Speaker
Description
The Breit frame provides a natural frame to analyze lepton--proton scattering events. In this reference frame, the parton model hard interactions between a quark and an exchanged boson defines the coordinate system such that the struck quark is back-scattered along the virtual photon momentum direction. In Quantum Chromodynamics (QCD), higher order perturbative or non-perturbative effects can change this picture drastically. The 1-jettiness event shape observable $\tau_1^b$ is mapping out the transition from single-jet events with $\tau_1^b=0$ up to extreme configurations where the current hemisphere is empty and hence $\tau_1^b=1$. The data sample was collected at the HERA $ep$ collider in the years 2003--2007 with center-of-mass energy of $\sqrt{s}=319$ GeV, corresponding to an integrated luminosity of 351.1 pb$^{-1}$. Triple differential cross sections are provided as a function of $\tau_1^b$, event virtuality $Q^2$, and inelasticity $y$, in the kinematic region $Q^2>150$ GeV$^2$. Double differential cross sections are measured, in contrast, integrated over $\tau_1^b$ and represent the inclusive neutral-current DIS cross section measured as a function of $Q^2$ and $y$. The data are compared to a variety of predictions and include classical and modern Monte Carlo event generators, predictions in fixed-order perturbative QCD where calculations up to $\mathcal{O}(\alpha_s^3)$ are available for $\tau_1^b$ or inclusive DIS, and resummed predictions at next-to-leading logarithmic accuracy matched to fixed order predictions at $\mathcal{O}(\alpha_s^2)$. The fraction of inclusive neutral-current DIS events with an empty hemisphere is also determined and is found to be $0.0112 \pm 3.9\%_\text{stat} \pm 4.5\%_\text{syst} \pm 1.6\%_\text{mod}$ in the selected kinematic region of $150
