The production of D*+-(2010) mesons in deep inelastic scattering has been measured in the ZEUS detector at HERA using an integrated luminosity of 37 pb^-1. The decay channels D*+ -> D0 pi+(+c.c.), with D0 -> K- pi+ or D0 ->K- pi- pi+ pi+, have been used to identify the D mesons. The e+p cross section for inclusive D*+- production with 1<Q^2<600 GeV^2 and 0.02<y<0.7 is 8.31 +- 0.31(stat.) +0.30-0.50(syst.) nb in the kinematic region 1.5< pT(D*+-)<15 GeV and |eta(D*+-)|<1.5. Differential cross sections are consistent with a next-to-leading-order perturbative-QCD calculation when using charm-fragmentation models which take into account the interaction of the charm quark with the proton remnant. The observed cross section is extrapolated to the full kinematic region in pT(D*+-) and eta(D*+-) in order to determine the charm contribution, F^ccbar_2(x,Q^2), to the proton structure function. The ratio F^ccbar_2/F_2 rises from ~10% at Q^2 ~1.8 GeV^2 to ~30% at Q^2 ~130 GeV^2 for x values in the range 10^-4 to 10-3.

The shapes of jets with transverse energies, E_T(jet), up to 45 GeV produced in neutral- and charged-current deep inelastic e+p scattering (DIS) at Q**2 > 100 GeV**2 have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the eta-phi plane with a cone radius of one unit. The jets become narrower as E_T(jet) increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in E_T(jet) and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in e+e- interactions and narrower than those in pbarp collisions for comparable E_T(jet). Since the jets in e+e- interactions and e+p DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions.

The charged multiplicity distributions and the mean charged multiplicity have been investigated in inclusive neutral current deep inelastic $ep$ scattering with the ZEUS detector at HERA, using an integrated luminosity of 38.6 pb$^{-1}$. The measurements were performed in the current region of the Breit frame, as well as in the current fragmentation region of the hadronic centre-of-mass frame. The KNO-scaling properties of the data were investigated and the energy dependence was studied using different energy scales. The data are compared to results obtained in $\epem$ collisions and to previous DIS measurements as well as to leading-logarithm parton-shower Monte Carlo predictions.

Three- and four-jet final states have been measured in photoproduction at HERA using the ZEUS detector with an integrated luminosity of 121 pb^-1. The results are presented for jets with transverse energy E_T^jet>6 GeV and pseudorapidity |eta^jet|<2.4, in the kinematic region given by the virtuality of the photon Q^2<1 GeV^2 and the inelasticity 0.2<y<0.85 and in two mass regions defined as 25<M_nj<50 GeV and M_nj>50 GeV, where M_nj is the invariant mass of the n-jet system. The four-jet photoproduction cross section has been measured for the first time and represents the highest-order process studied at HERA. Both the three- and four-jet cross sections have been compared with leading-logarithmic parton-shower Monte Carlo models, with and without multi-parton interactions. The three-jet cross sections have been compared to an order(alpha alpha_s^2) perturbative QCD calculation.

The cross section for dijet photoproduction at high transverse energies is presented as a function of the transverse energies and the pseudorapidities of the jets. The measurement is performed using a sample of ep-interactions corresponding to an integrated luminosity of 6.3 pb^(-1), recorded by the ZEUS detector.Jets are defined by applying a k_T-clustering algorithm to the hadrons observed in the final state. The measured cross sections are compared to next-to-leading order QCD calculations. In a kinematic regime where theoretical uncertainties are expected to be small, the measured cross sections are higher than these calculations.

Dijet cross sections are presented using photoproduction data obtained with the ZEUS detector during 1994. These measurements represent an extension of previous results, as the higher statistics allow cross sections to be measured at higher jet transverse energy (ETJ). Jets are identified in the hadronic final state using three different algorithms, and the cross sections compared to complete next-to-leading order QCD calculations. Agreement with these calculations is seen for the pseudorapidity dependence of the direct photon events with ETJ > 6 GeV and of the resolved photon events with ETJ > 11 GeV. Calculated cross sections for resolved photon processes with 6 GeV < ETJ < 11 GeV lie below the data.

Photoproduction at HERA is studied in $ep$ collisions, with the ZEUS detector, for $\gamma p$ centre-of-mass energies ranging from 130-270 GeV. A sample of events with two high-$p_T$ jets ($p_T > 6$ GeV, $\eta <1.6$) and a third cluster in the approximate direction of the electron beam is isolated using a clustering algorithm. These events are mostly due to resolved photoproduction. The third cluster is identified as the photon remnant. Its properties, such as the transverse and longitudinal energy flows around the axis of the cluster, are consistent with those commonly attributed to jets, and in particular with those found for the two jets in these events. The mean value of the photon remnant $p_T$ with respect to the beam axis is measured to be $2.1 \pm 0.2$ GeV, which demonstrates substantial mean transverse momenta for the photon remnant.

First inclusive measurements of isolated prompt photons in photoproduction at the HERA ep collider have been made with the ZEUS detector, using an integrated luminosity of 38.4 pb$^{-1}$. Cross sections are given as a function of the pseudorapidity and the transverse energy ($\eta^\gamma$, \eTg) of the photon, for $\eTg > $ 5 GeV in the $\gamma p$ centre-of-mass energy range 134-285 GeV. Comparisons are made with predictions from Monte Carlo models having leading-logarithm parton showers, and with next-to-leading-order QCD calculations, using currently available parameterisations of the photon structure. For forward $\eta^\gamma$ (proton direction) good agreement is found, but in the rear direction all predictions fall below the data.

The forward-jet cross section in deep inelastic ep scattering has been measured using the ZEUS detector at HERA with an integrated luminosity of 6.36 pb^-1. The jet cross section is presented as a function of jet transverse energy squared, E(T,jet)^2, and Q^2 in the kinematic ranges 10^-2<E(T,jet)^2/Q^2<10^2 and 2.5 10^-4<x<8.0 10^-2. Since the perturbative QCD predictions for this cross section are sensitive to the treatment of the log(E_T/Q)^2 terms, this measurement provides an important test. The measured cross section is compared to the predictions of a next-to-leading order pQCD calculation as well as to various leading-order Monte Carlo models. Whereas the predictions of all models agree with the measured cross section in the region of small E(T,Jet)^2/Q^2, only one model, which includes a resolved photon component, describes the data over the whole kinematic range.

Inclusive photoproduction of D*+- mesons has been measured for photon-proton centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of 37 pb^-1. Total and differential cross sections as functions of the D* transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the "massive charm" and "massless charm" schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.