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 data were recorded in 2003-2007 by the H1 detector at the HERA collider for \(ep\) collisions at \(\sqrt{s}=319~\text{GeV}\). The integrated luminosity amounts to \(351.1~\text{pb}^{-1}\).
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 \(Q^2 > 150~\text{GeV}^2\).
The data are compared to predictions from Monte Carlo event generators and NNLO pQCD calculations. These comparisons reveal sensitivity of the observable to QCD 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.
Furthermore, a peculiar event topology occurring at low Bjorken-\(x\) is investigated. Higher-order QCD effects can distort the simple picture of the Breit frame. This leads to configuration where all final state particles are located in one hemisphere, while the other one remains completely empty. We
present the first observation of these so-called empty hemisphere events.
The inclusive fraction is found to be \(0.0112\pm 0.0007\) in the considered phase space \(150 < Q^2 < 1500~\text{GeV}^2\), \(0.14 < y < 0.7\).