The gluon momentum density
xg(
x,
Q
2) of the proton was extracted at
Q
2 = 20 GeV
2 for small values of
x between 4 × 10
−4 and 10
−2 from the scaling violations of the proton structure function
F
2 ...measured recently by ZEUS in deep inelastic neutral current ep scattering at HERA. The extraction was performed in two ways. Firstly, using a global NLO fit to the ZEUS data on
F
2 at low
x constrained by measurementsfrom NMC at larger
x; and secondly using published approximate methods for the solution of the GLAP QCD evolution equations. Consistent results are obtained. A substantial increase of the gluon density is found at small
x in comparison with the NMC result obtained at larger values of
x.
Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identified using the cone algorithm. A cut on
x
γ
OBS, the fraction of the photon energy ...participating in the production of the two jets of highest transverse energy, is used to define cross sections sensitive to the parton distributions in the proton and in the photon. The dependence of the dijet cross sections on pseudorapidity has been measured for
x
γ
OBS ⩾ 0.75 and
x
γ
OBS < 0.75. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the ginon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations.
Inclusive jet differential cross sections for the reaction
ep → jet +
X at
Q
2 below 4 GeV
2 have been measured with the ZEUS detector at HERA using an integrated luminosity of 0.55 pb
−1. These ...cross sections are given in the kinematic region 0.2 <
y < 0.85, for jet pseudorapidities in the
ep-laboratory range −1 <
η
jet < 2 and refer to jets at the hadron level with a cone radius of one unit in the η - θ plane. These results correspond to quasi-real photoproduction at centre-of-mass energies in the range 130–270 GeV and, approximately, for jet pseudorapidities in the interval −3 <
η
jet(
λp CMS) < 0. These measurements cover a new kinematic regime of the partonic structure of the photon, at typical scales up to ∼300 GeV
2 and photon fractional momenta down to
x
γ
∼ 10
−2. Leading logarithm parton shower Monte Carlo calculations, which include both resolved and direct processes and use the predictions of currently available parametrisations of the photon parton distributions, describe in general the shape and magnitude of the measured
η
jet and
E
t
jet distributions.
Events with a large rapidity gap and total transverse energy greater than 5 GeV have been observed in quasi-real photoproduction at HERA with the ZEUS detector. The distribution of these events as a ...function of the
γp centre of mass energy is consistent with diffractive scattering. For total transverse energies above 12 GeV, the hadronic final states show predominantly a two-jet structure with each jet having a transverse energy greater than 4 GeV. For the two-jet events, little energy flow is found outside the jets. This observation is consistent with the hard scattering of a quasi-real photon with a colourless object in the proton.
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