as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <msup> <mrow> <mi>N</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msup> </mrow> </math> . Remarkably, lattice data for the action include a small coefficient whose presence likely strongly alters the phase transition dynamics. We give evidence, utilizing insights from softly broken supersymmetric Yang-Mills models, that the small coefficient originates from a deconfined phase instability just below the critical temperature. We predict the maximum achievable supercooling in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mi>SU</mi> </mrow> <mo stretchy="false">(</mo> <mi>N</mi> <mo stretchy="false">)</mo> </math> theories to be a few percent, which can be tested on the lattice. We briefly discuss the potentially significant suppression of the associated cosmological gravitational wave signals.

Abstract:

We present the projected sensitivity to non-relativistic Effective Field Theory (EFT) operators for dark matter (DM) direct detection using the QUEST-DMC experiment. QUEST-DMC employs superfluid Helium-3 as a target medium and measures energy deposition via nanomechanical resonators with SQUID-based readout to probe DM interactions. The experiment aims to explore new parameter space in the sub-GeV mass range, probing light DM and a broad range of interaction models. We analyse the sensitivity to a complete set of fourteen independent non-relativistic EFT operators, each parameterised by a Wilson coefficient that quantifies the strength of DM interactions with Standard Model particles. For each interaction channel, we determine the corresponding sensitivity ceiling due to attenuation of the DM flux incident on the detector, caused by DM scattering in the Earth and atmosphere. As a key component of this analysis, we provide the mapping between the non-relativistic EFT operators and the relativistic bilinear DM-nucleon interactions, and assess the interaction sensitivity to sub-GeV DM in the QUEST-DMC detector. Our findings demonstrate that QUEST-DMC provides a unique probe of DM interactions, particularly in previously unexplored parameter space for momentum- and velocity-dependent interactions, thereby expanding the search for viable DM candidates beyond traditional weakly interacting massive particles.