Measurements of deep inelastic scattering at high (x,Q[sup 2]) with the ZEUS detector at HERA

Parton densities in a nucleon

“…The experiment [1] shows that the deep inelastic structure function F 2 (x, Q 2 ) has a steep behaviour in the small x region (10 −2 > x > 10 −5 ), even for very small virtualities. Indeed, considering F 2 ∝ x −λ for small x, the experimental data go from λ = 0.15 for Q 2 = 0.85 GeV 2 to λ = 0.4 for Q 2 = 20 GeV 2 .…”

“…We decided to choose the value of r 2 ⊥ 0 = 1 GeV −2 as such scale. The argument comes from the new HERA data on the deep inelastic structure function F 2 (x, Q 2 ) [1] which show that the GRV parameterization works quite well starting from Q 2 ≥ 1 GeV 2 . Therefore, we consider the GRV structure functions at Q 2 = 1 GeV 2 both for gluons and quarks, as a lucky guess of the initial conditions for the QCD evolution that takes into account the unknown nonperturbative large distance physics, disregarding all physical motivation for such a guess.…”

“…σ(GG) is the gluon cross section inside the parton cascade and was evaluated in [7]. Using HERA data on photoproduction of J/Ψ meson [1] the value of R 2 was estimated as R 2 ≤ 5 GeV −2 [6]. Using the GRV parameterization [4] for the gluon structure function and the value of R 2 = 5 GeV −2 , we obtain that κ reaches 1 at HERA kinematic region ( see Fig.1 ), meaning shadowing corrections should not be neglected (see Ref.…”

The hadronic final state at HERA

“…Some of the first measurements at HERA established the potentially hard scattering nature of photoproduction through the observation of two jets with significant transverse energy (Ahmed et al, 1992;Derrick et al, 1992) in events in which no scattered electron was observed in the main detectors. The need for both directand resolved-photon interactions in describing photoproduction at HERA was also shown by comparing data with models of just one of the processes or the combined prediction.…”

“…The H1 forward proton spectrometer (FPS) (Adloff et al, 1999d;Van Esch et al, 2000) and ZEUS leading proton spectrometer (LPS) (Derrick et al, 1997) detected positively charged particles carrying a substantial fraction of the incoming proton energy. The particle trajectories were measured using a system of detectors that could be inserted very close (typically a few mm) to the proton beam, positioned between 64 m and 80 m and 24 m and 90 m from the interaction point for the FPS and LPS, respectively.…”