at Ambient Pressure*

The microscopic mechanism of Cooper pairing in a superconductor leaves its fingerprint on the symmetry of the order parameter. UTe₂ has been inferred to have a multi-component order parameter that entails exotic effects like time reversal symmetry breaking. However, recent experimental observations in newer-generation samples have raised questions about this interpretation, pointing to the need for a direct experimental probe of the order parameter symmetry. Here, we use pulse echo ultrasound to measure the elastic moduli of samples of UTe₂ that exhibit both one and two superconducting transitions. We demonstrate the absence of thermodynamic discontinuities in the shear elastic moduli of both single- and double-transition samples, providing direct evidence that UTe₂ has a single-component superconducting order parameter. We further show that the superconductivity is highly sensitive to compression strain along the a and c axes, but insensitive to strain along the b axis. This leads us to suggest a single-component, odd-parity order parameter−specifically the B_2u order parameter−as the most likely order parameter in UTe₂.