Observation of events with an empty hemisphere in the Breit frame and differential cross section measurement

The Breit frame provides a natural reference frame to analyse electron-proton scattering events when the process of interest is plainly considered as a photon-hadron interaction. In the Breit frame, the photon runs on the $z$ axis in the positive direction, and in the leading order picture the struck quark leaves the interaction on the $z$ axis, too. Higher-order QCD corrections change that picture and at sufficiently low $x$, a rather spectacular event signature is predicted with no radiation in the forward direction but all emissions are expected to be found in the backward direction, from where the photon approaches. We report on a first observation of those empty current hemisphere events in electron-proton collisions at the HERA collider using data recorded with the H1 detector at a center-of-mass energy of 319 GeV. The large data sample corresponds to an integrated luminosity 351.1 pb$^{- 1}$ and allows for a differential cross-section measurement of these events. The data are compared to selected predictions from Monte Carlo event generators.