The H1 Collaboration reports the first measurement of the 1-jettiness event shape observable $\tau_1^b$ in neutral-current deep-inelastic electron-proton scattering. The analysis is based on data recorded in 2003-2007 by the H1 detector at the HERA collider for $ep$ collisions at $\sqrt{s}=319~\text{GeV}$, with integrated luminosity of $351.1~\text{pb}^{-1}$.
The observable $\tau_1^b$ is equivalent to a thrust observable defined in the Breit frame. The triple differential cross section is presented as a function of $\tau_1^b$, event virtuality $Q^2$, and inelasticity $y$, in the kinematic region $Q^2 > 150~\text{GeV}$. The data are compared to predictions from Monte Carlo event generators and fixed-order calculations in next-to-next-to-leading order in perturbative Quantum Chromodynamics.
These comparisons reveal sensitivity of the observable to parton shower and resummation effects, the magnitude of the strong coupling constant, and proton parton distribution functions, as well as the modeling of hadronization and fragmentation.