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 leading to exotic effects. However, recent experiments on newer samples question this, necessitating a direct probe. Using pulse-echo ultrasound, the authors measured UTe₂'s elastic moduli in samples exhibiting one and two superconducting transitions. The absence of thermodynamic discontinuities in shear elastic moduli provides direct evidence for a single-component superconducting order parameter. Superconductivity's sensitivity to compression strain along the *a* and *c* axes, but not *b*, suggests a single-component, odd-parity B_2u order parameter.

Florian Theuss, Avi Shragai, Gael Grissonnanche, Ian M Hayes, Shanta R Saha, Yun Suk Eo, Alonso Suarez, Tatsuya Shishidou, Nicholas P Butch, Johnpierre Paglione, B. J. Ramshaw