The Data

The data presented here were taken with the ZEUS detector at HERA in 1994, during which period HERA operated with bunches of 820 GeV protons and 27.5 GeV positrons. The ZEUS detector has been described elsewhere [5]. DIS data were selected. The kinematic variables x, Q² and $\vec{\beta}\,$ were reconstructed using the Double Angle method [6].

The sizes of the bins in x and Q² were consistent with the measured resolutions and restricted to regions of high acceptance for tracks in the current region. They were based on the bins chosen in [7]. Tracks associated with the primary vertex and satisfying p_T > 150 MeV and |$\eta$| < 1.75, where $\eta$ is the pseudorapidity measured relative to the proton beam direction, were selected and boosted to the Breit frame. The scaled momentum, $x_p = 2p^{\mbox{{\tiny Breit}}}/Q$, of those reconstructed in the current region was then measured.

The data were corrected for the effects of the trigger and event selection cuts; event migration between (x, Q²) bins; QED radiative effects; track reconstruction efficiency; track selection cuts in p_T and $\eta$; track migration between current and target regions; and for the decay products of K⁰_S and $\Lambda$ decays being misassigned to the primary vertex. The most significant contribution to the resolution on the measurement of x_p was found to be from the uncertainty in the reconstruction of the boost vector, $\vec{\beta}\,$.